Michael Fogus

Chris Houser

FOREWORD BY STEVE YEGGE

M A N N I N G

The Joy of Clojure

The Joy of Clojure

MICHAEL FOGUS

CHRIS HOUSER

M A N N I N G

Greenwich

(74° w. long.)

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The Joy of Clojure ... wherein we teach you the joys of Clojure programming.

We have written this book for you, the adventurous programmer with prior

experience in Java or functional programming languages— especially Lisp.

Our aim is to enhance your programming knowledge in general,

and your understanding of Clojure in particular, by exploring

the philosophy, motivations, and semantics of the

Clojure programming language.

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brief contents

PART 1 FOUNDATIONS ......................................................................1

1

■

Clojure philosophy 3

2

■

Drinking from the Clojure firehose 20

3

■

Dipping our toes in the pool 43

PART 2 DATA TYPES . .......................................................................59

4 ■

On scalars 61

5 ■

Composite data types 76

PART 3 FUNCTIONAL PROGRAMMING............................................. 105

6 ■

Being lazy and set in your ways 107

7 ■

Functional programming 125

PART 4 LARGE-SCALE DESIGN ........................................................ 155

8

■

Macros 157

9

■

Combining data and code 177

10

■

Java.next 207

11

■

Mutation 234

PART 5 TANGENTIAL CONSIDERATIONS.......................................... 275

12 ■

Performance 277

13 ■

Clojure changes the way you think 292

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contents

foreword xvii

preface xix

acknowledgments xxi

about this book xxiii

PART 1 FOUNDATIONS .....................................................1

1 Clojure philosophy 3

1.1 The Clojure way 4

Simplicity 4 ■ Freedom to focus 5 ■ Empowerment 5

Clarity 6 ■ Consistency 7

1.2 Why a(nother) Lisp? 8

Beauty 9 ■ Extreme flexibility 9 ■ Code is data 11

1.3 Functional programming 12

A workable definition of functional programming 12

The implications of functional programming 12

1.4 Why Clojure isn’t especially object-oriented 13

Defining terms 13 ■ Imperative “baked in” 14 ■ Most of what

OOP gives you, Clojure provides 15

1.5 Summary 19

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x

CONTENTS

2 Drinking from the Clojure firehose 20

2.1 Scalars 21

Numbers 21 ■ Integers 22 ■ Floating-point numbers 22

Rationals 22 ■ Symbols 23 ■ Keywords 23 ■ Strings 23

Characters 23

2.2 Putting things together: collections 24

Lists 24 ■ Vectors 24 ■ Maps 25 ■ Sets 25

2.3 Making things happen: functions 25

Calling functions 25 ■ Defining functions 26 ■ Simplifying

function definitions with def and defn 27 ■ In-place functions

with #() 28

2.4 Vars 28

Declaring bindings using def 28

2.5 Locals, loops, and blocks 29

Blocks 29 ■ Locals 29 ■ Loops 30

2.6 Preventing things from happening: quoting 32

Evaluation 32 ■ Quoting 33 ■ Unquote 34

Unquote-splicing 35 ■ Auto-gensym 36

2.7 Leveraging Java via interop 36

Accessing static class members 36 ■ Creating Java class

instances 36 ■ Accessing Java instance members with the .

operator 37 ■ Setting Java instance properties 37

The .. macro 37 ■ The doto macro 38 ■ Defining classes 38

2.8 Exceptional circumstances 38

A little pitch and catch 38

2.9 Namespaces 39

Creating namespaces using ns 39 ■ Loading other namespaces

with :require 40 ■ Loading and creating mappings with :use 40

Creating mappings with :refer 41 ■ Loading Java classes with

:import 42

2.10 Summary 42

3 Dipping our toes in the pool 43

3.1 Truthiness 44

What’s truth? 44 ■ Don’t create Boolean objects 44

nil versus false 45

3.2 Nil pun with care 45

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CONTENTS

xi

3.3 Destructuring 47

Your assignment, should you choose to accept it 47

Destructuring with a vector 47 ■ Destructuring with a map 49

Destructuring in function parameters 50 ■ Destructuring versus

accessor methods 50

3.4 Using the REPL to experiment 51

Experimenting with seqs 51 ■ Experimenting with graphics 52

Putting it all together 54 ■ When things go wrong 54

Just for fun 56

3.5 Summary 56

PART 2 DATA TYPES .......................................................59

4 On scalars 61

4.1 Understanding precision 62

Truncation 62 ■ Promotion 63 ■ Overflow 64

Underflow 64 ■ Rounding errors 64

4.2 Trying to be rational 65

Why be rational? 66 ■ How to be rational 66 ■ Caveats of

rationality 67

4.3 When to use keywords 68

How are keywords different from symbols? 68 ■ Qualifying your

keywords 69

4.4 Symbolic resolution 70

Metadata 71 ■ Symbols and namespaces 71 ■ Lisp-1 72

4.5 Regular expressions—the second problem 73

Syntax 73 ■ Functions 74 ■ Beware of mutable matchers 75

4.6 Summary 75

5 Composite data types 76

5.1 Persistence, sequences, and complexity 77

“You keep using that word. I do not think it means what you think it

means.” 77 ■ Sequence terms and what they mean 78

Big-O 81

5.2 Vectors: creating and using them in all their varieties 82

Building vectors 82 ■ Large vectors 83 ■ Vectors as stacks 86

Using vectors instead of reverse 87 ■ Subvectors 88 ■ Vectors as

MapEntries 88 ■ What vectors aren’t 89

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xii

CONTENTS

5.3 Lists: Clojure’s code form data structure 90

Lists like Lisps like 90 ■ Lists as stacks 91 ■ What lists

aren’t 91

5.4 How to use persistent queues 91

A queue about nothing 92 ■ Putting things on 92 ■ Getting

things 93 ■ Taking things off 93

5.5 Persistent sets 94

Basic properties of Clojure sets 94 ■ Keeping your sets in order with

sorted-set 94 ■ contains? 95 ■ clojure.set 96

5.6 Thinking in maps 97

Hash maps 97 ■ Keeping your keys in order with sorted maps 99

Keeping your insertions in order with array maps 100

5.7 Putting it all together: finding the position of items 

in a sequence 101

Implementation 101

5.8 Summary 103

PART 3 FUNCTIONAL PROGRAMMING . ...........................105

6 Being lazy and set in your ways 107

6.1 On immutability 107

Defining immutability 108 ■ Being set in your ways—

immutability 109

6.2 Designing a persistent toy 110

6.3 Laziness 113

Familiar laziness with logical-and 113 ■ Understanding the lazy-

seq recipe 115 ■ Losing your head 117 ■ Employing infinite

sequences 118 ■ The delay and force macros 119

6.4 Putting it all together: a lazy quicksort 121

6.5 Summary 124

7 Functional programming 125

7.1 Functions in all their forms 126

First-class functions 126 ■ Higher-order functions 129

Pure functions 131 ■ Named arguments 132 ■ Constraining

functions with pre- and postconditions 133

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CONTENTS

xiii

7.2 Closures 135

7.3 Thinking recursively 141

Mundane recursion 142 ■ Tail calls and recur 143 ■ Don’t

forget your trampoline 146 ■ Continuation-passing style 148

7.4 Putting it all together: A* pathfinding 149

The A* implementation 151 ■ Notes about the A*

implementation 154

7.5 Summary 154

PART 4 LARGE-SCALE DESIGN. .......................................155

8 Macros 157

8.1

Data is code is data 158

Syntax-quote, unquote, and splicing 160 ■ Macro rules of

thumb 161

8.2 Defining control structures 161

Defining control structures without syntax-quote 162

Defining control structures using syntax-quote and unquoting 163

8.3 Macros combining forms 164

8.4 Using macros to change forms 165

8.5 Using macros to control symbolic resolution time 169

Anaphora 170 ■ (Arguably) useful selective name

capturing 171

8.6

Using macros to manage resources 171

8.7

Putting it all together: macros returning functions 173

8.8

Summary 176

9 Combining data and code 177

9.1 Namespaces 178

Creating namespaces 179 ■ Expose only what’s needed 180

Declarative inclusions and exclusions 182

9.2 Exploring Clojure multimethods with the Universal

Design Pattern 183

The parts 184 ■ Usage 185 ■ Multimethods to the rescue 185

Ad hoc hierarchies for inherited behaviors 186 ■ Resolving conflict

in hierarchies 187 ■ Arbitrary dispatch for true maximum

power 188

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xiv

CONTENTS

9.3 Types, protocols, and records 189

Records 189 ■ Protocols 192 ■ Building from a more primitive

base with deftype 200

9.4 Putting it all together: a fluent builder for chess moves 202

Java implementation 202 ■ Clojure implementation 204

9.5 Summary 206

10 Java.next 207

10.1

Generating objects on the fly with proxy 208

A simple dynamic web service 209

10.2 Clojure gen-class and GUI programming 212

Namespaces as class specifications 212 ■ Exploring user interface

design and development with Clojure 215

10.3 Clojure’s relationship to Java arrays 218

Types of arrays: primitive and reference 218 ■ Array mutability 220

That unfortunate naming convention 221 ■ Multidimensional

arrays 222 ■ Variadic method/constructor calls 222

10.4 All Clojure functions implement... 222

java.util.Comparator 223 ■ java.lang.Runnable 223

java.util.concurrent.Callable 224

10.5 Using Clojure data structures in Java APIs 224

java.util.List 225 ■ java.lang.Comparable 225

java.util.RandomAccess 226 ■ java.util.Collection 226

java.util.Set 227

10.6 definterface 227

Generating interfaces on the fly 227

10.7 Be wary of exceptions 229

A bit of background regarding exceptions 230 ■ Runtime versus

compile-time exceptions 230 ■ Handling exceptions 232

Custom exceptions 233

10.8 Summary 233

11 Mutation 234

11.1

Software transactional memory with multiversion

concurrency control and snapshot isolation 235

Transactions 236 ■ Embedded transactions 236 ■ The things

that STM makes easy 237 ■ Potential downsides 238

The things that make STM unhappy 239

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CONTENTS

xv

11.2 When to use Refs 240

Coordinated, synchronous change using alter 241 ■ Commutative

change with commute 244 ■ Vulgar change with ref-set 245

Fixing write-skew with ensure 245 ■ Refs under stress 246

11.3 When to use Agents 247

In-process versus distributed concurrency models 248

Controlling I/O with an Agent 249 ■ The difference between send

and send-off 251 ■ Error handling 252 ■ When not to use

Agents 254

11.4 When to use Atoms 255

Sharing across threads 255 ■ Using Atoms in transactions 256

11.5 When to use locks 258

Safe mutation through locking 259 ■ Using Java’s explicit

locks 260

11.6 When to use futures 261

Futures as callbacks 262

11.7 When to use promises 265

Parallel tasks with promises 265 ■ Callback API to blocking

API 266 ■ Deterministic deadlocks 267

11.8 Parallelism 268

pvalues 268 ■ pmap 269 ■ pcalls 269

11.9 Vars and dynamic binding 270

The binding macro 271 ■ Creating a named Var 271

Creating anonymous Vars 272 ■ Dynamic scope 273

11.10 Summary 274

PART 5 TANGENTIAL CONSIDERATIONS . ....................... 275

12 Performance 277

12.1

Type hints 278

Advantages of type adornment 278 ■ Type-hinting arguments

and returns 278 ■ Type-hinting objects 280

12.2 Transients 280

Ephemeral garbage 280 ■ Transients compare in efficiency to

mutable collections 281

12.3 Chunked sequences 282

Regaining one-at-a-time laziness 283

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xvi

CONTENTS

12.4 Memoization 284

Re-examining memoization 285 ■ A memoization protocol 285

12.5 Understanding coercion 287

First rule of coercion: don’t 288 ■ Corollary: you’re probably not

doing it right 288 ■ Second rule of coercion: don’t 290

Third rule of coercion: coerce a stable local 290 ■ Fourth rule of

coercion: watch your sizes 290 ■ Fifth rule of coercion: truncate

only as a goal 290

12.6 Summary 291

13 Clojure changes the way you think 292

13.1 DSLs 293

A ubiquitous DSL 293 ■ Putting parentheses around the

specification 294 ■ A note about Clojure’s approach to

DSLs 298

13.2 Testing 298

Some useful techniques 299 ■ Contracts programming 301

13.3 A lack of design patterns 303

Clojure’s first-class design patterns 303

13.4 Error handling and debugging 306

Error handling 306 ■ Debugging 308

13.5

Fare thee well 312

resources 313

index 319

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foreword

The authors of this book have taken an ambitious and aggressive approach to teach-

ing Clojure. You know how everyone loves to say they teach using the “drinking from a

fire hydrant” method? Well, at times it feels like these guys are trying to shove that fire

hydrant right up... let’s just say it’s a place where you don’t normally put a fire

hydrant. This isn’t intended as a first book on programming, and it may not be an

ideal first book on Clojure either. The authors assume you’re fearless and, impor-

tantly, equipped with a search engine. You’ll want to have Google handy as you go

through the examples. The authors blaze through many of the classics of both func-

tional programming and industry programming in a whirlwind tour of Clojure that

feels at times more like a class-five tropical storm. You’ll learn fast!

Our industry, the global programming community, is fashion-driven to a degree

that would embarrass haute couture designers from New York to Paris. We’re slaves to

fashion. Fashion dictates the programming languages people study in school, the lan-

guages employers hire for, the languages that get to be in books on shelves. A naive

outsider might wonder if the quality of a language matters a little, just a teeny bit at

least, but in the real world fashion trumps all.

So nobody could be more surprised than I that a Lisp dialect has suddenly become

fashionable again. Clojure has only been out for three years, but it’s gaining momen-

tum at a rate that we haven’t seen in a new language in decades. And it doesn’t even

have a “killer app” yet, in the way that browsers pushed JavaScript into the spotlight,

or Rails propelled Ruby. Or maybe the killer app for Clojure is the JVM itself. Every-

one’s fed up with the Java language, but understandably we don’t want to abandon

our investment in the Java Virtual Machine and its capabilities: the libraries, the con-

figuration, the monitoring, and all the other entirely valid reasons we still use it.

xvii

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xviii

FOREWORD

For those of us using the JVM or .NET, Clojure feels like a minor miracle. It’s an

astoundingly high-quality language, sure—in fact, I’m beginning to think it’s the best

I’ve ever seen—yet somehow it has still managed to be fashionable. That’s quite a

trick. It gives me renewed hope for the overall future of productivity in our industry.

We might just dig ourselves out of this hole we’re in and get back to where every proj-

ect feels like a legacy-free startup, just like it was in the early days of Java.

There are still open questions about Clojure’s suitability for production shops,

especially around the toolchain. That’s normal and expected for a new language. But

Clojure shows so much promise, such beautiful and practical design principles, that

everyone seems to be jumping in with both feet anyway. I certainly am. I haven’t had

this much fun with a new language since Java arrived on the scene 15 years ago. There

have been plenty of pretenders to the JVM throne, languages that promised to take

the Java platform to unprecedented new levels. But until now, none of them had the

right mix of expressiveness, industrial strength, performance, and just plain fun.

I think maybe it’s the “fun” part that’s helped make Clojure fashionable.

In some sense, all this was inevitable, I think. Lisp—the notion of writing your code

directly in tree form—is an idea that’s discovered time and again. People have tried all

sorts of crazy alternatives, writing code in XML or in opaque binary formats or using

cumbersome code generators. But their artificial Byzantine empires always fall into

disrepair or crush themselves into collapse while Lisp, the road that wanders through

time, remains simple, elegant, and pure. All we needed to get back on that road was a

modern approach, and Rich Hickey has given it to us in Clojure.

The Joy of Clojure just might help make Clojure as fun for you as it is for us.

STEVE YEGGE

GOOGLE

steve-yegge.blogspot.com

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preface

To fully appreciate Clojure, we hearken back to Paul Graham’s essay “Beating the

Averages,” an interesting look at the inner workings of his company Viaweb during

the years prior to being bought by Yahoo! Inc. in 1998. Though interesting as survey

of startup culture, the truly memorable part of the essay was the description of how

Viaweb used the programming language Lisp as an advantage over its competition.

How could a programming language more than 50 years old provide any market

advantage over Viaweb’s competitors, who were surely using modern enterprise tech-

nologies? Without repeating the exact terms of the essay, Graham makes a compelling

case for the capability of Lisp to facilitate a more agile programming environment.

Clojure is a dialect of Lisp directly supporting concurrent software development

using functional programming techniques, and like the Lisp described in “Beating

the Averages,” provides an environment conducive to agility. Clojure fosters agility in

ways that many popular programming languages can’t. Many programming languages

are bewitched with most or all of the following:

■

Verbosity

■

Unavoidable boilerplate

■

A long thought-code-feedback loop

■

Incidental complexity

■

Difficulties in extension

■

Deficiencies in supporting crucial programming paradigms

In contrast, Clojure provides a mixture of power and practicality fostering rapid devel-

opment cycles. But the benefits of Clojure don’t stop with its agile nature—as the

xix

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xx

PREFACE

clarion call declares, “Multicore is the new hot topic” (Mache Creeger in ACM Queue,

vol. 3, no. 7).

Though the idea of multicore processors isn’t in itself new, its importance is

becoming increasingly focused. Until recently, you could avoid concurrent and paral-

lel programming techniques and instead ride the ever-quickening processor wave to

better performance. Well, that ride is slowing to a stop, and Clojure is here to help.

Clojure provides a unique mix of functional programming and host symbiosis —an

embrace of and direct support for its host platform, in this case the Java Virtual

Machine. Additionally, the simplification and often elimination of the complexities

involved in coordinated state change have positioned Clojure as an important lan-

guage moving forward. All software developers must eventually address these prob-

lems as a matter of course, and the study, understanding, and eventual utilization of

Clojure is an essential path toward conquering them. From topics such as software

transactional memory to laziness to immutability, this book will guide you on your way

to understanding the “why” of Clojure, in addition to the “how.”

We’ll be your guides into a thoughtful understanding of the joyfulness in Clojure,

for we believe its art is prelude to a new age of software development.

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acknowledgments

The authors would like to jointly thank Rich Hickey, the creator of Clojure, for his

thoughtful creation, furthering the state of the art in language design. Without his

hard work, devotion, and vision, this book would never have been.

We’d also like to thank the brilliant members of the young Clojure community,

including but not limited to: Stuart Halloway, David Edgar Liebke, Christophe Grand,

Chas Emerick, Meikel Brandmeyer, Brian Carper, Bradford Cross, Sean Devlin, Tom

Faulhaber, Stephen Gilardi, Phil Hagelberg, Konrad Hinsen, George Jahad, David

Miller, David Nolen, Laurent Petit, and Stuart Sierra. Finally, we’d like to thank a few

early adopters who took the time to provide thoughtful feedback, including Jürgen

Hötzel, Robert “Uncle Bob” Martin, Grant Michaels, Mangala Sadhu Sangeet Singh

Khalsa, and Sam Aaron. And finally, we would like to thank Steve Yegge for agreeing

to write the foreword and for inspiring us over the years.

Manning sent out the manuscript for peer review at different stages of its develop-

ment and we would like to thank the following reviewers for their invaluable feed-

back: Art Gittleman, Stuart Caborn, Jeff Sapp, Josh Heyer, Dave Pawson, Andrew

Oswald, Federico Tomassetti, Matt Revelle, Rob Friesel, David Liebke, Pratik Patel,

Phil Hagelberg, Rich Hickey, Andy Dingley, Baishampayan Ghose, Chas Emerick,

John D’Emic, and Philipp K. Janert.

Thanks also to the team at Manning for their guidance and support, starting with

publisher Marjan Bace, associate publisher Michael Stephens, our development edi-

tor Susan Harkins, and the production team of Nicholas Chase, Benjamin Berg, Katie

Tennant, Dottie Marsico, and Mary Piergies. And again to Christophe Grand for a

final technical review of the mansucript during production.

xxi

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xxii

ACKNOWLEDGMENTS

FOGUS

I’d like to thank my beautiful wife Yuki for her unwavering patience during the writing

of this book. Without her I would’ve never made it through. I also owe a great debt to

Chris Houser, my coauthor and friend, for teaching me more about Clojure than I

ever would’ve thought possible. I’d also like to thank Dr. Larry Albright for introduc-

ing me to Lisp and the late Dr. Russel E. Kacher for inspiring in me a passion for

learning, curiosity, and reflection. Additionally, I’d like to thank the organizers of the

National Capital Area Clojure Users Group—Matthew Courtney, Russ Olsen, and

Gray Herter—for providing a place for others in the DC area to discover Clojure.

Finally, I’d like to thank my boys Keita and Shota for teaching me the true meaning of

love and that it’s not always about me.

CHOUSER

My most grateful thanks go to God, the source of all good things. To my parents,

thanks for your love and support—your spirit of exploration launched me on a life of

wonderful adventure. To my brother Bill, thanks for my earliest introduction to com-

puters and the joys and challenges of programming. To my wife Heather, thanks for

your constant encouragement from the very first moments of this book project to the

last. To my friend and coauthor Michael Fogus, thanks for the brilliant inspiration and

stunning breadth of knowledge you’ve brought to these pages.

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about this book

Why learn Clojure?

The only difference between Shakespeare and you was the size of his idiom list—not

the size of his vocabulary.

—Alan Perlis

When this book was conceived, our first instinct was to create a comprehensive com-

parison between Clojure and its host language, Java. After further reflection, we

reached the conclusion that such an approach would be disingenuous at best, and

disastrous at worst. Granted, some points of comparison can’t be avoided, but Java is

very different from Clojure and to try and distort one to explain the other would

respect neither. Therefore, we decided that a better approach would be to focus on

“The Clojure Way” of writing code.

When we become familiar with a programming language, the idioms and con-

structs of that language serve to define the way we think about and solve program-

ming tasks. It’s therefore natural that when faced with an entirely new language, we

find comfort in mentally mapping the new language onto the familiar old. But we

plead with you to leave all of your baggage behind; be you from Java, Lisp, Scheme,

C#, or Befunge, we ask you to bear in mind that Clojure is its own language and begs

an adherence to its own set of idioms. You’ll discover concepts that you can connect

between Clojure and languages you already know, but don’t assume that similar things

are entirely the same.

We’ll work hard to guide you through the features and semantics of Clojure to

help you build the mental model needed to use the language effectively. Most of the

samples in this book are designed to be run in Clojure’s interactive programming

xxiii

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xxiv

ABOUT THIS BOOK

environment, commonly known as the Read-Eval-Print Loop, or REPL, an extremely

powerful environment for experimentation and rapid prototyping.

By the time you’re done with this book, the Clojure way of thinking about and solv-

ing problems will be another comfortable tool in your toolbox. If we succeed, then

not only will you be a better Clojure programmer, but you’ll also start seeing your pro-

gramming language of choice—be it Java, C#, Python, Ruby, J, or Haskell—in an

entirely different light. This reassessment of topics that we often take for granted is

essential for personal growth.

Who should read this book?

Paths are made by walking.

—Franz Kafka

This book isn’t a beginner’s guide to Clojure. We start fast and don’t devote much

space to establishing a running Clojure environment, although we do provide some

guidance on page xxix. Additionally, this isn’t a book about Clojure’s implementation

details, but instead one about its semantical details. This is also not a “cookbook” for

Clojure, but instead a thorough investigation into the ingredients that Clojure pro-

vides for creating beautiful software. Often we’ll explain how these ingredients mix

and why they make a great match, but you won’t find complete recipes for systems.

Our examples directly address the discussion at hand and at times leave exposed wir-

ing for you to extend and thus further your own knowledge. It wouldn’t serve us, you,

or Clojure to try to awkwardly mold a comprehensive lesson into the guise of a book-

length project. Often, language books spend valuable time halfheartedly explaining

“real-world” matters totally unrelated to the language itself, and we wish to avoid this

trap. We strongly feel that if we show you the “why” of the language, then you’ll be bet-

ter prepared to take that knowledge and apply it to your real-world problems. In

short, if you’re looking for a book amenable to neophytes that will also show you how

to migrate Clojure into existing codebases, connect to NoSQL databases, and explore

other “real-world” topics, then we recommend the book Clojure in Action by Amit

Rathore (Manning, 2011).

Having said all of that, we do provide a short introduction to the language and feel

that for those of you willing to work hard to understand Clojure, this is indeed the

book for you. Additionally, if you already have a background in Lisp programming,

then much of the introductory material will be familiar, thus making this book ideal

for you. Though by no means perfect, Clojure has a nice combination of features that

fit together into a coherent system for solving programming problems. The way Clo-

jure encourages you to think about problems may be different than you’re used to,

requiring a bit of work to “get.” But once you cross that threshold, you too may experi-

ence a kind of euphoria, and in this book we’ll help you get there. These are exciting

times, and Clojure is the language we hope you’ll agree is an essential tool for navigat-

ing into the future.

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ABOUT THIS BOOK

xxv

Roadmap

We’re going to take you on a journey. Perhaps you’ve started on this journey yourself

by exploring Clojure beforehand. Perhaps you’re a seasoned Java or Lisp veteran and

are coming to Clojure for the first time. Perhaps you’re coming into this book from an

entirely different background. In any case, we’re talking to you. This is a self-styled

book for the adventurous and will require that you leave your baggage behind and

approach the enclosed topics with an open mind. In many ways, Clojure will change

the way you view programming, and in other ways it’ll obliterate your preconceived

notions. The language has a lot to say about how software should be designed and

implemented, and we’ll touch on these topics one by one throughout this book.

FOUNDATIONS

Every so often, a programming language comes along that can be considered founda-

tional. Occasionally a language is invented that shakes the foundations of the software

industry and dispels the collective preconceived notions of “good software practices.”

These foundational programming languages always introduce a novel approach to

software development, alleviating if not eliminating the difficult problems of their

time. Any list of foundational languages inevitably raises the ire of language propo-

nents who feel their preferences shouldn’t be ignored. But we’re willing to take this

risk and therefore list the following programming languages in this category.

Foundational programming languages

Year

Language

Inventor(s)

Interesting reading

1957

Fortran

John Backus

John Backus, “The History of Fortran I, II, and III,”

IEEE Annals of the History of Computing 20, no. 4

(1998).

1958

Lisp

John McCarthy

Richard P. Gabriel and Guy L. Steele Jr., “The Evolution

of Lisp” (1992), www.dreamsongs.com/Files/

HOPL2-Uncut.pdf.

1959

COBOL

Design by committee

Edsger Dijkstra, “EWD 498: How Do We Tell Truths

That Might Hurt?” in Selected Writings on Computing:

A Personal Perspective (New York: Springer-Verlag,

1982).

1968

Smalltalk

Alan Kay

Adele Goldberg, Smalltalk-80: The Language and Its

Implementation (Reading, MA: Addison-Wesley,

1983).

1972

C

Dennis Ritchie

Brian W. Kernighan and Dennis M. Ritchie, The C Pro-

gramming Language (Englewood Cliffs, NJ: Prentice

Hall, 1988).

1972

Prolog

Alain Colmerauer

Ivan Bratko, PROLOG: Programming for Artificial Intelli-

gence (New York: Addison-Wesley, 2000).

1975

Scheme

Guy Steele and 

Guy Steele and Gerald Sussman, the “Lambda

Gerald Sussman

Papers,” mng.bz/sU33.

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xxvi

ABOUT THIS BOOK

Foundational programming languages (continued)

Year

Language

Inventor(s)

Interesting reading

1983

C++

Bjarne Stroustrup

Bjarne Stroustrup, The Design and Evolution of C++

(Reading, MA: Addison-Wesley, 1994).

1986

Erlang

Telefonaktiebolaget 

Joe Armstrong, “A History of Erlang,” Proceedings of

L. M. Ericsson

the Third ACM SIGPLAN Conference on History of Pro-

gramming Languages (2007).

1987

Perl

Larry Wall

Larry Wall, Tom Christiansen, and Jon Orwant, Pro-

gramming Perl (Cambridge, MA: O’Reilly, 2000).

1990

Haskell

Simon Peyton Jones

Miran Lipovacˇa, “Learn You a Haskell for Great

Good!” http://learnyouahaskell.com/.

1995

Java

Sun Microsystems

David Bank, “The Java Saga,” Wired 3.12 (1995).

2007

Clojure?

Rich Hickey

You’re reading it.

Like them or not, there’s little dispute that the listed programming languages have

greatly influenced the way that software is constructed. Whether Clojure should be

included in this category remains to be seen, but Clojure does borrow heavily from

many of the foundational languages and also from other influential programming

languages to boot.

Chapter 1 starts our journey and provides some of the core concepts embraced by

Clojure. These concepts should be well understood by the time you’ve finished the

chapter. Along the way, we’ll show illustrative code samples highlighting the concepts

at hand (and sometimes even pretty pictures). Much of what’s contained in chapter 1

can be deemed “The Clojure Philosophy,” so if you’ve ever wondered what inspired

and constitutes Clojure, we’ll provide that for you.

Chapter 2 provides a fast introduction to specific features and syntax of Clojure.

Chapter 3 will address general Clojure programming idioms that aren’t easily cate-

gorized. From matters of truthiness and style to considerations of packaging and nil,

chapter 3 is a mixed bag. All of the topics are important in their own right, and to

understand them is in many ways a start to understanding a large portion of idiomatic

Clojure source code.

DATA TYPES

The discussion on scalar data types in chapter 4 will be relatively familiar to most pro-

grammers, but some important points beg our attention, arising from Clojure’s inter-

esting nature as a functional programming language hosted on the Java Virtual

Machine. Java programmers reading this book will recognize the points made concern-

ing numerical precision (section 4.1), and Lisp programmers will recognize the discus-

sion on Lisp-1 versus Lisp-2 (section 4.4). Programmers will appreciate the practical

inclusion of regular expressions as first-class syntactical elements (section 4.5). Finally,

long-time Clojure programmers may find that the discussion of rationals and keywords

(sections 4.2 and 4.3, respectively) sheds new light on these seemingly innocent types.

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ABOUT THIS BOOK

xxvii

Regardless of your background, chapter 4 will provide crucial information in under-

standing the nature of Clojure’s underappreciated scalar types.

Clojure’s novel persistent data structures will be covered in chapter 5; this should

be enlightening to anyone wishing to look more deeply into them. Persistent data

structures lie at the heart of Clojure’s programming philosophy and must be under-

stood to fully grasp the implications of Clojure’s design decisions. We’ll only touch

briefly on the implementation details of these persistent structures, because they’re

less important than understanding why and how to use them.

FUNCTIONAL PROGRAMMING

Chapter 6 will deal with the nebulous notions of immutability, persistence, and lazi-

ness. We’ll explore Clojure’s use of immutability as the key element in supporting con-

current programming. We’ll likewise show how, in the presence of immutability, many

of the problems associated with coordinated state change disappear. Regarding lazi-

ness, we’ll explore the ways that Clojure leverages it to reduce the memory footprint

and speed execution times. Finally, we’ll cover the interplay between immutability and

laziness. For programmers coming from languages that allow unconstrained mutation

and strict evaluation of expressions, chapter 6 may prove to be an initially mind-

bending experience. But with this mind-bending comes enlightenment, and you’ll

likely never view your preferred programming languages in the same light.

Chapter 7 will tackle Clojure’s approach to functional programming full-on. For

those of you coming from a functional programming background, much of the chap-

ter will be familiar, although Clojure will present its own unique blend. But like every

programming language dubbed “functional,” Clojure’s implementation will provide a

different lens by which to view your previous experience. For those of you wholly

unfamiliar with functional programming techniques, chapter 7 will likely be mind-

bending. In coming from a language that centers on object hierarchies and impera-

tive programming techniques, the notion of functional programming seems alien. But

we believe Clojure’s decision to base its programming model in the functional para-

digm to be the correct one, and we hope that you’ll agree.

LARGE-SCALE DESIGN

Clojure can be used as the primary language for any application scale, and the discus-

sion of macros in chapter 8 might change your ideas regarding how to develop soft-

ware. Clojure as a Lisp embraces macros, and we’ll lead you through the process of

understanding them and realizing that with great power comes great responsibility.

In chapter 9, we’ll guide you through the use of Clojure’s built-in mechanisms for

combining and relating code and data. From namespaces to multimethods to types

and protocols, we’ll explain how Clojure fosters the design and implementation of

large-scale applications.

Clojure is a symbiotic programming language, meaning that it’s intended to run

atop a host environment. For now, the host of choice is the Java Virtual Machine, but

the future bodes well for Clojure becoming host-agnostic. In any case, Clojure pro-

vides top-notch functions and macros for interacting directly with the host platform.

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xxviii

ABOUT THIS BOOK

In chapter 10, we’ll discuss the ways that Clojure interoperates with its host, focusing

on the JVM throughout.

Clojure is built to foster the sane management of program state, which in turn

facilitates concurrent programming, as you’ll see in chapter 11. Clojure’s simple yet

powerful state model alleviates most of the headaches involved in such complicated

tasks, and we’ll show you how and why to use each. Additionally, we’ll address the mat-

ters not directly solved by Clojure, such as how to identify and reduce those elements

that should be protected using Clojure’s reference types.

TANGENTIAL CONSIDERATIONS

The final part of this book will discuss topics that are equally important: the design

and development of your application viewed through the lens of the Clojure Philoso-

phy. In chapter 12, we’ll discuss ways to improve your application’s performance in

single-threaded applications. Clojure provides many mechanisms for improving per-

formance, and we’ll delve into each, including their usage and caveats where applica-

ble. And to wrap up our book, in chapter 13, we’ll address the ways that Clojure

changes the ways that you look at tangential development activities, such as the defini-

tion of your application domain language, testing, error-handling, and debugging.

Code conventions

The source code used throughout this book is formatted in a straightforward and

pragmatic fashion. Any source code listings inlined within the text, for example

(:lemonade :fugu), will be formatted using a fixed-width font and highlighted.

Source code snippets outlined as blocks of code will be offset from the left margin,

formatted in a fixed-width font, and highlighted to stand out:

(def population {::zombies 2700 ::humans 9})

(def per-capita (/ (population ::zombies) (population ::humans)))

(println per-capita "zombies for every human!")

Whenever a source code snippet indicates the result of an expression, the result will be

prefixed by the characters ;=>. This particular sequence serves a threefold purpose:

■

It helps the result stand out from the code expressions.

■

It indicates a Clojure comment.

■

Because of this, whole code blocks can be easily copied from an EBook or PDF

version of this book and pasted into a running Clojure REPL:

(def population {::zombies 2700 ::humans 9})

(/ (population ::zombies) (population ::humans))

;=> 300

Additionally, any expected display in the REPL that’s not a returned value (such as

exceptions or printouts) will be denoted with a leading ; prior to the actual return

value:

(println population)

; {:user/zombies 2700, :user/humans 9}

;=> nil

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ABOUT THIS BOOK

xxix

In the previous example, the map displayed as {:user/zombies 2700, :user/humans

9} is the printed value, whereas nil denotes the returned value from the println

function. If no return value is shown after an expression, then you can assume that it’s

either nil or negligible to the example at hand.

READING CLOJURE CODE When reading Clojure code, skim it when read-

ing left-to-right, paying just enough attention to note important bits of

context (defn, binding, let, and so on). When reading from the inside

out, pay careful attention to what each expression returns to be passed to

the next outer function. This is much easier than trying to remember the

whole outer context when reading the innermost expressions.

All code formatted as either inline or block-level is intended to be typed or pasted

exactly as written into Clojure source files or a REPL. We generally won’t show the Clo-

jure prompt user> because it’ll cause copy/paste to fail. Finally, we’ll at times use the

ellipsis ... to indicate an elided result or printout.

Code annotations accompany many of the listings, highlighting important con-

cepts. In some cases, numbered bullets link to explanations that follow the listing.

Getting Clojure

If you don’t currently have Clojure, then we recommend you retrieve the Clojure

REPL package (Cljr) created by David Edgar Liebke, located at http://joyofclo-

jure.com/cljr and installing it via the following instructions.

PREREQUISITES

■

Java version 1.6 and later

■

An Internet connection

INSTRUCTIONS

Run the following from your operating system’s console:

java -jar cljr-installer.jar

If your chosen download method appended a .zip file extension to the Cljr package,

then the following is fine:

java -jar cljr-installer.jar.zip

You’ll see output from Cljr indicating its installation and package download progress.

Once it has completed, you’ll see instructions for running Clj similar to the following:

Cljr has been successfully installed. Add $HOME/.cljr/bin to your PATH:

$ export PATH=$HOME/.cljr/bin:$PATH

Run 'cljr help' for a list of available commands.

Following the steps displayed, run Cljr.

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xxx

ABOUT THIS BOOK

REPL

The Cljr package runs a Clojure REPL (Read/Eval/Print Loop) for version 1.2.0—the

same version corresponding to this book. When you launch the Cljr program, you’ll

see the window shown in the figure below.

The Cljr REPL is similar to the stock Clojure REPL, but with additional convenient

features as explained at http://github.com/fogus/cljr.

The book won’t proceed under the assumption that you’re using Cljr but will work

regardless of your own personal REPL setup—as long as you’re running Clojure ver-

sion 1.2.

DOWNLOADING CODE EXAMPLES

Source code for all working examples in this book is available for download from the

publisher’s website at www.manning.com/TheJoyofClojure.

Author Online

Purchase of The Joy of Clojure includes free access to a private web forum run by Man-

ning Publications where you can make comments about the book, ask technical ques-

tions, and receive help from the authors and from other users. To access the forum

and subscribe to it, point your web browser to www.manning.com/TheJoyofClojure.

This page provides information on how to get on the forum once you are registered,

what kind of help is available, and the rules of conduct on the forum.

Manning’s commitment to our readers is to provide a venue where a meaningful

dialogue between individual readers and between readers and the authors can take

place. It is not a commitment to any specific amount of participation on the part of

the authors, whose contribution to the AO remains voluntary (and unpaid). We sug-

gest you try asking the authors some challenging questions lest their interest stray!

The Author Online forum and the archives of previous discussions will be accessi-

ble from the publisher’s website as long as the book is in print.

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ABOUT THIS BOOK

xxxi

About the cover illustration

The figure on the cover of The Joy of Clojure is captioned “The Confidence Man,” which,

in 19th century France, could mean anything from a healer or medicine man to a card

shark or money lender or traveling salesman. The illustration is taken from a 19th-

century edition of Sylvain Maréchal’s four-volume compendium of regional dress cus-

toms published in France. Each illustration is finely drawn and colored by hand. The

rich variety of Maréchal’s collection reminds us vividly of how culturally apart the

world’s towns and regions were just 200 years ago. Isolated from each other, people

spoke different dialects and languages. In the streets or in the countryside, it was easy

to identify where they lived and what their trade or station in life was just by their dress.

Dress codes have changed since then and the diversity by region, so rich at the

time, has faded away. It is now hard to tell apart the inhabitants of different conti-

nents, let alone different towns or regions. Perhaps we have traded cultural diversity

for a more varied personal life—certainly for a more varied and fast-paced technolog-

ical life.

At a time when it is hard to tell one computer book from another, Manning cele-

brates the inventiveness and initiative of the computer business with book covers

based on the rich diversity of regional life of two centuries ago, brought back to life by

Maréchal’s pictures.

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Part 1

Foundations

Even the most elaborate mansion must begin with a firm if humble founda-

tion. We begin here by pouring a foundation of knowledge on which you’ll be

able to build a solid understanding about Clojure’s less familiar ways. This foun-

dation includes, among other things, the philosophy of programming underly-

ing Clojure, sturdy walls of data and functions, and REPLs and nil puns.

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Clojure philosophy

This chapter covers

 The Clojure way

 Why a(nother) Lisp?

 Functional programming

 Why Clojure isn’t especially object-oriented

Learning a new language generally requires significant investment of thought and

effort, and it is only fair that programmers expect each language they consider

learning to justify that investment. Clojure was born out of creator Rich Hickey’s

desire to avoid many of the complications, both inherent and incidental, of manag-

ing state using traditional object-oriented techniques. Thanks to a thoughtful

design based in rigorous programming language research, coupled with a fervent

look toward practicality, Clojure has blossomed into an important programming

language playing an undeniably important role in the current state of the art in lan-

guage design. On one side of the equation, Clojure utilizes Software Transactional

Memory (STM), agents, a clear distinction between identity and value types, arbi-

trary polymorphism, and functional programming to provide an environment con-

ducive to making sense of state in general, and especially in the face of

concurrency. On the other side, Clojure shares a symbiotic relationship with the

3

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4

CHAPTER 1 Clojure philosophy

Java Virtual Machine, thus allowing prospective developers to avoid the costs of main-

taining yet another infrastructure while leveraging existing libraries.

In the grand timeline of programming language history, Clojure is an infant; but

its colloquialisms (loosely translated as “best practices” or idioms) are rooted1 in 50

years of Lisp, as well as 15 years of Java history. Additionally, the enthusiastic commu-

nity that has exploded since its introduction has cultivated its own set of unique idi-

oms. As mentioned in the preface, the idioms of a language help to define succinct

representations of more complicated expressions. Although we will certainly cover idi-

omatic Clojure code, we will also expand into deeper discussions of the “why” of the

language itself.

In this chapter, we’ll discuss the weaknesses in existing languages that Clojure was

designed to address, how it provides strength in those areas, and many of the design

decisions Clojure embodies. We’ll also look at some of the ways existing languages

have influenced Clojure, and define terms that will be used throughout the book.

1.1

The Clojure way

We’ll start slowly.

Clojure is an opinionated language—it doesn’t try to cover all paradigms or pro-

vide every checklist bullet-point feature. Instead it provides the features needed to

solve all kinds of real-world problems the Clojure way. To reap the most benefit from

Clojure, you’ll want to write your code with the same vision as the language itself. As

we walk through the language features in the rest of the book, we’ll mention not just

what a feature does, but why it’s there and how best to take advantage of it.

But before we get to that, we’ll first take a high-level view of some of Clojure’s most

important philosophical underpinnings. Figure 1.1 lists some broad goals that Rich

Hickey had in mind while designing Clojure and some of the more specific decisions

that are built into the language to support

these goals.

simplicity

As the figure illustrates, Clojure’s

pure

broad goals are formed from a confluence

functions

of supporting goals and functionality,

freedom

which we will touch on in the following

separation

empowerment

to focus

concise

of

concerns

subsections.

clarity

consistent

1.1.1

Simplicity

expressive

practical

direct

interop

It’s hard to write simple solutions to com-

plex problems. But every experienced

programmer has also stumbled on areas

Figure 1.1 Broad goals of Clojure: this figure

where we’ve made things more complex

shows some of the concepts that underlie the

than necessary, what you might call

Clojure philosophy, and how they intersect.

1 While drawing on the traditions of Lisps (in general) and Java, Clojure in many ways stands as a direct chal-

lenge to them for change.

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The Clojure way

5

incidental complexity as opposed to complexity that’s essential to the task at hand (Mose-

ley 2006). Clojure strives to let you tackle complex problems involving a wide variety

of data requirements, multiple concurrent threads, independently developed librar-

ies, and so on without adding incidental complexity. It also provides tools reducing

what at first glance may seem like essential complexity. The resulting set of features

may not always seem simple, especially when they’re still unfamiliar, but as you read

through this book we think you’ll come to see how much complexity Clojure helps

strip away.

One example of incidental complexity is the tendency of modern object-oriented

languages to require that every piece of runnable code be packaged in layers of class

definitions, inheritance, and type declarations. Clojure cuts through all this by cham-

pioning the pure function, which takes a few arguments and produces a return value

based solely on those arguments. An enormous amount of Clojure is built from such

functions, and most applications can be too, which means that there’s less to think

about when trying to solve the problem at hand.

1.1.2

Freedom to focus

Writing code is often a constant struggle against distraction, and every time a language

requires you to think about syntax, operator precedence, or inheritance hierarchies, it

exacerbates the problem. Clojure tries to stay out of your way by keeping things as sim-

ple as possible, not requiring you to go through a compile-and-run cycle to explore an

idea, not requiring type declarations, and so on. It also gives you tools to mold the lan-

guage itself so that the vocabulary and grammar available to you fit as well as possible

to your problem domain—Clojure is expressive. It packs a punch, allowing you to per-

form highly complicated tasks succinctly without sacrificing comprehensibility.

One key to delivering this freedom is a commitment to dynamic systems. Almost

everything defined in a Clojure program can be redefined, even while the program is

running: functions, multimethods, types, type hierarchies, and even Java method

implementations. Though redefining things on the fly might be scary on a production

system, it opens a world of amazing possibilities in how you think about writing pro-

grams. It allows for more experimentation and exploration of unfamiliar APIs, and it

adds an element of fun that can sometimes be impeded by more static languages and

long compilation cycles.

But Clojure’s not just about having fun. The fun is a by-product of giving program-

mers the power to be more productive than they ever thought imaginable.

1.1.3

Empowerment

Some programming languages have been created primarily to demonstrate some nug-

get of academia or to explore certain theories of computation. Clojure is not one of

these. Rich Hickey has said on numerous occasions that Clojure has value to the

degree that it lets you build interesting and useful applications.

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6

CHAPTER 1 Clojure philosophy

To serve this goal, Clojure strives to be practical—a tool for getting the job done. If

a decision about some design point in Clojure had to weigh the trade-offs between the

practical solution and a clever, fancy, or theoretically pure solution, usually the practi-

cal solution won out. Clojure could try to shield you from Java by inserting a compre-

hensive API between the programmer and the libraries, but this could make the use of

third-party Java libraries more clumsy. So Clojure went the other way: direct, wrapper-

free, compiles-to-the-same-bytecode access to Java classes and methods. Clojure strings

are Java strings; Clojure function calls are Java method calls—it’s simple, direct, and

practical.

The decision to use the Java Virtual Machine (JVM) itself is a clear example of this

practicality. The JVM has some technical weaknesses such as startup time, memory

usage, and lack of tail-call optimization 2 (TCO). But it’s also an amazingly practical plat-

form—it’s mature, fast, and widely deployed. It supports a variety of hardware and

operating systems and has a staggering number of libraries and support tools avail-

able, all of which Clojure can take advantage of because of this supremely practical

decision.

With direct method calls, proxy, gen-class, gen-interface (see chapter 10),

reify, definterface, deftype, and defrecord (see section 9.3), Clojure works hard

to provide a bevy of interoperability options, all in the name of helping you get your

job done. Practicality is important to Clojure, but many other languages are practical

as well. You’ll start to see some ways that Clojure really sets itself apart by looking at

how it avoids muddles.

1.1.4

Clarity

When beetles battle beetles in a puddle paddle battle and the beetle battle puddle is a

puddle in a bottle they call this a tweetle beetle bottle puddle paddle battle muddle.

—Dr. Seuss

Consider what might be described as a simple snippet of code in a language like

Python:

x = [5]

process(x)

x[0] = x[0] + 1

After executing this code, what’s the value of x? If you assume process doesn’t change

the contents of x at all, it should be [6], right? But how can you make that assump-

tion? Without knowing exactly what process does, and whatever function it calls does,

and so on, you can’t be sure at all.

Even if you’re sure process doesn’t change the contents of x, add multithreading

and now you have another whole set of concerns. What if some other thread changes

2

Don’t worry if you don't know what tail-call optimization is. Also don’t worry if you do know what TCO is and

think the JVM’s lack of it is a critical flaw for a Lisp or functional language such as Clojure. All your concerns

will be addressed in section 7.3. Until then, just relax.

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The Clojure way

7

x between the first and third lines? Worse yet, what if something is setting x at the

moment the third line is doing its assignment—are you sure your platform guarantees

an atomic write to that variable, or is it possible that the value will be a corrupted mix

of multiple writes? We could continue this thought exercise in hopes of gaining some

clarity, but the end result would be the same—what you have ends up not being clear

at all, but the opposite: a muddle.

Clojure strives for code clarity by providing tools to ward off several different kinds

of muddles. For the one just described, it provides immutable locals and persistent

collections, which together eliminate most of the single- and multithreaded issues all

at once.

You can find yourself in several other kinds of muddles when the language you’re

using merges unrelated behavior into a single construct. Clojure fights this by being

vigilant about separation of concerns. When things start off separated, it clarifies your

thinking and allows you to recombine them only when and to the extent that doing so

is useful for a particular problem. Table 1.1 contrasts common approaches that merge

concepts together in some other languages with separations of similar concepts in

Clojure that will be explained in greater detail throughout this book.

Table 1.1 Separation of concerns in Clojure

Conflated

Separated

Where

Object with mutable fields

Values from identities

Chapter 4 and section 5.1

Class acts as namespace for

Function namespaces from type

Sections 8.2 and 8.3

methods

namespaces

Inheritance hierarchy made of

Hierarchy of names from data and

Chapter 8

classes

functions

Data and methods bound

Data objects from functions

Sections 6.1 and 6.2 and

together lexically

chapter 8

Method implementations embed-

Interface declarations from func-

Sections 8.2 and 8.3

ded throughout class inheritance

tion implementations

chain

It can be hard at times to tease apart these concepts in our own minds, but accom-

plishing it can bring remarkable clarity and a sense of power and flexibility that’s

worth the effort. With all these different concepts at your disposal, it’s important that

the code and data you work with express this variety in a consistent way.

1.1.5

Consistency

Clojure works to provide consistency in two specific ways: consistency of syntax and of

data structures.

Consistency of syntax is about the similarity in form between related concepts. One

simple but powerful example of this is the shared syntax of the for and doseq macros.

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8

CHAPTER 1 Clojure philosophy

They don’t do the same thing—for returns a lazy seq whereas doseq is for generating

side effects—but both support the same mini-language of nested iteration, destructur-

ing, and :when and :while guards. The similarities stand out when comparing the fol-

lowing examples:

(for [x [:a :b], y (range 5) :when (odd? y)] [x y])

;=> ([:a 1] [:a 3] [:b 1] [:b 3])

(doseq [x [:a :b], y (range 5) :when (odd? y)] (prn x y))

; :a 1

; :a 3

; :b 1

; :b 3

;=> nil

The value of this similarity is having to learn only one basic syntax for both situations,

as well as the ease with which you can convert any particular usage of one form to the

other if that becomes necessary.

Likewise, the consistency of data structures is the deliberate design of all of Clo-

jure’s persistent collection types to provide interfaces as similar to each other as possi-

ble, as well as to make them as broadly useful as possible. This is actually an extension

of the classic Lisp “code is data” philosophy. Clojure data structures aren’t used just

for holding large amounts of application data, but also to hold the expression ele-

ments of the application itself. They’re used to describe destructuring forms and to

provide named options to various built-in functions. Where other object-oriented lan-

guages might encourage applications to define multiple incompatible classes to hold

different kinds of application data, Clojure encourages the use of compatible map-like

objects.

The benefit of this is that the same set of functions designed to work with Clojure

data structures can be applied to all these contexts: large data stores, application

code, and application data objects. You can use into to build any of these types, seq to

get a lazy seq to walk through them, filter to select elements of any of them that sat-

isfy a particular predicate, and so on. Once you’ve grown accustomed to having the

richness of all these functions available everywhere, dealing with a Java or C++ applica-

tion’s Person or Address class will feel constraining.

Simplicity, freedom to focus, empowerment, consistency, and clarity.

Nearly every element of the Clojure programming language is designed to pro-

mote these goals. When writing Clojure code, if you keep in mind the desire to maxi-

mize simplicity, empowerment, and the freedom to focus on the real problem at

hand, we think you’ll find Clojure provides you the tools you need to succeed.

1.2

Why a(nother) Lisp?

By relieving the brain of all unnecessary work, a good notation sets it free to concen-

trate on more advanced problems.

—Alfred North Whitehead

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Why a(nother) Lisp?

9

Go to any open source project hosting site and perform a search for the term “Lisp

interpreter.” You’ll likely get a cyclopean mountain3 of results from this seemingly

innocuous term. The fact of the matter is that the history of computer science is lit-

tered (Fogus 2009) with the abandoned husks of Lisp implementations. Well-inten-

tioned Lisps have come and gone and been ridiculed along the way, and still

tomorrow the search results will have grown almost without bounds. Bearing in mind

this legacy of brutality, why would anyone want to base their brand-new programming

language on the Lisp model?

1.2.1

Beauty

Lisp has attracted some of the brightest minds in the history of computer science. But

an argument from authority is insufficient, so you shouldn’t judge Lisp on this alone.

The real value in the Lisp family of languages can be directly observed through the

activity of using it to write applications. The Lisp style is one of expressivity and

empowerment, and in many cases outright beauty. Joy awaits the Lisp neophyte. The

original Lisp language as defined by John McCarthy in his earth-shattering essay

“Recursive Functions of Symbolic Expressions and Their Computation by Machine,

Part I” (McCarthy 1960) defined the whole language in terms of only seven functions

and two special forms: atom, car, cdr, cond, cons, eq, quote, lambda, and label.

Through the composition of those nine forms, McCarthy was able to describe the

whole of computation in a way that takes your breath away. Computer programmers

are perpetually in search of beauty, and more often than not, this beauty presents

itself in the form of simplicity. Seven functions and two special forms. It doesn’t get

more beautiful than that.

1.2.2

Extreme flexibility

Why has Lisp persevered for more than 50 years while countless other languages have

come and gone? There are probably complex reasons, but chief among them is likely

the fact that Lisp as a language genotype (Tarver 2008) fosters language flexibility in

the extreme. Newcomers to Lisp are sometimes unnerved by its pervasive use of

parentheses and prefix notation, which is different than non-Lisp programming lan-

guages. The regularity of this behavior not only reduces the number of syntax rules

you have to remember, but also makes the writing of macros trivial. We’ll look at mac-

ros in more detail in chapter 8, but to whet your appetite we’ll take a brief look at one

now. It’s an example that we’ll get working on in a moment:

(defn query [max]

(SELECT [a b c]

(FROM X

(LEFT-JOIN Y :ON (= X.a Y.b)))

(WHERE (AND (< a 5) (< b ~max)))))

3 ...of madness.

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10

CHAPTER 1 Clojure philosophy

We hope some of those words look familiar to you, because this isn’t a book on SQL.

Regardless, our point here is that Clojure doesn’t have SQL support built in. The

words SELECT, FROM, and so forth aren’t built-in forms. They’re also not regular func-

tions, because if SELECT were, then the use of a, b, and c would be an error, because

they haven’t been defined yet.

So what does it take to define a domain-specific language (DSL) like this in Clo-

jure? Well, it’s not production-ready code and doesn’t tie into any real database serv-

ers; but with just one macro and the three functions shown in listing 1.1, the

preceding query returns these handy values:

(query 5)

;=> ["SELECT a, b, c FROM X LEFT JOIN Y ON (X.a = Y.b)

WHERE ((a < 5) AND (b < ?))"

[5]]

Note that some words such as FROM and ON are taken directly from the input expres-

sion, whereas others such as ~max and AND are treated specially. The max that was given

the value 5 when the query was called is extracted from the literal SQL string and pro-

vided in a separate vector, perfect for using in a prepared query in a way that will

guard against SQL-injection attacks. The AND form was converted from the prefix nota-

tion of Clojure to the infix notation required by SQL.

Listing 1.1 A domain-specific language for embedding SQL queries in Clojure

(ns joy.sql

(:use [clojure.string :as str :only []])

Use core 

(defn expand-expr [expr]

string functions

(if (coll? expr)

(if (= (first expr) ùnquote)

Handle unsafe literals

"?"

(let [[op & args] expr]

(str "(" (str/join (str " " op " ")

(map expand-expr args)) ")")))

Convert prefix

expr))

to infix

(declare expand-clause)

Support each

(def clause-map

kind of clause

{'SELECT (fn [fields & clauses]

(apply str "SELECT " (str/join ", " fields)

(map expand-clause clauses)))

'FROM (fn [table & joins]

(apply str " FROM " table

(map expand-clause joins)))

'LEFT-JOIN (fn [table on expr]

(str " LEFT JOIN " table

" ON " (expand-expr expr)))

'WHERE (fn [expr]

Call 

(str " WHERE " (expand-expr expr)))})

appropriate

(defn expand-clause [[op & args]]

converter

(apply (clause-map op) args))

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Why a(nother) Lisp?

11

(defmacro SELECT [& args]

Provide main

[(expand-clause (cons 'SELECT args))

entrypoint macro

(vec (for [n (tree-seq coll? seq args)

:when (and (coll? n) (= (first n) ùnquote))]

(second n)))])

But the point here isn’t that this is a particularly good SQL DSL—more complete ones

are available.4 Our point is that once you have the skill to easily create a DSL like this,

you’ll recognize opportunities to define your own that solve much narrower,

application-specific problems than SQL does. Whether it’s a query language for an

unusual non-SQL datastore, a way to express functions in some obscure math disci-

pline, or some other application we as authors can’t imagine, having the flexibility to

extend the base language like this, without losing access to any of the language’s own

features, is a game-changer.

Although we shouldn’t get into too much detail about the implementation, take

a brief look at listing 1.1 and follow along as we discuss important aspects of its

implementation.

Reading from the bottom up, you’ll notice the main entry point, the SELECT

macro. This returns a vector of two items—the first is generated by calling expand-

clause, which returns the converted query string, whereas the second is another vec-

tor of expressions marked by ~ in the input. The ~ is known as unquote and we discuss

its more common uses in chapter 8. Also note the use of tree-seq here to succinctly

extract items of interest from a tree of values, namely the input expression.

The expand-clause function takes the first word of a clause, looks it up in the

clause-map, and calls the appropriate function to do the actual conversion from Clo-

jure s-expression to SQL string. The clause-map provides the specific functionality

needed for each part of the SQL expression: inserting commas or other SQL syntax,

and sometimes recursively calling expand-clause when subclauses need to be con-

verted. One of these is the WHERE clause, which handles the general conversion of pre-

fix expressions to the infix form required by SQL by delegating to the expand-expr

function.

Overall, the flexibility of Clojure demonstrated in this example comes largely from

the fact that macros accept code forms, such as the SQL DSL example we showed, and

can treat them as data—walking trees, converting values, and more. This works not

only because code can be treated as data, but because in a Clojure program, code is

data.

1.2.3

Code is data

The notion of “code is data” is difficult to grasp at first. Implementing a programming

language where code shares the same footing as its comprising data structures presup-

poses a fundamental malleability of the language itself. When your language is repre-

sented as the inherent data structures, the language itself can manipulate its own

4 One of note is ClojureQL at http://gitorious.org/clojureql.

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12

CHAPTER 1 Clojure philosophy

structure and behavior (Graham 1995). You may have visions of Ouroboros after read-

ing the previous sentence, and that wouldn’t be inappropriate, because Lisp can be

likened to a self-licking lollypop—more formally defined as homoiconicity. Lisp’s

homoiconicity takes a great conceptual leap in order to fully grasp, but we’ll lead you

toward that understanding throughout this book in hopes that you too will come to

realize the inherent power.

There’s a joy in learning Lisp for the first time, and if that’s your experience com-

ing into this book then we welcome you—and envy you.

1.3

Functional programming

Quick, what does functional programming mean? Wrong answer.

Don’t be too discouraged, however—we don’t really know the answer either. Func-

tional programming is one of those computing terms5 that has a nebulous definition.

If you ask 100 programmers for their definition, you’ll likely receive 100 different

answers. Sure, some definitions will be similar, but like snowflakes, no two will be

exactly the same. To further muddy the waters, the cognoscenti of computer science

will often contradict one another in their own independent definitions. Likewise, the

basic structure of any definition of functional programming will be different depend-

ing on whether your answer comes from someone who favors writing their programs

in Haskell, ML, Factor, Unlambda, Ruby, or Qi. How can any person, book, or lan-

guage claim authority for functional programming? As it turns out, just as the multi-

tudes of unique snowflakes are all made mostly of water, the core of functional

programming across all meanings has its core tenets.

1.3.1

A workable definition of functional programming

Whether your own definition of functional programming hinges on the lambda calcu-

lus, monadic I/O, delegates, or java.lang.Runnable, your basic unit of currency is

likely to be some form of procedure, function, or method—herein lies the root. Func-

tional programming concerns and facilitates the application and composition of func-

tions. Further, for a language to be considered functional, its notion of function must

be first-class. The functions of a language must be able to be stored, passed, and

returned just like any other piece of data within that language. It’s beyond this core

concept that the definitions branch toward infinity, but thankfully, it’s enough to start.

Of course, we’ll also present a further definition of Clojure’s style of functional pro-

gramming that includes such topics as purity, immutability, recursion, laziness, and

referential transparency, but those will come later in chapter 7.

1.3.2

The implications of functional programming

Object-oriented programmers and functional programmers will often see and solve a

problem in different ways. Whereas an object-oriented mindset will foster the

5 Quick, what’s the definition of combinator? How about cloud computing? Enterprise? SOA? Web 2.0? Real-

world? Hacker? Often it seems that the only term with a definitive meaning is “yak shaving.”

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Why Clojure isn’t especially object-oriented

13

approach of defining an application domain as a set of nouns (classes), the functional

mind will see the solution as the composition or verbs (functions). Though both pro-

grammers may in all likelihood generate equivalent results, the functional solution

will be more succinct, understandable, and reusable. Grand claims indeed! We hope

that by the end of this book you’ll agree that functional programming fosters ele-

gance in programming. It takes a shift in mindset to start from thinking in nouns to

arrive at thinking in verbs, but the journey will be worthwhile. In any case, we think

there’s much that you can take from Clojure to apply to your chosen language—if

only you approach the subject with an open mind.

1.4

Why Clojure isn’t especially object-oriented

Elegance and familiarity are orthogonal.

—Rich Hickey

Clojure was born out of frustration provoked in large part by the complexities of con-

current programming, complicated by the weaknesses of object-oriented program-

ming in facilitating it. This section explores these weaknesses and lays the groundwork

for why Clojure is functional and not object-oriented.

1.4.1

Defining terms

Before we begin, it’s useful to define terms.6

The first important term to define is time. Simply put, time refers to the relative

moments when events occur. Over time, the properties associated with an entity—

both static and changing, singular or composite—will form a concrescence (White-

head 1929) and be logically deemed its identity. It follows from this that at any given

time, a snapshot can be taken of an entity’s properties defining its state. This notion of

state is an immutable one because it’s not defined as a mutation in the entity itself, but

only as a manifestation of its properties at a given moment in time. Imagine a child’s

flip book, as seen in figure 1.2, to understand the terms fully.

It’s important to note that in the canon of object-oriented programming, there’s

no clear distinction between state and identity. In other words, these two ideas are

Figure 1.2 The Runner: a child’s flip book serves to

illustrate Clojure’s notions of state, time, and identity. The

book itself represents the identity. Whenever you wish to

show a change in the illustration, you draw another picture

and add it to the end of your flip book. The act of flipping the

pages therefore represents the states over time of the image

within. Stopping at any given page and observing the

particular picture represents the state of the Runner at that

moment in time.

6 These terms are also defined and elaborated on in Rich Hickey’s presentation, “Are We There Yet?” (Hickey

2009).

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14

CHAPTER 1 Clojure philosophy

Figure 1.3 The Mutable Runner: modeling state change with

mutation requires that you stock up on erasers. Your book

becomes a single page, requiring that in order to model

changes, you must physically erase and redraw the parts of

the picture requiring change. Using this model, you should see

that mutation destroys all notion of time, and state and

identity become one.

conflated into what’s commonly referred to as mutable state. The classical object-

oriented model allows unrestrained mutation of object properties without a willing-

ness to preserve historical states. Clojure’s implementation attempts to draw a clear

separation between an object’s state and identity as they relate to time. To state the dif-

ference to Clojure’s model in terms of the aforementioned flip book, the mutable

state model is different, as seen in figure 1.3.

Immutability lies at the cornerstone of Clojure, and much of the implementation

ensures that immutability is supported efficiently. By focusing on immutability, Clo-

jure eliminates entirely the notion of mutable state (which is an oxymoron) and instead

expounds that most of what’s meant by objects are instead values. Value by definition

refers to an object’s constant representative7 amount, magnitude, or epoch. You

might ask yourself: what are the implications of the value-based programming seman-

tics of Clojure?

Naturally, by adhering to a strict model of immutability, concurrency suddenly

becomes a simpler (although not simple) problem, meaning if you have no fear that

an object’s state will change, then you can promiscuously share it without fear of con-

current modification. Clojure instead isolates value change to its reference types, as

we’ll show in chapter 11. Clojure’s reference types provide a level of indirection to an

identity that can be used to obtain consistent, if not always current, states.

1.4.2

Imperative “baked in”

Imperative programming is the dominant programming paradigm today. The most

unadulterated definition of an imperative programming language is one where a

sequence of statements mutates program state. During the writing of this book (and

likely for some time beyond), the preferred flavor of imperative programming is the

object-oriented style. This fact isn’t inherently bad, because there are countless suc-

cessful software projects built using object-oriented imperative programming tech-

niques. But from the context of concurrent programming, the object-oriented

imperative model is self-cannibalizing. By allowing (and even promoting) unre-

strained mutation via variables, the imperative model doesn’t directly support concur-

rency. Instead, by allowing a maenadic approach to mutation, there are no guarantees

that any variable contains the expected value. Object-oriented programming takes this

one step further by aggregating state in object internals. Though individual methods

may be thread-safe through locking schemes, there’s no way to ensure a consistent

7 Some entities have no representative value—Pi is an example. But in the realm of computing, where we’re

ultimately referring to finite things, this is a moot point.

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Why Clojure isn’t especially object-oriented

15

object state across multiple method calls without expanding the scope of potentially

complex locking scheme(s). Clojure instead focuses on functional programming,

immutability, and the distinction between state, time, and identity. But object-oriented

programming isn’t a lost cause. In fact, there are many aspects that are conducive to

powerful programming practice.

1.4.3

Most of what OOP gives you, Clojure provides

It should be made clear that we’re not attempting to mark object-oriented program-

mers as pariahs. Instead, it’s important that we identify the shortcomings of object-

oriented programming (OOP) if we’re ever to improve our craft. In the next few sub-

sections we’ll also touch on the powerful aspects of OOP and how they’re adopted,

and in some cases improved, by Clojure.

POLYMORPHISM AND THE EXPRESSION PROBLEM

Polymorphism is the ability of a function or method to have different definitions

depending on the type of the target object. Clojure provides polymorphism via both

multimethods and protocols, and both mechanisms are more open and extensible

than polymorphism in many languages.

Listing 1.2 Clojure’s polymorphic protocols

(defprotocol Concatenatable

(cat [this other]))

(extend-type String

Concatenatable

(cat [this other]

(.concat this other)))

(cat "House" " of Leaves")

;=> "House of Leaves"

What we’ve done in listing 1.2 is to define a protocol named Concatenatable that

groups one or more functions (in this case only one, cat) that define the set of func-

tions provided. That means the function cat will work for any object that fully satisfies

the protocol Concatenatable. We then extend this protocol to the String class and

define the specific implementation—a function body that concatenates the argument

other onto the string this. We can also extend this protocol to another type:

(extend-type java.util.List

Concatenatable

(cat [this other]

(concat this other)))

(cat [1 2 3] [4 5 6])

;=> (1 2 3 4 5 6)

So now the protocol has been extended to two different types, String and

java.util.List, and thus the cat function can be called with either type as its first

argument —the appropriate implementation will be invoked.

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16

CHAPTER 1 Clojure philosophy

Note that String was already defined (in this case by Java itself) before we defined

the protocol, and yet we were still able to successfully extend the new protocol to it.

This isn’t possible in many languages. For example, Java requires that you define all

the method names and their groupings (known as interfaces) before you can define a

class that implements them, a restriction that’s known as the expression problem.

THE EXPRESSION PROBLEM The expression problem refers to the desire to

implement an existing set of abstract methods for an existing concrete class

without having to change the code that defines either. Object-oriented lan-

guages allow you to implement an existing abstract method in a concrete class

you control (interface inheritance), but if the concrete class is outside your

control, the options for making it implement new or existing abstract meth-

ods tend to be sparse. Some dynamic languages such as Ruby and JavaScript

provide partial solutions to this problem by allowing you to add methods to

an existing concrete object, a feature sometimes known as monkey-patching.

A Clojure protocol can be extended to any type where it makes sense, even those that

were never anticipated by the original implementor of the type or the original

designer of the protocol. We’ll dive deeper into Clojure’s flavor of polymorphism in

chapter 9, but we hope now you have a basic idea of how it works.

SUBTYPING AND INTERFACE-ORIENTED PROGRAMMING

Clojure provides a form of subtyping by allowing the creation of ad-hoc hierarchies.

We’ll delve into leveraging the ad-hoc hierarchy facility later, in section 9.2. Likewise,

Clojure provides a capability similar to Java’s interfaces via its protocol mechanism. By

defining a logically grouped set of functions, you can begin to define protocols to which

data-type abstractions must adhere. This abstraction-oriented programming model is key

in building large-scale applications, as you’ll discover in section 9.3 and beyond.

ENCAPSULATION

If Clojure isn’t oriented around classes, then how does it provide encapsulation?

Imagine that you need a simple function that, given a representation of a chessboard

and a coordinate, returns a simple representation of the piece at the given square. To

keep the implementation as simple as possible, we’ll use a vector containing a set of

characters corresponding to the colored chess pieces, as shown next.

Listing 1.3 A simple chessboard representation in Clojure

(ns joy.chess)

(defn initial-board []

[\r \n \b \q \k \b \n \r

\p \p \p \p \p \p \p \p

Lowercase dark

\- \- \- \- \- \- \- \-

\- \- \- \- \- \- \- \-

\- \- \- \- \- \- \- \-

\- \- \- \- \- \- \- \-

\P \P \P \P \P \P \P \P

Uppercase light

\R \N \B \Q \K \B \N \R])

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Why Clojure isn’t especially object-oriented

17

There’s no need to complicate matters with the

8

\r

\n

\b

\q

\k

\b

\n

\r

chessboard representation; chess is hard

0

1

2

3

4

5

6

7

enough. This data structure in the code corre-

7

\p

\p

\p

\p

\p

\p

\p

\p

8

9

10

11

12

13

14

15

sponds directly to an actual chessboard in the

6

16

17

18

19

20

21

22

23

starting position, as shown in figure 1.4.

5

From the figure, you can gather that the

24

25

26

27

28

29

30

31

black pieces are lowercase characters and white 4

32

33

34

35

36

37

38

39

pieces are uppercase. This kind of structure is 3

likely not optimal, but it’s a good start. You can

40

41

42

43

44

45

46

47

ignore the actual implementation details for 2

\P

\P

\P

\P

\P

\P

\P

\P

48

49

50

51

52

53

54

55

now and focus on the client interface to query

\R

\N

\B

\Q

\K

\B

\N

\R

1

56

57

58

59

60

61

62

63

the board for square occupations. This is a per-

a b c d e f g h

fect opportunity to enforce encapsulation to

avoid drowning the client in board implementa-

Figure 1.4 

The corresponding chessboard layout

tion details. Fortunately, programming lan-

guages with closures automatically support a form of encapsulation (Crockford 2008)

to group functions with their supporting data.8

The functions in listing 1.4 are self-evident in their intent9 and are encapsulated at

the level of the namespace joy.chess through the use of the defn- macro that creates

namespace private functions. The command for using the lookup function in this case

would be (joy.chess/lookup (initial-board) "a1").

Listing 1.4 Querying the squares of a chessboard

(def *file-key* \a)

(def *rank-key* \0)

(defn- file-component [file]

Calculate file

(- (int file) (int *file-key*)))

(horizontal) projection

(defn- rank-component [rank]

Calculate rank

(* 8 (- 8 (- (int rank) (int *rank-key*)))))

(vertical) projection

(defn- index [file rank]

(+ (file-component file) (rank-component rank)))

Project 1D layout onto

(defn lookup [board pos]

logical 2D chessboard

(let [[file rank] pos]

(board (index file rank))))

Clojure’s namespace encapsulation is the most prevalent form of encapsulation that

you’ll encounter when exploring idiomatic source code. But the use of lexical clo-

sures provides more options for encapsulation: block-level encapsulation, as shown in

listing 1.5, and local encapsulation, both of which effectively aggregate unimportant

details within a smaller scope.

8

This form of encapsulation is described as the module pattern. But the module pattern as implemented with

JavaScript provides some level of data hiding also, whereas in Clojure—not so much.

9

And as a nice bonus, these functions can be generalized to project a 2D structure of any size to a 1D represen-

tation—which we leave to you as an exercise.

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18

CHAPTER 1 Clojure philosophy

Listing 1.5 Using block-level encapsulation

(letfn [(index [file rank]

(let [f (- (int file) (int \a))

r (* 8 (- 8 (- (int rank) (int \0))))]

(+ f r)))]

(defn lookup [board pos]

(let [[file rank] pos]

(board (index file rank)))))

It's often a good idea to aggregate relevant data, functions, and macros at their most

specific scope. You’d still call lookup as before, but now the ancillary functions aren’t

readily visible to the larger enclosing scope—in this case, the namespace joy.chess.

In the preceding code, we’ve taken the file-component and rank-component func-

tions and the *file-key* and *rank-key* values out of the namespace proper and

rolled them into a block-level index function defined with the body of the letfn

macro. Within this body, we then define the lookup function, thus limiting the client

exposure to the chessboard API and hiding the implementation specific functions and

forms. But we can further limit the scope of the encapsulation, as shown in the next

listing, by shrinking the scope even more to a truly function-local context.

Listing 1.6 Local encapsulation

(defn lookup2 [board pos]

(let [[file rank] (map int pos)

[fc rc] (map int [\a \0])

f (- file fc)

r (* 8 (- 8 (- rank rc)))

index (+ f r)]

(board index)))

Finally, we’ve now pulled all of the implementation-specific details into the body of

the lookup2 function itself. This localizes the scope of the index function and all aux-

iliary values to only the relevant party—lookup2. As a nice bonus, lookup2 is simple

and compact without sacrificing readability. But Clojure eschews the notion of data-

hiding encapsulation featured prominently in most object-oriented languages.

NOT EVERYTHING IS AN OBJECT

Finally, another downside to object-oriented programming is the tight coupling

between function and data. In fact, the Java programming language forces you to

build programs entirely from class hierarchies, restricting all functionality to contain-

ing methods in a highly restrictive “Kingdom of Nouns” (Yegge 2006). This environ-

ment is so restrictive that programmers are often forced to turn a blind eye to

awkward attachments of inappropriately grouped methods and classes. It’s because of

the proliferation of this stringent object-centric viewpoint that Java code tends toward

being verbose and complex (Budd 1995). Clojure functions are data, yet this in no

way restricts the decoupling of data and the functions that work upon them. Many of

what programmers perceive to be classes are data tables that Clojure provides via

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Summary

19

maps10 and records. The final strike against viewing everything as an object is that

mathematicians view little (if anything) as objects (Abadi 1996). Instead, mathematics

is built on the relationships between one set of elements and another through the

application of functions.

1.5

Summary

We’ve covered a lot of conceptual ground in this chapter, but it was necessary in order

to define the terms used throughout the remainder of the book. Likewise, it’s impor-

tant to understand Clojure’s underpinnings in order to frame the discussion for the

rest of the book. If you’ve taken in the previous sections and internalized them, then

congratulations: you have a solid basis for proceeding to the rest of the book. But if

you’re still not sure what to make of Clojure, it’s okay—we understand that it may be a

lot to take in all at once. Understanding will come gradually as we piece together Clo-

jure’s story. For those of you coming from a functional programming background,

you’ll likely have recognized much of the discussion in the previous sections, but per-

haps with some surprising twists. Conversely, if your background is more rooted in

object-oriented programming, then you may get the feeling that Clojure is very differ-

ent than you’re accustomed to. Though in many ways this is true, in the coming chap-

ters you’ll see how Clojure elegantly solves many of the problems that you deal with on

a daily basis. Clojure approaches solving software problems from a different angle

than classical object-oriented techniques, but it does so having been motivated by

their fundamental strengths and shortcomings.

With this conceptual underpinning in place, it’s time to make a quick run through

Clojure’s technical basics and syntax. We’ll be moving fairly quickly, but no faster than

necessary to get to the deeper topics in following chapters. So hang on to your REPL,

here we go...

10 See section 5.6 for more discussion on this idea.

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Drinking from

the Clojure firehose

This chapter covers

 Scalars

 Putting things together: collections

 Making things happen: functions

 Vars

 Locals, loops, and blocks

 Preventing things from happening: quoting

 Leveraging Java via interop

 Exceptional circumstances

 Namespaces

This chapter provides a quick tour of the bare necessities—the things you’ll need

to know to understand the rest of this book. If you’ve been programming with Clo-

jure for a while, this may be a review, but otherwise it should give you everything

you need to start writing Clojure code. In most cases throughout this chapter, the

examples provided will be perfunctory in order to highlight the immediate point.

20

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Scalars

21

Later in the book we’ll build on these topics and many more, so don’t worry if you

don’t quite grasp every feature now—you’ll get there.

Interaction with Clojure is often performed at the Read-Eval-Print Loop (REPL).

When starting a new REPL session, you’re presented with a simple prompt:

user>

The user prompt refers to the top-level namespace of the default REPL. It’s at this

point that Clojure waits for input expressions. Valid Clojure expressions consist of

numbers, symbols, keywords, booleans, characters, functions, function calls, macros,

strings, literal maps, vectors, and sets. Some expressions, such as numbers, strings, and

keywords, are self-evaluating—when entered, they evaluate to themselves. The Clojure

REPL also accepts source comments, which are marked by the semicolon ; and con-

tinue to a newline:

user> 42 ; numbers evaluate to themselves

;=> 42

user> "The Misfits" ; strings do too

;=> "The Misfits"

user> :pyotr ; as do keywords

;=> :pyotr

Now that we’ve seen several scalar data types, we’ll take a closer look at each of them.

2.1

Scalars

The Clojure language has a rich set of data types. Like most programming languages,

it provides scalar types such as integers, strings, and floating-point numbers, each rep-

resenting a single unit of data. Clojure provides several different categories of scalar

data types: integers, floats, rationals, symbols, keywords, strings, characters, booleans,

and regex patterns. In this section, we’ll address most of these1 categories in turn, pro-

viding examples of each.

2.1.1

Numbers

A number can consist of only the digits 0-9, a decimal point (.), a sign (+ or -), and an

optional e for numbers written in exponential notation. In addition to these elements,

numbers in Clojure can take either octal or hexadecimal form and also include an

optional M, that flags a number as a decimal requiring arbitrary precision: an impor-

tant aspect of numbers in Clojure. In many programming languages, the precision2 of

numbers is restricted by the host platform, or in the case of Java and C#, defined by

the language specification. Clojure on the other hand uses the host language’s primi-

tive numbers when appropriate, but rolls over to the arbitrarily precise versions when

needed, or when explicitly specified.

1

We won’t look at regular expression patterns here, but for details on everything regex-related you can flip for-

ward to section 4.6.

2

With caveats, as we’ll describe in chapter 4.

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22

CHAPTER 2 Drinking from the Clojure firehose

2.1.2

Integers

Integers comprise the whole number set, both positive and negative. Any number

starting with an optional sign or digit followed exclusively by digits is considered and

stored as an integer. Integers in Clojure can theoretically take an infinitely large value,

although in practice the size is limited by the memory available. The following num-

bers are recognized by Clojure as integers:

42

+9

-107

991778647261948849222819828311491035886734385827028118707676848307166514

The following illustrates the use of decimal, hexadecimal, octal, radix-32, and binary

literals, respectively, all representing the same number:

[127 0x7F 0177 32r3V 2r01111111]

;=> [127 127 127 127 127]

The radix notation supports up to base 36. Adding signs to the front of each of the

integer literals is also legal.

2.1.3

Floating-point numbers

Floating-point numbers are the decimal expansion of rational numbers. Like Clo-

jure’s implementation of integers, the floating-point values are arbitrarily precise.3

Floating-point numbers can take the traditional form of some number of digits and

then a decimal point, followed by some number of digits. But floating-point numbers

can also take an exponential form (scientific notation) where a significant part is fol-

lowed by an exponent part separated by a lower or uppercase E. The following num-

bers are examples of valid floating-point numbers:

1.17

+1.22

-2.

366e7

32e-14

10.7e-3

Numbers are largely the same across most programming languages, so we’ll move on

to some scalar types that are more unique to Lisp and Lisp-inspired languages.

2.1.4

Rationals

Clojure provides a rational type in addition to integer and floating-point numbers.

Rational numbers offer a more compact and precise representation of a given value

over floating-point. Rationals are represented classically by an integer numerator and

denominator, and that’s exactly how they’re represented in Clojure. The following

numbers are examples of valid rational numbers:

3 With some caveats, as we’ll discuss in section 4.1.

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Scalars

23

22/7

7/22

1028798300297636767687409028872/88829897008789478784

-103/4

Something to note about rational numbers in Clojure is that they’ll be simplified if

they can—the rational 100/4 will resolve to the integer 25.

2.1.5

Symbols

Symbols in Clojure are objects in their own right, but are often used to represent

another value. When a number or a string is evaluated, you get back exactly the same

object, but when a symbol is evaluated, you’ll get back whatever value that symbol is

referring to in the current context. In other words, symbols are typically used to refer

to function parameters, local variables, globals, and Java classes.

2.1.6

Keywords

Keywords are similar to symbols, except that they always evaluate to themselves. You’re

likely to see the use of keywords far more in Clojure than symbols. The form of a key-

word’s literal syntax is as follows:

:chumby

:2

:?

:ThisIsTheNameOfaKeyword

Although keywords are prefixed by a colon :, it’s only part of the literal syntax and not

part of the name itself. We go into further detail about keywords in section 4.3.

2.1.7

Strings

Strings in Clojure are represented similarly to the way they’re used in many program-

ming languages: a string is any sequence of characters enclosed within a set of double

quotes, including newlines, as shown:

"This is a string"

"This is also a

String"

Both will be stored as written, but when printed at the REPL, multiline strings will

include escapes for the literal newline characters like "This is also a\n String".

2.1.8

Characters

Clojure characters are written with a literal syntax prefixed with a backslash and are

stored as Java Character objects, as shown:

\a ; The character lowercase a

\A ; The character uppercase A

\u0042 ; The unicode character uppercase B

\\ ; The back-slash character \

\u30DE ; The unicode katakana character ?

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24

CHAPTER 2 Drinking from the Clojure firehose

And that’s it for Clojure’s scalar data types. In the next section, we’ll discuss Clojure’s

collection data types, which is where the real fun begins.

2.2

Putting things together: collections

We’ll cover the collection types in greater detail in chapter 5, but because Clojure pro-

grams are made up of various kinds of literal collections, it’s helpful to at least glance

at the basics of lists, vectors, maps, and sets.

2.2.1

Lists

Lists are the classic collection type in List Processing languages, and Clojure is no

exception. Literal lists are written with parentheses:

(yankee hotel foxtrot)

When a list is evaluated, the first item of the list —yankee in this case—will be resolved

to a function, macro, or special form. If yankee is a function, the remaining items in the

list will be evaluated in order, and the results will be passed to yankee as its parameters.

FORMS A form is any Clojure object meant to be evaluated, including but not

limited to lists, vectors, maps, numbers, keywords, and symbols. A special form

is a form with special syntax or special evaluation rules that are typically not

implemented using the base Clojure forms. An example of a special form is

the . (dot) operator used for Java interoperability purposes.

If on the other hand yankee is a macro or special form, the remaining items in the list

aren’t necessarily evaluated, but are processed as defined by the macro or operator.

Lists can contain items of any type, including other collections. Here are some

more examples:

(1 2 3 4)

()

(:fred ethel)

(1 2 (a b c) 4 5)

Note that unlike some Lisps, the empty list in Clojure, written as (), isn’t the same as

nil.

2.2.2

Vectors

Like lists, vectors store a series of values. There are several differences described in

section 5.4, but for now only two are important. First, vectors have a literal syntax

using square brackets:

[1 2 :a :b :c]

The other important difference is that when evaluated, vectors simply evaluate each

item in order. No function or macro call is performed on the vector itself, though if a

list appears within the vector, that list is evaluated following the normal rules for a list.

Like lists, vectors are type heterogeneous, and as you might guess, the empty vector []

isn’t the same as nil.

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Making things happen: functions

25

2.2.3

Maps

Maps store unique keys and one value per key—similar to what some languages and

libraries call dictionaries or hashes. Clojure actually has several types of maps with differ-

ent properties, but don’t worry about that for now. Maps can be written using a literal

syntax with alternating keys and values inside curly braces. Commas are frequently

used between pairs, but are just whitespace like they are everywhere else in Clojure:

{1 "one", 2 "two", 3 "three"}

Like vectors, every item in a map literal (each key and each value) is evaluated before

the result is stored in the map. Unlike vectors, the order in which they’re evaluated

isn’t guaranteed. Maps can have items of any type for both keys and values, and the

empty map {} isn’t the same as nil.

2.2.4

Sets

Sets are probably the least common collection type that has a literal syntax. Sets store

zero or more unique items. They’re written using curly braces with a leading hash:

#{1 2 "three" :four 0x5}

Again, the empty set #{} isn’t the same as nil.

That’s all for now regarding the basic collection types, but chapter 4 will cover in-

depth the idiomatic uses of each, including their relative strengths and weaknesses.

2.3

Making things happen: functions

Functions in Clojure are a first-class type, meaning that they can be used the same as

any value. Functions can be stored in Vars, held in lists and other collection types, and

passed as arguments to and even returned as the result of other functions.

2.3.1

Calling functions

Clojure borrows its function calling conventions from Lisp, also known as prefix notation:

(+ 1 2 3)

;=> 6

The immediately obvious advantage of prefix over infix notation used by C-style lan-

guages4 is that the former allows any number of operands per operator, whereas infix

allows only two. Another, less obvious advantage to structuring code as prefix notation

is that it completely eliminates the problem of operator precedence. Clojure makes

no distinction between operator notation and regular function calls—all Clojure con-

structs, functions, macros, and operators are formed using prefix, or fully parenthe-

sized, notation. This uniform structure forms the basis for the incredible flexibility

that Lisp-like languages provide.

4 Of course, Java uses infix notation in only a few instances. The remainder of the language forms tend toward

C-style ad hoc debauchery.

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CHAPTER 2 Drinking from the Clojure firehose

2.3.2

Defining functions

An anonymous (unnamed) Clojure function can be defined as a special form. A spe-

cial form is a Clojure expression that’s part of the core language, but not created in

terms of functions, types, or macros.

An example of a function taking two elements that returns a set of those elements

would be defined as

(fn mk-set [x y] #{x y})

;=> #<user$eval__1$mk_set__2 user$eval__1$mk_set__2@d3576a2>

Entering this function definition in a Clojure REPL gives us a seemingly strange result.

This is because the REPL is showing its internal name for the function object returned

by the fn special form. This is far from satisfying, given that now that the function has

been defined, there’s no apparent way to execute it. It should be noted that the mk-set

symbol is optional and doesn’t correspond to a globally accessible name for the func-

tion, but instead to a name internal to the function itself used for self-calls. Recall from

the previous section that the function call form is always (some-function arguments):

((fn [x y] #{x y}) 1 2)

;=> #{1 2}

The second form to define functions allows for arity overloading of the invocations of

a function. Arity refers to the differences in the argument count that a function will

accept. Changing our previous simple set-creating function to accept either one or

two arguments would be represented as

(fn

([x] #{x})

([x y] #{x y}))

The difference from the previous form is that we can now have any number of argu-

ment/body pairs as long as the arity of the arguments differ. Naturally, the execution

of such a function for one argument would be

((fn

([x] #{x})

([x y] #{x y})) 42)

;=> #{42}

As you saw, arguments to functions are bound one-for-one to symbols during the func-

tion call, but there is a way for functions to accept a variable5 number of arguments:

((fn arity2 [x y] [x y]) 1 2 3)

;=> java.lang.IllegalArgumentException: Wrong number of args passed

Clearly, calling the arity2 function with three arguments won’t work. But what if we

wanted it to take any number of arguments? The way to denote variable arguments is

to use the & symbol followed by a symbol. Every symbol in the arguments list before

5 The implementation details of Clojure prevent the creation of functions with an arity larger than 20, but in

practice this should rarely, if ever, be an issue.

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Making things happen: functions

27

the & will still be bound one-for-one to the same number of arguments passed during

the function call. But any additional arguments will be aggregated in a sequence

bound to the symbol following the & symbol:

((fn arity2+ [x y & z] [x y z]) 1 2)

;=> [1 2 nil]

((fn arity2+ [x y & z] [x y z]) 1 2 3 4)

;=> [1 2 (3 4)]

((fn arity2+ [x y & z] [x y z]) 1)

;=> java.lang.IllegalArgumentException: Wrong number of args passed

Of course, arity2+ still requires at least two arguments. But this isn’t satisfactory, as it

quickly becomes clear that to write programs using only this form would be cumber-

some, repetitive, and overly verbose. Thankfully, Clojure provides another, more con-

venient form to create named functions.

2.3.3

Simplifying function definitions with def and defn

The def special form is a way to assign a symbolic name to a piece of Clojure data. Clo-

jure functions are first-class; they’re equal citizens with data, allowing assignment to

Vars, storage in collections, and as arguments to (or returned from) other functions.

This is different from programming languages where functions are functions and data

are data, and there’s a world of capability available to the latter that’s incongruous to

the former.

Therefore, in order to associate a name with our previous function using def,

we’d use

(def make-a-set

(fn

([x] #{x})

([x y] #{x y})))

And we could now call it in a more intuitive way:

(make-a-set 1)

;=> #{1}

(make-a-set 1 2)

;=> #{1 2}

There’s another way to define functions in Clojure using the defn macro. While cer-

tainly a much nicer way to define and consequently refer to functions by name, using

def as shown is still cumbersome to use. Instead, the simplest defn syntax is a conve-

nient and concise way to create named functions that looks similar to the original fn

form, and allow an additional documentation string:

(defn make-a-set

"Takes either one or two values and makes a set from them"

([x] #{x})

([x y] #{x y}))

The function can again be called the same as we saw before.

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CHAPTER 2 Drinking from the Clojure firehose

2.3.4

In-place functions with #()

Clojure provides a shorthand notation for creating an anonymous function using the

#() reader feature. In a nutshell, reader features are analogous to preprocessor direc-

tives in that they signify that some given form should be replaced with another at read

time. In the case of the #() form, it’s effectively replaced with the special form fn. In

fact, anywhere that it’s appropriate to use #(), it’s likewise appropriate for the fn spe-

cial form.

The #() form can also accept arguments that are implicitly declared through the

use of special symbols prefixed with %:

(def make-a-list_ #(list %))

(def make-a-list1 #(list %1))

(def make-a-list2 #(list %1 %2))

(def make-a-list3 #(list %1 %2 %3))

(def make-a-list3+ #(list %1 %2 %3 %&))

(make-a-list_ 1)

;=> (1)

(make-a-list3+ 1 2 3 4 5)

;=> (1 2 3 (4 5))

The %& argument in make-a-list3+ is used to specify the variable arguments as dis-

cussed previously.

2.4

Vars

Programmers are typically accustomed to dealing with variables and mutation. Clo-

jure’s closest analogy to the variable is the Var. A Var is named by a symbol and holds

a single value. Its value can be changed while the program is running, but this is best

reserved for the programmer making manual changes. A Var’s value can also be

shadowed by a thread local value, though this doesn’t change its original value or

root binding.

2.4.1

Declaring bindings using def

Using def is the most common way to create Vars in Clojure:

(def x 42)

Using def to associate the value 42 to the symbol x creates what’s known as a root

binding —a binding that’s the same across all threads, unless otherwise rebound rela-

tive to specific threads. By default, all threads start with the root binding, which is

their associated value in the absence of a thread-bound value.

The trivial case is that the symbol x is bound to the value 42. Because we used def

to create the Var’s root binding, we should observe that even other threads will view

the same root binding by default:

(.start (Thread. #(println "Answer: " x)))

; Answer: 42

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Locals, loops, and blocks

29

Vars don’t require a value; instead we can simply declare them and defer the responsi-

bility of binding their values to individual threads:6

(def y)

y

;=> java.lang.IllegalStateException: Var user/y is unbound.

Functions and vars theoretically provide all you need to implement any algorithm,

and some languages leave you with exactly these “atomic” constructs.

2.5

Locals, loops, and blocks

Clojure’s function and value binding capabilities provide a basis for much of what a

developer needs to start getting operational code, but a large part of the story is miss-

ing. Clojure also provides capabilities for creating local value bindings, looping con-

structs, and aggregating blocks of functionality.

2.5.1

Blocks

Use the do form when you have a series or block of expressions that need to be treated

as one. All the expressions will be evaluated, but only the last one will be returned:

(do

6

(+ 5 4)

3)

;=> 3

The expressions 6 and (+ 5 4) are perfectly valid and legal. The addition in (+ 5 4)

is even done, but the value is thrown away—only the final expression 3 is returned.

The middle bits of the do form are typically where the side-effects occur.

2.5.2

Locals

Clojure doesn’t have local variables, but it does have locals; they just can’t vary. Locals

are created and their scope defined using a let form, which starts with a vector that

defines the bindings, followed by any number of expressions that make up the body.

The vector starts with a binding form (usually just a symbol), which is the name of a

new local. This is followed by an expression whose value will be bound to this new

local for the remainder of the let form. You can continue pairing binding names and

expressions to create as many locals as you need. All of them will be available in the

body of the let:

(let [r 5

pi 3.1415

r-squared (* r r)]

(println "radius is" r)

(* pi r-squared))

6 We’ll talk more about per-thread bindings in chapter 11.

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30

CHAPTER 2 Drinking from the Clojure firehose

The body is sometimes described as an “implicit do” because it follows the same rules:

you may include any number of expressions and all will be evaluated, but only the

value of the last one is returned.

All of the binding forms in the previous example are simple symbols: r, pi, and r-

squared. More complex binding expressions can be used to pull apart expressions

that return collections. This feature is called destructuring: see section 2.9 for details.

Because they’re immutable, locals can’t be used to accumulate results; instead,

you’d use a high level function or loop/recur form.

2.5.3

Loops

The classic way to build a loop in a Lisp is a recursive call, and it’s in Clojure as well.

Using recursion sometimes requires thinking about your problem in a different way

than imperative languages encourage; but recursion from a tail position is in many

ways like a structured goto, and has more in common with an imperative loop than it

does with other kinds of recursion.

RECUR

Clojure has a special form called recur that’s specifically for tail recursion:

(defn print-down-from [x]

(when (pos? x)

(println x)

(recur (dec x))))

This is nearly identical to how you’d structure a while loop in an imperative language.

One significant difference is that the value of x isn’t decremented somewhere in the

body of the loop. Instead, a new value is calculated as a parameter to recur, which

immediately does two things: rebinds x to the new value and returns control to the top

of print-down-from.

If the function has multiple arguments, the recur call must as well, just as if you

were calling the function by name instead of using the recur special form. And just as

with a function call, the expressions in the recur are evaluated in order first and only

then bound to the function arguments simultaneously.

The previous example doesn’t concern itself with return values; it’s just about the

println side effects. Here’s a similar loop that builds up an accumulator and returns

the final result:

(defn sum-down-from [sum x]

(if (pos? x)

(recur (+ sum x) (dec x))

sum))

The only ways out of the function are recur, which isn’t really a way out, and sum. So

when x is no longer positive, the function will return the value of sum:

(sum-down-from 0 10)

;=> 55

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Locals, loops, and blocks

31

You may have noticed that the two preceding functions used different blocks: the first

when and the second if. You’ll often see one or the other used as a conditional, but

it’s not always immediately apparent why. In general, the reasons to use when are

 No else-part is associated with the result of a conditional.

 You require an implicit do in order to perform side-effects.

The reasons for the use of if would therefore be the inverse of those listed.

LOOP

Sometimes you want to loop back not to the top of the function, but to somewhere

inside. For example, in sum-down-from you might prefer that callers not have to pro-

vide an initial value for sum. To help, there’s a loop form that acts exactly like let but

provides a target for recur to jump to. It’s used like this:

(defn sum-down-from [initial-x]

(loop [sum 0, x initial-x]

(if (pos? x)

(recur (+ sum x) (dec x))

sum)))

Upon entering the loop form, the locals sum and x are initialized, just as they would

be for a let.

A recur always loops back to the closest enclosing loop or fn, so in this case it’ll go

to the loop. The loop locals are rebound to the values given in recur. The looping

and rebinding will continue until finally x is no longer positive. The return value of

the whole loop expression is sum, just as it was for the earlier function.

TAIL POSITION

Now that we’ve looked at a couple examples of how to use recur, we must discuss an

important restriction. The recur form can only appear in the tail position of a func-

tion or loop. So what’s a tail position? Succinctly, a form is in the tail position of an

expression when its value may be the return value of the whole expression. Consider

this function:

(defn absolute-value [x]

(if (pos? x)

x ; "then" clause

(- x))) ; "else" clause

It takes a single parameter and names it x. If x is already a positive number, then x is

returned; otherwise the opposite of x is returned.

The if form is in the function’s tail position because whatever it returns, the whole

function will return. The x in the “then” clause is also in a tail position of the function.

But the x in the “else” clause is not in the function’s tail position because the value of x

is passed to the - function, not returned directly. The else clause as a whole (- x) is in

a tail position.

If you try to use the recur form somewhere other than a tail position, Clojure will

remind you at compile time:

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32

CHAPTER 2 Drinking from the Clojure firehose

(fn [x] (recur x) (println x))

; java.lang.UnsupportedOperationException:

; Can only recur from tail position

You’ve seen how Clojure provides core functionality available to most popular pro-

gramming languages, albeit from a different bent. But in the next section, we’ll cover

the notion of quoting forms, which are in many ways unique to the Lisp family of lan-

guages and may seem alien to programmers coming from classically imperative and/

or object-oriented languages.

2.6

Preventing things from happening: quoting

Clojure has two quoting forms: quote and syntax-quote. Both are simple bits of syntax

you can put in front of a form in your program. They’re the primary ways for includ-

ing literal scalars and composites in your Clojure program without evaluating them as

code. But before quoting forms can make sense, you need a solid understanding of

how expressions are evaluated.

2.6.1

Evaluation

When a collection is evaluated, each of its contained items is evaluated first:7

(cons 1 [2 3])

If you enter this at the REPL, the form as a whole will be evaluated. In this specific

example, the function cons “constructs” a new sequence with its first argument in the

front of the sequence provided as its second. Because the form is a list, each of the

items will be evaluated first. A symbol, when evaluated, is resolved to a local, a Var, or a

Java class name. If a local or a Var, its value will be returned:

cons

;=> #<core$cons__3806 clojure.core$cons__3806@24442c76>

Literal scalar values evaluate to themselves—evaluating one just returns the same

thing:

1

;=> 1

The evaluation of another kind of collection, a vector, starts again by evaluating the

items it contains. Because they’re literal scalars, nothing much happens. Once that’s

done, evaluation of the vector can proceed. Vectors, like scalars and maps, evaluate to

themselves:

[2 3]

;=> [2 3]

Now that all the items of the original list have been evaluated (to a function, the

number 1, and the vector [2 3]), evaluation of the whole list can proceed. Lists are

7 ...unless it’s a list that starts with the name of a macro or special form. We’ll get to that later.

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Preventing things from happening: quoting

33

evaluated differently from vectors and maps: they call functions, or trigger special

forms, as shown:

(cons 1 [2 3])

;=> (1 2 3)

Whatever function was at the head of the list, cons in this case, is called with the

remaining items of the list as arguments.

2.6.2

Quoting

Using a special form looks like calling a function—a symbol as the first item of a list:

(quote tena)

Each special form has its own evaluation rules. The quote special form simply pre-

vents its argument from being evaluated at all. Though the symbol tena by itself might

evaluate to the value of a Var with the value 9, when it’s inside a quote form, it won’t:

(def tena 9)

(quote tena)

;=> tena

Instead, the whole form evaluates to just the symbol itself. This works for arbitrarily

complex arguments to quote: nested vectors, maps, even lists that would otherwise be

function calls, macro calls, or even more special forms. The whole thing is returned:

(quote (cons 1 [2 3]))

;=> (cons 1 [2 3])

There are a few reasons you might use the quote form, but by far the most common is

so that you can use a literal list as a data collection without having Clojure try to call a

function. We’ve been careful to use vectors in the examples so far in this section

because vectors are never themselves function calls. But if we wanted to use a list

instead, a naive attempt would fail:

(cons 1 (2 3))

; java.lang.ClassCastException:

; java.lang.Integer cannot be cast to clojure.lang.IFn

That’s Clojure telling us that an integer (the number 2 here) can’t be used as a func-

tion. So we have to prevent the form (2 3) from being treated like a function call—

exactly what quote is for:

(cons 1 (quote (2 3)))

;=> (1 2 3)

In other Lisps, this need is so common that they provide a shortcut: a single quote.

Although it’s used less in Clojure, it’s still provided. The previous example can also be

written as

(cons 1 '(2 3))

;=> (1 2 3)

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CHAPTER 2 Drinking from the Clojure firehose

And look at that: one less pair of parens—always welcome in a Lisp. Remember

though that quote affects all of its argument, not just the top level. So even though it

worked in the preceding examples to replace [] with '(), this may not always give you

the results you want:

[1 (+ 2 3)] ;=> [1 5]

'(1 (+ 2 3)) ;=> (1 (+ 2 3))

Finally, note that the empty list () already evaluates to itself; it doesn’t need to be

quoted. Quoting the empty list isn’t idiomatic Clojure.

SYNTAX-QUOTE

Like the quote, syntax-quote prevents its argument and subforms from being evalu-

ated. Unlike quote, it has a few extra features that make it ideal for constructing col-

lections to be used as code.

Syntax-quote is written as a single back-quote:

`(1 2 3)

;=> (1 2 3)

It doesn’t expand to a simple form like quote, but to whatever set of expressions is

required to support the following features.8

SYMBOL AUTO-QUALIFICATION

A symbol can begin with a namespace and a slash. These can be called qualified symbols:

clojure.core/map

clojure.set/union

i.just.made.this.up/quux

Syntax-quote will automatically qualify all unqualified symbols in its argument:

`map

;=> clojure.core/map

Ìnteger

;=> java.lang.Integer

`(map even? [1 2 3])

;=> (clojure.core/map clojure.core/even? [1 2 3])

If the symbol doesn’t name a Var or class that exists yet, syntax-quote will use the cur-

rent namespace:

ìs-always-right

;=> user/is-always-right

This behavior will come in handy in chapter 8, when we discuss macros.

2.6.3

Unquote

As you discovered, the quote special form prevents its argument, and all of its sub-

forms, from being evaluated. But there will come a time when you’ll want some of its

8 A future version of Clojure is likely to expand the back-quote to syntax-quote at read time and implement the

rest of syntax-quote’s features as a macro or special form.

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Preventing things from happening: quoting

35

constituent forms to be evaluated. The way to accomplish this feat is to use what’s

known as an unquote. An unquote is used to demarcate specific forms as requiring

evaluation by prefixing them with the symbol ~ within the body of a syntax-quote:

`(+ 10 (* 3 2))

;=> (clojure.core/+ 10 (clojure.core/* 3 2))

`(+ 10 ~(* 3 2))

;=> (clojure.core/+ 10 6)

What just happened? The final form uses an unquote to evaluate the subform (* 3

2), which of course performs a multiplication of 3 and 2, thus inserting the result into

the outermost syntax-quoted form. The unquote can be used to denote any Clojure

expression as requiring evaluation:

`(1 2 ~3)

;=> (1 2 3)

(let [x 2]

`(1 ~x 3))

;=> (1 2 3)

`(1 ~(2 3))

;=> java.lang.ClassCastException: java.lang.Integer

Whoops! By using the unquote, we’ve told Clojure that the marked form should be

evaluated. But the marked form here is (2 3), and what happens when Clojure

encounters an expression like this? It attempts to evaluate it as a function! Therefore,

care needs to be taken with unquote to ensure that the form requiring evaluation is of

the form that you expect. The more appropriate way to perform the previous task

would thus be

(let [x '(2 3)] `(1 ~x))

;=> (1 (2 3))

This provides a level of indirection such that the expression being evaluated is no lon-

ger (2 3) but x. But this new way breaks the pattern of the previous examples that

returned a list of (1 2 3).

2.6.4

Unquote-splicing

Clojure provides a handy feature to solve exactly the problem posed earlier. A variant

of unquote called unquote-splicing works similarly to unquote, but a little differently:

(let [x '(2 3)] `(1 ~@x))

;=> (1 2 3)

Note the @ in ~@, which tells Clojure to unpack the sequence x, splicing it into the

resulting list rather than inserting it as a nested list.

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CHAPTER 2 Drinking from the Clojure firehose

2.6.5

Auto-gensym

Sometimes you need an unqualified symbol, such as for a parameter or let local

name. The easiest way to do this inside a syntax-quote is to append a # to the symbol

name. This will cause Clojure to generate a new unqualified symbol:

`potion#

;=> potion__211__auto__

Sometimes even this isn’t enough, either because you need to refer to the same sym-

bol in multiple syntax-quotes or because you want to capture a particular unqualified

symbol.

Until this point, we’ve covered many of the basic features making Clojure a unique

flavor of Lisp. But one of the main goals that Clojure excels at meeting is that of

interoperability with a host language and runtime, namely Java and the Java Virtual

Machine.

2.7

Leveraging Java via interop

Clojure is symbiotic with its host,9 providing its rich and powerful features, while Java

provides an object model, libraries, and runtime support. In this section, we’ll take a

brief look at how Clojure allows you to access Java classes and class members, and how

you can create instances and access their members.

2.7.1

Accessing static class members

Clojure provides powerful mechanisms for accessing, creating, and mutating Java

classes and instances. The trivial case is accessing static class properties:

java.util.Locale/JAPAN

;=> #<Locale ja_JP>

Idiomatic Clojure prefers that you access static class members using a syntax like

accessing a namespace-qualified Var:

(Math/sqrt 9)

;=> 3.0

The preceding call is to the java.lang.Math#sqrt static method.

2.7.2

Creating Java class instances

Creating Java class instances is likewise a trivial matter with Clojure. The new special

form closely mirrors the Java model:

(new java.util.HashMap {"foo" 42 "bar" 9 "baz" "quux"})

;=> #<HashMap {baz=quux, foo=42, bar=9}>

The second, more succinct, Clojure form to create instances is actually the idiomatic

form:

9 We’ll focus on the Java Virtual Machine throughout this book, but Clojure has also been hosted on the .NET

Common Language Runtime (CLR) and JavaScript (http://clojurescript.n01se.net/repl/).

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Leveraging Java via interop

37

(java.util.HashMap. {"foo" 42 "bar" 9 "baz" "quux"})

;=> #<HashMap {baz=quux, foo=42, bar=9}>

As you can see, the class name was followed by a dot in order to signify a constructor

call.

2.7.3

Accessing Java instance members with the . operator

To access instance properties, precede the property or method name with a dot:

(.x (java.awt.Point. 10 20))

;=> 10

This returns the value of the field x from the Point instance given.

To access instance methods, the dot form allows an additional argument to be

passed to the method:

(.divide (java.math.BigDecimal. "42") 2M)

;=> 21M

The preceding example calls the #divide method on the class BigDecimal.

2.7.4

Setting Java instance properties

In the absence of mutators in the form setXXX, Java instance properties can be set via

the set! function:

(let [origin (java.awt.Point. 0 0)]

(set! (.x origin) 15)

(str origin))

;=> "java.awt.Point[x=15,y=0]"

The first argument to set! is the instance member access form.

2.7.5

The .. macro

When working with Java, it’s common practice to chain together a sequence of

method calls on the return type of the previous method call:

new java.util.Date().toString().endsWith("2010") /* Java code */

Using Clojure’s dot special form, the following code is equivalent:

(.endsWith (.toString (java.util.Date.)) "2010") ; Clojure code

;=> true

Though correct, the preceding code is difficult to read and will only become more so

when we lengthen the chain of method calls. To combat this, Clojure provides the ..

macro, which can simplify the call chain as follows:

(.. (java.util.Date.) toString (endsWith "2010"))

The preceding .. call closely follows the equivalent Java code and is much easier to

read. Bear in mind, you might not see .. used often in Clojure code found in the wild

outside of the context of macro definitions. Instead, Clojure provides the -> and ->>

macros, which can be used similarly to the .. macro but are also useful in non-interop

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38

CHAPTER 2 Drinking from the Clojure firehose

situations, thus making them the preferred method call facilities in most cases. The ->

and ->> macros are covered in more depth in the introduction to chapter 8.

2.7.6

The doto macro

When working with Java, it’s also common to initialize a fresh instance by calling a set

of mutators:

java.util.HashMap props = new java.util.HashMap(); /* More java code. */

props.put("HOME", "/home/me"); /* Sorry. */

props.put("SRC", "src");

props.put("BIN", "classes");

But using this method is overly verbose and can be streamlined using the doto macro,

which takes the form

(doto (java.util.HashMap.)

(.put "HOME" "/home/me")

(.put "SRC" "src")

(.put "BIN" "classes"))

;=> #<HashMap {HOME=/home/me, BIN=classes, SRC=src}>

Though these Java and Clojure comparisons are useful, it shouldn’t be assumed that

their compiled structures are the same.

2.7.7

Defining classes

Clojure provides the reify and deftype macros as possible ways to create realizations

of Java interfaces, but we’ll defer covering them until chapter 9. Additionally, Clojure

provides a macro named proxy that can be used to implement interfaces and extend

base classes on the fly. Similarly, using the gen-class macro, you can generate statically

named classes. More details about proxy and gen-class are available in chapter 10.

2.8

Exceptional circumstances

We’ll now talk briefly about Clojure’s facilities for handling exceptions. Like Java, Clo-

jure provides a couple of forms for throwing and catching runtime exceptions:

namely throw and catch, respectively.

2.8.1

A little pitch and catch

The mechanism to throw an exception is fairly straightforward:

(throw (Exception. "I done throwed"))

;=> java.lang.Exception: I done throwed

The syntax for catching exceptions in Clojure is similar to that of Java:

(defn throw-catch [f]

[(try

(f)

(catch ArithmeticException e "No dividing by zero!")

(catch Exception e (str "You are so bad " (.getMessage e)))

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Namespaces

39

(finally (println "returning... ")))])

(throw-catch #(/ 10 5))

; returning...

;=> [2]

(throw-catch #(/ 10 0))

; returning...

;=> ["No dividing by zero!"]

(throw-catch #(throw (Exception. "foo")))

; returning...

;=> ["You are so bad foo"]

The major difference between the way that Java handles exceptions compared to Clo-

jure is that Clojure doesn’t adhere to checked exception requirements. In the next,

final section of our introduction to Clojure, we present namespaces, which might look

vaguely familiar if you’re familiar with Java or Common Lisp.

2.9

Namespaces

Clojure’s namespaces provide a way to bundle related functions, macros, and values.

In this section, we’ll briefly talk about how to create namespaces and how to reference

and use things from other namespaces.

2.9.1

Creating namespaces using ns

To create a new namespace, you can use the ns macro:

(ns joy.ch2)

Whereupon your REPL prompt will now display as:

joy.ch2=>

This prompt shows that you’re working within the context of the joy.ch2 namespace.

Clojure also provides a Var *ns* that holds the value of the current namespace. Any

Var created will be a member of the current namespace:

(defn hello [] (println "Hello Cleveland!"))

(defn report-ns [] (str "The current namespace is " *ns*))

(report-ns)

;=> "The current namespace is joy.ch2"

Entering a symbol within a namespace will cause Clojure to attempt to look up its

value within the current namespace:

hello

;=> #<ch2$hello joy.ch2$hello@2af8f5>

You can create new namespaces at any time:

(ns joy.another)

Again, you’ll notice that your prompt has changed, indicating that the new context is

joy.another. Attempting to run report-ns will no longer work:

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CHAPTER 2 Drinking from the Clojure firehose

(report-ns)

; java.lang.Exception:

; Unable to resolve symbol: report-ns in this context

This is because report-ns exists in the joy.ch1 namespace and is only accessible via

its fully qualified name joy.ch2/report-ns. But this will only work for namespaces

created locally or those previously loaded, which we’ll discuss next.

2.9.2

Loading other namespaces with :require

Creating a namespace is straightforward, but how do you load namespaces? Clojure

provides a convenience directive :require to take care of this task. Observe the

following:

(ns joy.req

(:require clojure.set))

(clojure.set/intersection #{1 2 3} #{3 4 5})

;=> #{3}

Using :require indicates that you want the clojure.set namespace loaded, but you

don’t want the mappings of symbols to functions in the joy.req namespace. You can

also use the :as directive to create an additional alias to clojure.set:

(ns joy.req-alias

(:require [clojure.set :as s]))

(s/intersection #{1 2 3} #{3 4 5})

;=> #{3}

The qualified namespace form looks the same as a call to a static class method. The

difference is that a namespace symbol can only be used as a qualifier, whereas a class

symbol can also be referenced independently:

clojure.set

; java.lang.ClassNotFoundException: clojure.set

java.lang.Object

;=> java.lang.Object

The vagaries of namespace mappings from symbols to Vars both qualified and unqual-

ified have the potential for confusion between class names and static methods in the

beginning, but the differences will begin to feel natural as you progress. In addition,

idiomatic Clojure code will tend to use my.Class and my.ns for naming classes and

namespaces respectively, to help eliminate potential confusion.

2.9.3

Loading and creating mappings with :use

Sometimes you’ll want to create mappings from Vars in another namespace to names

in your own, in order to avoid calling each function or macro with the qualifying

namespace symbol. To create these unqualified mappings, Clojure provides the :use

directive:

(ns joy.use-ex

(:use [clojure.string :only [capitalize]]))

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Namespaces

41

(map capitalize ["kilgore" "trout"])

;=> ("Kilgore" "Trout")

The :use directive indicates that only the function capitalize should be mapped in

the namespace joy.use-ex. Specifying the Vars that you’d like explicit mappings for

is good practice in Clojure, as it avoids creating many unnecessary names within a

namespace. Unnecessary names increase the odds of names clashes, which you’ll see

next. A similar directive to :use for managing precise mappings is :exclude

(ns joy.exclusion

(:use [clojure.string :exclude [capitalize]]))

; WARNING: replace already refers to: #'clojure.core/replace in namespace:

; joy.exclusion, being replaced by: #'clojure.string/replace

; WARNING: reverse already refers to: #'clojure.core/reverse in namespace:

; joy.exclusion, being replaced by: #'clojure.string/reverse

(map capitalize ["kilgore" "trout"])

; java.lang.Exception: Unable to resolve symbol: capitalize in this context

The :exclude directive indicates that we wanted to map names for all of clojure.

string’s Vars except for capitalize. Indeed, any attempt to use capitalize directly

throws an exception. But it’s still accessible via clojure.string/capitalize. The rea-

son for this accessibility is because :use implicitly performs a :require directive in

addition to creating mappings. As you might’ve noticed, the creation of the joy.

exclusion namespace signaled two warnings. The reason was that the clojure.

string namespace defines two functions reverse and replace that are already

defined in the clojure.core namespace—which was already loaded by using ns.

Therefore, when either of those functions are used, the last Var definition wins:

(reverse "abc")

;=> "cba"

(clojure.core/reverse "abc")

(\c \b \a)

The clojure.string version of reverse takes precedence over the clojure.core ver-

sion, which may or may not be what we wanted. You should always strive to eliminate

the warnings that Clojure presents in these cases. The most obvious strategy for resolv-

ing these particular warnings would be to use the :require directive to create a

namespace alias with :as as we showed in the previous section.

2.9.4

Creating mappings with :refer

Clojure also provides a :refer directive that works almost exactly like :use except

that it only creates mappings for libraries that have already been loaded:

(ns joy.yet-another

(:refer joy.ch1))

(report-ns)

;=> "The current namespace is #<Namespace joy.yet-another>"

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CHAPTER 2 Drinking from the Clojure firehose

The use of :refer in this way creates a mapping from the name report-ns to the

actual function located in the namespace joy.ch2 so that the function can be called

normally. You could also set an alias for the same function using the :rename keyword

taking a map, as shown:

(ns joy.yet-another

(:refer joy.ch1 :rename {hello hi}))

(hi)

; Hello Cleveland!

Any namespaces referenced must already be loaded implicitly by being previously

defined or by being one of Clojure’s core namespaces, or explicitly loaded through the

use of :require. It should be noted that :rename also works with the :use directive.

2.9.5

Loading Java classes with :import

To use unqualified Java classes within any given namespace, they should be imported

via the :import directive, as shown:

(ns joy.java

(:import [java.util HashMap]

[java.util.concurrent.atomic AtomicLong]))

(HashMap. {"happy?" true})

;=> #<HashMap {happy?=true}>

(AtomicLong. 42)

;=> 42

As a reminder, any classes in the Java java.lang package are automatically imported

when namespaces are created. We’ll discuss namespaces in more detail in sections 9.1

and 10.2.

2.10

Summary

We named this chapter “Drinking from the Clojure firehose”—and you’ve made it

through! How does it feel? We’ve only provided an overview of the topics needed to

move on to the following chapters instead of a full-featured language tutorial. Don’t

worry if you don’t fully grasp the entirety of Clojure the programming language;

understanding will come as you work your way through the book.

In the next chapter, we’ll take a step back and delve into some topics that can’t be

easily categorized, but that deserve attention because of their ubiquity. It’ll be short

and sweet and give you a chance to take a breath before moving into the deeper dis-

cussions on Clojure later in the book.

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Dipping our toes

in the pool

This chapter covers

 Truthiness

 Nil punning

 Destructuring

 Use the REPL to experiment

Deeper and broader topics will be covered in later chapters, but now’s a good time

to pick through an eclectic selection of smaller topics. The topics covered in this

chapter stand alone but are important. Covering them now will be a fun way to start

digging into practical matters of how to use Clojure.

We’ve covered a lot of conceptual ground in the previous chapter and built our

Clojure lexicon. In this chapter, we’ll take a bit of a detour into some fundamental

underpinnings driving idiomatic Clojure source code. First we’ll explore Clojure’s

straightforward notions of Truthiness,1 or the distinctions between values

1 As a deviation from the definition coined by Stephen Colbert in his television show The Colbert Report. Ours

isn’t about matters of gut feeling but rather about matters of Clojure’s logical truth ideal.

43

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44

CHAPTER 3 Dipping our toes in the pool

considered logical true and those considered logical false. Much of idiomatic Clojure

code is built with matters of Truthiness in mind, and we’ll discuss Clojure’s extremely

simple rules. After this we’ll then move on to the notion of nil punning, or treating an

empty sequence as nil. Those of you coming from a background in Lisp may recog-

nize the term, but Clojure handles nil punning differently. We’ll discuss the idioms

related to nil punning in Clojure and their rationale. We’ll then cover destructuring—

a powerful mechanism for pulling apart collection types and binding their constituent

parts as individual values. Using destructuring within your own code can often lead to

extremely concise and elegant solutions, and we’ll provide some examples to illustrate

this. Finally, we’ll sit down and pair-program together to gain an appreciation for the

power of Clojure’s Read-Eval-Print Loop (REPL).

3.1

Truthiness

Truthfulness may be an important virtue, but it doesn’t come up much in program-

ming. On the other hand, Truthiness, or the matter of logical truth values in Clojure, is

critical.

Clojure has one Boolean context: the test expression of the if form. Other forms

that expect Booleans—and, or, when, and so forth—are macros built on top of if. It’s

here that Truthiness matters.

3.1.1

What’s truth?

Every value looks like true to if, except for false and nil. That means that values

which some languages treat as false—zero-length strings, empty lists, the number zero,

and so on—are all treated as true in Clojure:

(if true :truthy :falsey) ;=> :truthy

(if [] :truthy :falsey) ;=> :truthy

(if nil :truthy :falsey) ;=> :falsey

(if false :truthy :falsey) ;=> :falsey

This may feel uncomfortable to you, depending on your background. But because

branches in a program’s logic are already one of the most likely places for complexity

and bugs, Clojure has opted for a simple rule. There’s no need to check a class’s defi-

nition to see if it acts like “false” when you think it should (as is sometimes required in

Python, for example). Every object is “true” all the time, unless it’s nil or false.

3.1.2

Don’t create Boolean objects

It’s possible to create an object that looks a lot like, but isn’t actually, false.

Java has left a landmine for you here, so take a moment to look at it so that you can

step past it gingerly and get on with your life:

(def evil-false (Boolean. "false")) ; NEVER do this

This creates a new instance of Boolean—and that’s already wrong! Because there are

only two possible values of Boolean, an instance of each has already been made for

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Nil pun with care

45

you—they’re named true and false.2 But here you’ve gone and done it anyway,

created a new instance of Boolean and stored it in a Var named evil-false. It looks

like false:

evil-false

;=> false

Sometimes it even acts like false:

(= false evil-false)

;=> true

But once it gains your trust, it’ll show you just how wicked it is by acting like true:

(if evil-false :truthy :falsey)

;=> :truthy

Java’s own documentation warns against the creation of this evil thing, and now you’ve

been warned again. If you just want to parse a string, use the Boolean class’s static

valueOf method instead of its constructor. This is the right way:

(if (Boolean/valueOf "false") :truthy :falsey)

;=> :falsey

3.1.3

nil versus false

Rarely do you need to differentiate between the two false values, but if you do, you can

use nil? and false?:

(when (nil? nil) "Actually nil, not false")

;=> "Actually nil, not false"

Keeping in mind the basic rule that everything in Clojure is truthy unless it’s false or

nil is an astonishingly powerful concept, allowing for elegant solutions. Often pro-

gramming languages have complicated semantics for Truthiness, but Clojure manages

to avoid those matters nicely. You’ll see this simplicity leveraged throughout this book

and in all examples of idiomatic Clojure source code.

Building on that theme, we’ll now discuss the matter of nil punning, which may or

may not surprise you given your background.

3.2

Nil pun with care

Because empty collections act like true in Boolean contexts, we need an idiom for

testing whether there’s anything in a collection to process. Thankfully, Clojure pro-

vides just such a technique:

(seq [1 2 3])

;=> (1 2 3)

(seq [])

;=> nil

2 Clojure’s true and false instances are the same as Java’s Boolean/TRUE and Boolean/FALSE, respectively.

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46

CHAPTER 3 Dipping our toes in the pool

The seq function returns a sequence view of a collection, or nil if the collection is

empty. In a language like Common Lisp, an empty list acts as a false value and can be

used as a pun (a term with the same behavior) for such in determining a looping ter-

mination. As you saw in section 2.3, Clojure’s empty sequences are instead truthy, and

therefore to use one as a pun for falsity will lead to heartache and despair. One solu-

tion that might come to mind is to use empty? in the test, leading to the awkward

phrase (when-not (empty? s) ...). Though it would work, this isn’t idiomatic. A

better solution would be to use seq as a termination condition, as in the following

function print-seq:

(defn print-seq [s]

(when (seq s)

(prn (first s))

(recur (rest s))))

(print-seq [1 2])

; 1

; 2

;=> nil

(print-seq [])

;=> nil

There are a number of points to take away from this example. First, the use of seq as a

terminating condition is the idiomatic way to test whether a sequence is empty. If we

tested just s instead of (seq s), then the terminating condition wouldn’t occur even

for empty collections, leading to an infinite loop.

PREFER DOSEQ An important point not mentioned is that it would be best to

use doseq in this case, but that wouldn’t allow us to illustrate our overarching

points: the Clojure forms named with do at the start (doseq, dotimes, do, and

so on) are intended for side-effects in their bodies and generally return nil as

their results.

Second, rest is used to consume the sequence on the recursive call, which can return

a sequence that’s either empty or not empty (has elements). Clojure also provides a

next function that returns a seq of the rest, or (seq (rest s)), and thus never returns

an empty sequence, but nil instead. But rest is appropriate here because we’re using

seq explicitly in each subsequent iteration. Finally, print-seq is a template for most

functions in Clojure, in that it shows that we should generally not assume seq has been

called on our collection arguments, but instead call seq within the function itself and

process based on its result. Using this approach fosters a more generic handling of col-

lections, a topic that we explore in great detail in chapter 5. In the meantime, it’s

important to keep in mind the difference between empty collections and false values;

otherwise your attempts at nil punning may cause groans all around.

To top off our trifecta of core Clojure concepts, we next explore the most powerful

of the three—destructuring. You’ll see just how powerful this mini-language within

Clojure can be toward developing elegant and often beautiful solutions.

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Destructuring

47

3.3

Destructuring

In the previous section, we briefly described Clojure’s destructuring facility as a mini-

language embedded within Clojure. Destructuring allows us to positionally bind locals

based on an expected form for a composite data structure. In this section, we’ll

explore how destructuring can be used to pull apart composite structures into bind-

ings through the lens of a simple rolodex example project.

PATTERN MATCHING Destructuring is loosely related to pattern matching

found in Haskell, KRC, or Scala, but much more limited in scope. For more

full-featured pattern matching in Clojure, consider using http://

github.com/dcolthorp/matchure, which may in the future be included in contrib as clojure.core.match.

3.3.1

Your assignment, should you choose to accept it

You’ve heard that the rolodex project has been overdue, but now every developer

assigned to it is out sick. The QA team is ready to go, but one function is still missing

and it’s a show-stopper. You’re told to drop everything and write the function ASAP.

The design? Take a vector of length 3 that represents a person’s first, middle, and

last names and return a string that will sort in the normal way, like “Steele, Guy Lewis”.

What are you waiting for? Why aren’t you done yet?!?!

(def guys-whole-name ["Guy" "Lewis" "Steele"])

(str (nth guys-whole-name 2) ", "

(nth guys-whole-name 0) " "

(nth guys-whole-name 1)))

;=> "Steele, Guy Lewis"

Alas, by the time you’ve finished typing guys-whole-name for the fourth time, it’s too

late. The customers have cancelled their orders, and the whole department is bound

to be downsized.

If only you’d known about destructuring.

Okay, so you’re not likely to lose your job because your function is twice as many

lines as it needs to be, but still that’s a lot of code repeated in a pretty small function.

And using index numbers instead of named locals makes the purpose of the function

more obscure than necessary.

Destructuring solves both these problems by allowing you to place a collection of

names in a binding form where normally you’d put just a single name. One kind of

binding form is the list of parameters given in a function definition.

3.3.2

Destructuring with a vector

So let’s try that again, but use destructuring with let to create more convenient locals

for the parts of Guy’s name:

(let [[f-name m-name l-name] guys-whole-name]

(str l-name ", " f-name " " m-name))

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CHAPTER 3 Dipping our toes in the pool

Positional destructuring

This positional destructuring doesn’t work on maps and sets because they’re not

logically3 aligned sequentially. Thankfully, positional destructuring will work with

Java’s java.util.regex.Matcher and anything implementing the CharSequence

and java.util.RandomAccess interfaces.

This is the simplest form of destructuring, where you want to pick apart a sequential

thing (a vector of strings in this case, though a list or other sequential collection

would work as well), giving each item a name.3

We don’t need it here, but we can also use an ampersand in a destructuring vector

to indicate that any remaining values of the input should be collected into a (possibly

lazy) seq:

(let [[a b c & more] (range 10)]

(println "a b c are:" a b c)

(println "more is:" more))

; a b c are: 0 1 2

; more is: (3 4 5 6 7 8 9)

;=> nil

Here the locals a, b, and c are created and bound to the first three values of the range.

Because the next symbol is an ampersand, the remaining values are made available as

a seq bound to more. The name more is pretty common for this purpose, but isn’t spe-

cial—you’ll often see etc or xs instead, or some other name that makes sense in a par-

ticular context.

The final feature of vector destructuring is :as, which can be used to bind a local

to the entire collection. It must be placed after the & local, if there is one, at the end of

the destructuring vector:

(let [range-vec (vec (range 10))

[a b c & more :as all] range-vec]

(println "a b c are:" a b c)

(println "more is:" more)

(println "all is:" all))

; a b c are: 0 1 2

; more is: (3 4 5 6 7 8 9)

; all is: [0 1 2 3 4 5 6 7 8 9]

;=> nil

We made range-vec a vector in this example, and the directive :as binds the input

collection as-is, entirely unmolested, so that the vector stays a vector. This is in contrast

to &, which bound more to a seq, not a vector.

3 Technically, positional destructuring might make sense with sorted sets and maps, but alas it doesn’t operate

as such.

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Destructuring

49

3.3.3

Destructuring with a map

Perhaps passing a name as a three-part vector wasn’t a good idea in the first place. It

might be better stored in a map:

(def guys-name-map

{:f-name "Guy" :m-name "Lewis" :l-name "Steele"})

But now we can’t use a vector to pick it apart effectively. Instead, we use a map:

(let [{f-name :f-name, m-name :m-name, l-name :l-name} guys-name-map]

(str l-name ", " f-name " " m-name))

A couple things about this example may jump out at you. One might be that it still

seems repetitive—we’ll get to that in a moment.

Another might be that it looks a bit unexpected to have the keywords like :f-name

on the right-hand side of each pair even though the input map had keywords on the

left. There are a couple reasons for that. The first is to help keep the pattern of the

name on the left getting the value specified by the thing on the right. That is, the new

local f-name gets the value looked up in the map by the key :f-name, just as the whole

map gets its value from guys-name-map in the earlier def form.

The second reason is because it allows us to conjure up other destructuring fea-

tures by using forms that would otherwise make no sense. Because the item on the left

of each pair will be a new local name, it must be a symbol or possibly a nested destruc-

turing form. But one thing it can’t be is a keyword, unless the keyword is a specially

supported feature such as :keys, :strs, :syms, :as, and :or.

We’ll discuss the :keys feature first because it nicely handles the repetitiveness we

mentioned earlier. It allows us to rewrite our solution like this:

(let [{:keys [f-name m-name l-name]} guys-name-map]

(str l-name ", " f-name " " m-name))

So by using :keys instead of a binding form, we’re telling Clojure that the next form

will be a vector of names that it should convert to keywords such as :f-name in order

to look up their values in the input map. Similarly, if we had used :strs, Clojure

would be looking for items in the map with string keys such as "f-name", and :syms

would indicate symbol keys.

The directives :keys, :strs, :syms, and regular named bindings can appear in

any combination and in any order. But sometimes you’ll want to get at the original

map—in other words, the keys that you didn’t name individually by any of the meth-

ods just described. For that, you want :as, which works just like it does with vector

destructuring:

(let [{f-name :f-name, :as whole-name} guys-name-map]

whole-name)

;=> {:f-name "Guy", :m-name "Lewis", :l-name "Steele"}

If the destructuring map looks up a key that’s not in the source map, it’s normally

bound to nil, but you can provide different defaults with :or:

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CHAPTER 3 Dipping our toes in the pool

(let [{:keys [title f-name m-name l-name], :or {title "Mr."}} guys-name-map]

(println title f-name m-name l-name))

; Mr. Guy Lewis Steele

;=> nil

ASSOCIATIVE DESTRUCTURING

One final technique worth mentioning is associative destructuring. Using a map to

define a number of destructure bindings isn’t limited to maps. We can also destruc-

ture a vector by providing a map declaring the local name as indices into them, as

shown:

(let [{first-thing 0, last-thing 3} [1 2 3 4]]

[first-thing last-thing])

;=> [1 4]

We’ll explore associative destructuring later in section 6.1 when we discuss Clojure’s

support for named structures. You’ve seen the shapes that destructuring takes within

the let form, but you’re not limited to that exclusively, as we’ll explore next.

3.3.4

Destructuring in function parameters

All the preceding examples use let to do their destructuring, but exactly the same

features are available in function parameters. Each function parameter can destruc-

ture a map or sequence:

(defn print-last-name [{:keys [l-name]}]

(println l-name))

(print-last-name guys-name-map)

; Steele

;=> nil

Note that function arguments can include an ampersand as well, but this isn’t the

same as destructuring. Instead, that’s part of their general support for multiple func-

tion bodies, each with its own number of parameters.

3.3.5

Destructuring versus accessor methods

In many object-oriented languages, you might create new classes to manage your

application data objects, each with its own set of getter and setter methods. It’s idiom-

atic in Clojure to instead build your application objects by composing maps and vec-

tors as necessary. This makes destructuring natural and straightforward. So anytime

you find that you’re calling nth on the same collection a few times, or looking up con-

stants in a map, or using first or next, consider using destructuring instead.

Now that we’ve made it through the cursory introduction to Clojure, let’s take

some time to pair-program (Williams 2002). In the next section, we’ll take many of

the bare necessities that you’ve just learned and walk through the creation of a couple

interesting functions for drawing pretty pictures within Clojure’s REPL.

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Using the REPL to experiment

51

3.4

Using the REPL to experiment

Most software development projects include a stage where you’re not sure what needs

to happen next. Perhaps you need to use a library or part of a library you’ve never

touched before. Or perhaps you know what your input to a particular function will be,

and what the output should be, but you aren’t sure how to get from one to other. In

more static languages, this can be time-consuming and frustrating; but by leveraging

the power of the Clojure REPL, the interactive command prompt, it can actually be fun.

3.4.1

Experimenting with seqs

Say someone suggests to you that coloring every pixel of a canvas with the xor of its x

and y coordinates might produce an interesting image. It shouldn’t be too hard, so

you can jump right in. You’ll need to perform an operation on every x and y in a pair

of ranges. Do you know how range works?

(range 5)

;=> (0 1 2 3 4)

That should do nicely for one coordinate. To nest seqs, for often does the trick. But

again, rather than writing code and waiting until you have enough to warrant compil-

ing and testing, you can just try it:

(for [x (range 2) y (range 2)] [x y])

;=> ([0 0] [0 1] [1 0] [1 1])

There are the coordinates that will form your input. Now you need to xor them:

(xor 1 2)

;=> java.lang.Exception: Unable to resolve symbol: xor in this context

Bother—no function named xor. Fortunately, Clojure provides find-doc, which

searches not just function names but also their doc strings for the given term:

(find-doc "xor")

; -------------------------

; clojure.core/bit-xor

; ([x y])

; Bitwise exclusive or

;=> nil

So the function you need is called bit-xor:

(bit-xor 1 2)

;=> 3

Perfect! Now you want to adjust your earlier for form to return the bit-xor along

with the x and y. The easiest way to do this will depend on what tool is hosting your

REPL. In many, you can just press the up-arrow key on your keyboard a couple of times

to bring back the earlier for form. You’re not going to want to retype things to make

minor adjustments, so take a moment right now to figure out a method you like that

will allow you to make a tweak like this by inserting the bit-xor call:

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CHAPTER 3 Dipping our toes in the pool

(for [x (range 2) y (range 2)] [x y (bit-xor x y)])

;=> ([0 0 0] [0 1 1] [1 0 1] [1 1 0])

That looks about right. Now you’re about to shift gears to pursue the graphics side of

this problem, so tuck that bit of code away in a function so it’ll be easy to use later:

(defn xors [max-x max-y] (for [x (range max-x) y (range max-y)] [x y (bit-

xor x y)]))

(xors 2 2)

;=> ([0 0 0] [0 1 1] [1 0 1] [1 1 0])

You might even save that into a .clj file somewhere, if you haven’t already.

3.4.2

Experimenting with graphics

Clojure’s REPL isn’t just for playing around; it’s also great for experimenting with Java

libraries. We believe that there’s no better environment for exploring a Java API than

Clojure’s REPL. To illustrate, poke around with java.awt, starting with a Frame:

(def frame (java.awt.Frame.))

;=> #'user/frame

That should’ve created a Frame, but no window appeared. Did it work at all?

frame

;=> #<Frame java.awt.Frame[frame0,0,22,0x0,invalid,hidden,...]>

Well, you have a Frame object, but perhaps the reason you can’t see it is hinted at by

the word hidden in the #<Frame...> printout. Perhaps the Frame has a method you

need to call to make it visible. One way to find out would be to check the Javadoc of

the object, but because you’re at the REPL already, let’s try something else. You’ve

already seen how the for macro works, so maybe you can check a class for which

methods it has to see whether one that you can use is available:

(for [method (seq (.getMethods java.awt.Frame))

:let [method-name (.getName method)]

:when (re-find #"Vis" method-name)]

method-name)

;=> ("setVisible" "isVisible")

The for macro takes a :let flag and bindings vector that works similarly to the let

special form that you use to bind the local method-name to the result of calling the

method .getName on each method in turn. The :when is used to limit the elements

used in its body to only those that return a truthy value in the expression after the

directive. Using these directives allows you to iterate through the methods and build a

seq of those whose names match a regular expression #"Vis". We’ll cover Clojure’s

regular expression syntax in section 3.5.

THE CONTRIB FUNCTION SHOW The clojure-contrib library also has a function

show in the clojure.contrib.repl-utils namespace that allows for more

useful printouts of class members than we show using for.

Your query returned two potential methods, so try out each of them:

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Using the REPL to experiment

53

(.isVisible frame)

;=> false

That’s false, as you might’ve suspected. Will setting it to true make any difference?

(.setVisible frame true)

;=> nil

It did, but it’s so tiny! Not to worry, as a Frame class also has a .setSize method that

you can use:

(.setSize frame (java.awt.Dimension. 200 200))

;=> nil

And up pops a blank window for you to draw on. At this point, we’ll guide you

through the rest of this section, but keep in mind that Java’s official API might be of

interest should you choose to extend the example program.

THE JAVADOC FUNCTION As of Clojure 1.2, a javadoc function is automati-

cally available at the REPL to query and view official API documentation:

(javadoc frame)

This should return a string corresponding to a URL and open a browser win-

dow for just the right page of documentation. Prior to Clojure 1.2, this func-

tion was in clojure.contrib.repl-utils.

What you need to draw into your Frame is its graphics context, which can be fetched

as shown:

(def gfx (.getGraphics frame))

;=> #'user/gfx

Then, to actually draw, you can try out the fillRect method of that graphics context.

If you’re trying this yourself, make sure the blank window is positioned so that it’s

unobscured while you’re typing into your REPL:

(.fillRect gfx 100 100 50 75)

And just like that, you’re drawing on the screen interactively. You should see a single

black rectangle in the formerly empty window. Exciting, isn’t it? You could be a kid

playing with turtle graphics for the first time, it’s so much fun. But what it needs now is

a dash of color:

(.setColor gfx (java.awt.Color. 255 128 0))

(.fillRect gfx 100 150 75 50)

Now there should be an orange rectangle as well. Perhaps the coloring would make

Martha Stewart cry, but you now have tried out all the basic building blocks you’ll

need to complete the original task: you have a function that returns a seq of coordi-

nates and their xor values, you have a window you can draw into, and you know how to

draw rectangles of different colors. Bear in mind that if you move the actual frame

with the mouse, your beautiful graphics will disappear. This is just an artifact of this

limited experiment and can be avoided using the full Java Swing capabilities.

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CHAPTER 3 Dipping our toes in the pool

3.4.3

Putting it all together

What’s left to do? Use the graphics functions you just saw to draw the xor values you

created earlier:

(doseq [[x y xor] (xors 200 200)]

(.setColor gfx (java.awt.Color. xor xor xor))

(.fillRect gfx x y 1 1))

The xors function you created earlier generates a seq of

vectors, if you remember, where each vector has three

elements: the x and y for your coordinates and the xor

value that goes with them. The first line here uses

destructuring to assign each of those three values to new

locals x, y, and xor, respectively.

The second line sets the “pen” color to a gray level

based on the xor value, and the final line draws a single-

pixel rectangle at the current coordinates. The resulting

Figure 3.1 Visualization of

graphic is shown in figure 3.1.

xor. This is the graphic drawn

But just because you’ve succeeded doesn’t mean you

by the six or so lines of code

have to quit. You’ve built up some knowledge and a bit of

we’ve looked at so far—a

visual representation of

a toolbox, so why not play with it a little?

Clojure’s bit-xor function.

3.4.4

When things go wrong

For example, the pattern appears to cut off in the middle—perhaps you’d like to see a

bit more. Re-enter that last expression, but this time try larger limits:

(doseq [[x y xor] (xors 500 500)]

(.setColor gfx (java.awt.Color. xor xor xor))

(.fillRect gfx x y 1 1))

; java.lang.IllegalArgumentException:

; Color parameter outside of expected range: Red Green Blue

Whoops. Something went wrong, but what exactly? This gives you a perfect opportu-

nity to try out one final REPL tool. When an exception is thrown from something you

try at the REPL, the result is stored in a Var named *e. This allows you to get more

detail about the expression, such as the stack trace:

(.printStackTrace *e)

; java.lang.IllegalArgumentException: Color parameter outside of

; expected range: Red Green Blue

; at clojure.lang.Compiler.eval(Compiler.java:4639)

; at clojure.core$eval__5182.invoke(core.clj:1966)

; at clojure.main$repl__7283$read_eval_print__7295.invoke(main.clj:180)

; ...skipping a bit here...

; Caused by: java.lang.IllegalArgumentException: Color parameter

; outside of expected range: Red Green Blue

; at java.awt.Color.testColorValueRange(Color.java:298)

; at java.awt.Color.<init>(Color.java:382)

; ...skipping a bit more...

; ... 11 more

;=> nil

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Using the REPL to experiment

55

That’s a lot of text, but don’t panic. Learning to read Java stack traces will be useful, so

let’s pick it apart.

The first thing to understand is the overall structure of the trace—there are two

“causes.” The original or root cause of the exception is listed last—this is the best

place to look first.4 The name and text of the exception there are the same as the

REPL printed for us in the first place, though they won’t always be. So let’s look at that

next line:

at java.awt.Color.testColorValueRange(Color.java:298)

Like most lines in the stack trace, this has four parts: the name of the class, the name

of the method, the filename, and finally the line number:

at <class>.<method or constructor>(<filename>:<line>)

In this case, the function name is testColorValueRange, which is defined in Java’s

own Color.java file. Unless this means more to you than it does to us, let’s move on to

the next line:

at java.awt.Color.<init>(Color.java:382)

It appears that it was the Color’s constructor (called <init> in stack traces) that called

that test function you saw earlier. So now the picture is pretty clear—when you con-

structed a Color instance, it checked the values you passed in, decided they were

invalid, and threw an appropriate exception.

If this weren’t enough, you could continue walking down the stack trace until the

line

... 11 more

This is your cue to jump up to the cause listed before this one to find out what the

next 11 stack frames were.

To fix your invalid Color argument, you can just adjust the xors function to return

only legal values using the rem function, which returns the remainder so you can keep

the results under 256:

(defn xors [xs ys]

(for [x (range xs) y (range ys)]

[x y (rem (bit-xor x y) 256)]))

Note that you’re redefining an existing function here. This is perfectly acceptable and

well-supported behavior. Before moving on, create a function that takes a graphics

object and clears it:

(defn clear [g] (.clearRect g 0 0 200 200))

Calling (clear gfx) will clear the frame, allowing the doseq form you tried before to

work perfectly.

4

This is a runtime exception, the most common kind. If you misuse a macro or find a bug in one, you may see

compile-time exceptions. The trace will look similar but will have many more references to Compiler.java. For

these traces, the most recent exception (listed first) may be the only one that identifies the filename and line

number in your code that’s at fault.

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CHAPTER 3 Dipping our toes in the pool

3.4.5

Just for fun

The bit-xor function does produce an interesting image, but perhaps you wonder

what other functions might look like. Try adding another parameter to xors so that

you can pass in whatever function you’d like to look at. Because it’s not just bit-xor

anymore, change the name while you’re at it:

(defn f-values [f xs ys]

(for [x (range xs) y (range ys)]

[x y (rem (f x y) 256)]))

You might as well wrap your call to setSize, clear, and the doseq form in a function

as well:

(defn draw-values [f xs ys]

(clear gfx)

(.setSize frame (java.awt.Dimension. xs ys))

(doseq [[x y v] (f-values f xs ys)]

(.setColor gfx (java.awt.Color. v v v))

(.fillRect gfx x y 1 1)))

This allows you to try out different functions and ranges quite easily. More nice exam-

ples are shown in figure 3.2, resulting from the following:

(draw-values bit-and 256 256)

(draw-values + 256 256)

(draw-values * 256 256)

If this were the beginning or some awkward middle stage of a large project, you’d

have succeeded in pushing past this troubling point and could now take the functions

you’ve built and drop them into the larger project.

By trying everything out at the REPL, you’re encouraged to try smaller pieces

rather than larger ones. The smaller the piece, the shorter the distance down an

incorrect path you’re likely to go. Not only does this reduce the overall development

time, but it provides developers more frequent successes that can help keep morale

and motivation high through even

tough stages of a project. But trial-and-

error exploration isn’t enough. An

intuitive basis in Clojure is also

needed to become highly effective.

Throughout this book, we’ll help you

to build your intuition in Clojure Figure 3.2 Three possible results from draw-

values. The draw-values function you’ve written

through discussions of its idioms and can be used to create a variety of graphics. Here are

its motivating factors and rationale.

examples, from left to right, of bit-and, +, and *.

3.5

Summary

We started slowly in this chapter in order to take a breather from the sprint that was

chapter 2. Truthiness in Clojure observes a simple rule: every object is true all the

time, unless it’s nil or false. Second, in many Lisp-like languages, the empty list ()

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Summary

57

and the truth value nil are analogous—this is known as nil-punning—but in Clojure

this isn’t the case. Instead, idiomatic Clojure employs the (seq (rest _)) idiom in

the form of the next function to provide a mechanism fostering “form follows func-

tion” and also to eliminate errors associated with falsety/empty-seq disparity. Finally,

destructuring provides a powerful mechanism, a mini-language for binding if you will,

for partially or entirely pulling apart the constituent components of composite types.

Our trek through the REPL illustrated the power in having the whole language

(Graham 2001) at your disposal. As a Clojure programmer, you’ll spend a lot of time

in the REPL, and pretty soon you won’t know how you lived without it.

In the next chapter, we’ll touch on matters concerning Clojure’s seemingly inno-

cent scalar data types. Although in most cases these scalars will expose powerful pro-

gramming techniques, be forewarned: as you’ll see, the picture isn’t always rosy.

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Part 2

Data types

Clojure has squirreled away interesting tidbits even among its data types.

The scalar types include some less common items such as keywords and rational

numbers, and the composite types are all immutable. In this part, we’ll explore

all of them in detail.

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On scalars

It requires a very unusual mind to

undertake the analysis of the obvious.

—Alfred North Whitehead

This chapter covers

 Understanding precision

 Trying to be rational

 When to use keywords

 Symbolic resolution

 Regular expressions—the second problem

So far, we’ve covered a somewhat eclectic mix of theoretical and practical concerns.

This brings us now to a point where we can dive deeper into a fundamental topic:

how Clojure deals with scalar values, including numeric, symbolic, and regular

expression values, and how they behave as data and sometimes as code.

A scalar data type is one that can only hold one value at a time of a number, sym-

bol, keyword, string, or character. Most of the use cases for Clojure’s scalar data

types will be familiar to you. But there are some nuances that should be observed.

Clojure’s scalar data types exist in an interesting conceptual space. Because of its

symbiotic nature, some of the scalar type behaviors walk a conceptual line between

61

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CHAPTER 4 On scalars

pure Clojure semantics and host semantics. This chapter provides a rundown of some

of the idiomatic uses of Clojure’s scalar data types as well as some pitfalls that you

might encounter. In most cases, Clojure will shield you from the quirks of its host, but

there are times when they’ll demand attention. Clojure’s scalar types have the poten-

tial to act like Sybil—sweet and kind one moment, vicious and vile the next—requir-

ing some thought to handle properly. We’ll also talk about this duality and address its

limitations and possible mitigation techniques. Additionally, we’ll address the age-old

topic of Lisp-1 versus Lisp-2 implementations and how Clojure approaches the matter.

Finally, we’ll talk briefly about Clojure’s regular expression literals and how they’re

typically used.

We’ll first cover matters of numerical precision and how the Java Virtual Machine

works to thwart your attempts at mathematical nirvana.

4.1

Understanding precision

Numbers in Clojure are by default as precise1 as they need to be. Given enough mem-

ory, you could store the value of Pi accurately up to a billion places and beyond; in

practice, values that large are rarely needed. But it’s sometimes important to provide

perfect accuracy at less-precise values. When dealing with raw Clojure functions and

forms, it’s a trivial matter to ensure such accuracy; it’s handled automatically. Because

Clojure encourages interoperability with its host platform, the matter of accuracy

becomes less than certain. This section will talk about real matters of precision related

to Clojure’s support for the Java Virtual Machine. As it pertains to programming lan-

guages,2 numerical precision is proportional to the mechanisms used for storing

numerical representations. The Java language specification describes the internal rep-

resentation of its primitive types thus limiting their precision. Depending on the class

of application specialization, a programmer could go an entire career and never be

affected by Java’s precision limitations. But many industries require perfect accuracy

of arbitrarily precise computations, and it’s here that Clojure can provide a great

boon; but with this power come some pointy edges, as we’ll discuss shortly.

4.1.1

Truncation

Truncation refers to the limiting of accuracy for a floating-point number based on a

deficiency in the corresponding representation. When a number is truncated, its pre-

cision is limited such that the maximum number of digits of accuracy is bound by the

number of bits that can “fit” into the storage space allowed by its representation. For

floating-point values, Clojure truncates by default. Therefore, if high precision is

required for your floating-point operations, then explicit typing is required, as seen

with the use of the M literal in the following:

1

In a future version of Clojure, this arbitrary precision won’t be the default, but will require explicit flagging

(with the aforementioned M for decimal numbers and N for longs). Additionally, overflow of primitive num-

bers will always signal an exception.

2

As opposed to arithmetic precision.

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Understanding precision

63

(let [imadeuapi 3.14159265358979323846264338327950288419716939937M]

(println (class imadeuapi))

imadeuapi)

; java.math.BigDecimal

;=> 3.14159265358979323846264338327950288419716939937M

(let [butieatedit 3.14159265358979323846264338327950288419716939937]

(println (class butieatedit))

butieatedit)

; java.lang.Double

;=> 3.141592653589793

As we show, the local butieatedit is truncated because the default Java double type is

insufficient. On the other hand, imadeuapi uses Clojure’s literal notation, a suffix

character M, to declare a value as requiring arbitrary decimal representation. This is

one possible way to mitigate truncation for a immensely large range of values, but as

we’ll explore in section 4.2, it’s not a guarantee of perfect precision.

4.1.2

Promotion

Clojure is able to detect when overflow occurs, and will promote the value to a numer-

ical representation that can accommodate larger values. In many cases, promotion

results in the usage of a pair of classes used to hold exceptionally large values. This pro-

motion within Clojure is automatic, as the primary focus is first correctness of numeri-

cal values, then raw speed. It’s important to remember that this promotion will occur,

as shown in the following listing, and your code should accommodate3 this certainty.

Listing 4.1 Automatic promotion in Clojure

(def clueless 9)

(class clueless)

;=> java.lang.Integer

(class (+ clueless 9000000000000000))

;=> java.lang.Long

(class (+ clueless 90000000000000000000))

;=> java.math.BigInteger

(class (+ clueless 9.0))

;=> java.lang.Double

Java itself has a bevy of contexts under which automatic type conversion will occur, so

we advise you to familiarize yourself with those (Lindholm 1999) when dealing with

Java native libraries.

3 In the example, it’s important to realize that the actual class of the value is changing, so any functions or meth-

ods reliant on specific types might not work as expected.

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CHAPTER 4 On scalars

4.1.3

Overflow

Integer and long values in Java are subject to overflow errors. When an integer calcu-

lation results in a value that’s larger than 32 bits of representation will allow, the bits of

storage will “wrap” around. When you’re operating in Clojure, overflow won’t be an

issue for most cases, thanks to promotion. But when dealing with numeric operations

on primitive types, overflow can occur. Fortunately in these instances an exception

will occur rather than propagating inaccuracies:

(+ Integer/MAX_VALUE Integer/MAX_VALUE)

;=> java.lang.ArithmeticException: integer overflow

Clojure provides a class of unchecked integer and long mathematical operations that

assume that their arguments are primitive types. These unchecked operations will

overflow if given excessively large values:

(unchecked-add (Integer/MAX_VALUE) (Integer/MAX_VALUE))

;=> -2

You should take care with unchecked operations, because there’s no way to detect

overflowing values and no reliable way to return from them. Use the unchecked func-

tions only when overflow is desired.

4.1.4

Underflow

Underflow is the inverse of overflow, where a number is so small that its value collapses

into zero. Simple examples of underflow for float and doubles can be demonstrated:

(float 0.0000000000000000000000000000000000000000000001)

;=> 0.0

1.0E-430

;=> 0.0

Underflow presents a danger similar to overflow, except that it occurs only with

floating-point numbers.

4.1.5

Rounding errors

When the representation of a floating-point value isn’t sufficient for storing its actual

value, then rounding errors will occur (Goldberg 1994). Rounding errors are an espe-

cially insidious numerical inaccuracy, as they have a habit of propagating throughout a

computation and/or build over time, leading to difficulties in debugging. There’s a

famous case involving the failure of a Patriot missile caused by a rounding error, result-

ing in the death of 28 U.S. soldiers in the first Gulf War (Skeel 1992). This occurred

due to a rounding error in the representation of a count register’s update interval. The

timer register was meant to update once every 0.1 seconds, but because the hardware

couldn’t represent 0.1 directly, an approximation was used instead. Tragically, the

approximation used was subject to rounding error. Therefore, over the course of 100

hours, the rounding accumulated into a timing error of approximately 0.34 seconds.

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Trying to be rational

65

Listing 4.2 Illustrating the Patriot missile tragedy

(let [aprox-interval (/ 209715 2097152)

Patriot’s approx 0.1

actual-interval (/ 1 10)

Clojure’s accurate 0.1

hours (* 3600 100 10)

actual-total (double (* hours actual-interval))

aprox-total (double (* hours aprox-interval))]

(- actual-total aprox-total))

;=> 0.34332275390625

In the case of the Patriot missile, the deviation of 0.34 seconds was enough to cause a

catastrophic software error, resulting in its ineffectiveness. When human lives are at

stake, the inaccuracies wrought from rounding errors are unacceptable. For the most

part, Clojure will be able to maintain arithmetic accuracies within a certain range, but

you shouldn’t take for granted that such will be the case when interacting with Java

libraries.

One way to contribute to rounding errors is to introduce doubles and floats into

an operation. In Clojure, any computation involving even a single double will result in

a value that’s a double:

(+ 0.1M 0.1M 0.1M 0.1 0.1M 0.1M 0.1M 0.1M 0.1M 0.1M)

;=> 0.9999999999999999

Can you spot the double?

This discussion was Java-centric, but Clojure’s ultimate goal is to be platform-

agnostic, and the problem of numerical consistency across platforms is a nontrivial

matter. It’s still unknown whether the preceding points will be universal across host

platforms, so please bear in mind that they should be reexamined when using Clojure

outside the context of the JVM. Now that we’ve identified the root issues when dealing

with numbers in Clojure, we’ll dive into a successful mitigation technique for dealing

with them—rationals.

4.2

Trying to be rational

Clojure provides a data type representing a rational number, and all of its core mathe-

matical functions operate with rational numbers. Clojure’s rationals allow for arbi-

trarily large numerators and denominators. We won’t go into depth about the

limitations of floating-point operations, but the problem can be summarized simply.

Given a finite representation of an infinitely large set, a determination must be made

which finite subset is represented. In the case of standard floating-point numbers as

representations of real numbers, the distribution of represented numbers is logarith-

mic (Kuki 1973) and not one-for-one. What does this mean in practice? It means that

requiring more accuracy in your floating-point operations increases the probability

that the corresponding representation won’t be available. In these circumstances,

you’ll have to settle for approximations. But Clojure’s rational number type provides a

way to retain perfect accuracy when needed.

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66

CHAPTER 4 On scalars

4.2.1

Why be rational?

Of course, Clojure provides a decimal type that’s boundless relative to your computer

memory, so why wouldn’t you just use those? In short, you can, but decimal operations

can be easily corrupted, especially when working with existing Java libraries (Kahan

1998) taking and returning primitive types. Additionally, in the case of Java, its under-

lying BigDecimal class is finite in that it uses a 32-bit integer to represent the number

of digits to the right of the decimal place. This can represent an extremely large range

of values perfectly, but it’s still subject to error:

1.0E-430000000M

;=> 1.0E-430000000M

1.0E-4300000000M

;=> java.lang.RuntimeException: java.lang.NumberFormatException

Even if you manage to ensure that your BigDecimal values are free from floating-point

corruption, you can never protect them from themselves. At some point or another, a

floating-point calculation will encounter a number such as 2/3 that will always require

rounding, leading to subtle, yet propagating errors. Finally, floating-point arithmetic

is neither associative nor distributive, which may lead to the shocking results shown in

this listing.

Listing 4.3 Floating-point arithmetic isn’t associative or distributive.

(def a 1.0e50)

(def b -1.0e50)

(def c 17.0e00)

(+ (+ a b) c)

;=> 17.0

Associativity should

(+ a (+ b c))

guarantee 17.0 also

;=> 0.0

(let [a (float 0.1)

b (float 0.2)

c (float 0.3)]

(=

(* a (+ b c))

Distributive should

(+ (* a b) (* a c))))

guarantee equality

;=> false

Therefore, for absolutely precise calculations, rationals are the best choice.4

4.2.2

How to be rational

Aside from the rational data type, Clojure provides functions that can help to main-

tain your sanity: ratio?, rational?, and rationalize. Additionally, taking apart ratio-

nals is also a trivial matter.

4 In the case of irrational numbers like Pi, all bets are off.

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Trying to be rational

67

The best way to ensure that your calculations remain as accurate as possible is to

ensure that they’re all done using rational numbers. As shown in the following listing,

the shocking results from using floating-point numbers have been eliminated.

Listing 4.4 Being rational preserves associativity and distributive natures.

(def a (rationalize 1.0e50))

(def b (rationalize -1.0e50))

(def c (rationalize 17.0e00))

(+ (+ a b) c)

;=> 17

(+ a (+ b c))

Associativity

;=> 17

preserved

(let [a (rationalize 0.1)

b (rationalize 0.2)

c (rationalize 0.3)]

(=

(* a (+ b c))

Distributive

(+ (* a b) (* a c))))

nature preserved

;=> true

To ensure that your numbers remain rational, you can use rational? to check

whether a given number is one and then use rationalize to convert it to one. There

are a few rules of thumb to remember if you want to maintain perfect accuracy in your

computations:

1

Never use Java math libraries unless they return results of BigDecimal, and even

then be suspicious.

2

Don’t rationalize values that are Java float or double primitives.

3

If you must write your own high-precision calculations, do so with rationals.

4

Only convert to a floating-point representation as a last resort.

Finally, you can extract the constituent parts of a rational using the numerator and

denominator functions:

(numerator (/ 123 10))

;=> 123

(denominator (/ 123 10))

;=> 10

You might never need perfect accuracy in your calculations. When you do, Clojure pro-

vides tools for maintaining sanity, but the responsibility to maintain rigor lies with you.

4.2.3

Caveats of rationality

Like any tool, Clojure’s rational type is a double-edged sword. The calculation of ratio-

nal math, though accurate, isn’t nearly as fast as with floats or doubles. Each operation

in rational math has an overhead cost (such as finding the least common denomina-

tor) that should be accounted for. It does you no good to use rational operations if

speed is a primary concern above accuracy.

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68

CHAPTER 4 On scalars

That covers the numerical scalars, so we’ll move on to two data types that you may

not be familiar with unless you happen to come from a background in the Lisp family

of languages: keywords and symbols.

4.3

When to use keywords

The purpose of Clojure keywords, or symbolic identifiers, can sometimes lead to confu-

sion for first-time Clojure programmers, because their analogue isn’t often found5 in

other languages. This section will attempt to alleviate the confusion and provide some

tips for how keywords are typically used.

4.3.1

How are keywords different from symbols?

Keywords always refer to themselves. What this means is that the keyword :magma

always has the value :magma, whereas the symbol ruins may refer to any legal Clojure

value or reference.

AS KEYS

Because keywords are self-evaluating and provide fast equality checks, they’re almost

always used in the context of map keys. An equally important reason to use keywords

as map keys is that they can be used as functions, taking a map as an argument, to per-

form value lookups:

(def population {:zombies 2700, :humans 9})

(:zombies population)

;=> 2700

(println (/ (:zombies population)

(:humans population))

"zombies per capita")

; 300 zombies per capita

This leads to much more concise code.

AS ENUMERATIONS

Often, Clojure code will use keywords as enumeration values, such as :small,

:medium, and :large. This provides a nice visual delineation within the source code.

AS MULTIMETHOD DISPATCH VALUES

Because keywords are used often as enumerations, they’re ideal candidates for dis-

patch values for multimethods, which we’ll explore in more detail in section 9.1.

AS DIRECTIVES

Another common use for keywords is to provide a directive to a function, multi-

method, or macro. A simple way to illustrate this is to imagine a simple function pour,

shown in listing 4.5, that takes two numbers and returns a lazy sequence of the range

of those numbers. But there’s also a mode for this function that takes a keyword :tou-

jours, which will instead return an infinite lazy range starting with the first number

and continuing “forever.”

5 Ruby has a symbol type that acts, looks, and is used similarly to Clojure keywords.

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When to use keywords

69

Listing 4.5 Using a keyword as a function directive

(defn pour [lb ub]

(cond

(= ub :toujours) (iterate inc lb)

:else (range lb ub)))

(pour 1 10)

;=> (1 2 3 4 5 6 7 8 9)

(pour 1 :toujours)

; ... runs forever

An illustrative bonus with pour is that the macro cond itself uses a directive :else to

mark the default conditional case. In this case, cond uses the fact that the keyword

:else is truthy; any keyword (or truthy value) would’ve worked just as well.

4.3.2

Qualifying your keywords

Keywords don’t belong to any specific namespace, although they may appear to if

namespace qualification is used:

::not-in-ns

;=> :user/not-in-ns

The prefix portion of the keyword marked as :user/ only looks like it’s denoting an

owning namespace; in fact, it’s a prefix gathered from the current namespace by the

Clojure reader. Observe the use of arbitrary prefixing:

(ns another)

:user/in-another

;=> :user/in-another

:haunted/name

;=> :haunted/name

In the first case, we created a namespace another and created a keyword :user/in-

another that appears to belong to the user namespace, but in fact is prefixed. In the

second example, we created a keyword :haunted/name showing that the prefix doesn’t

have to belong to a namespace at all, given that one named haunted certainly doesn’t

exist. But the fact that keywords aren’t members of any given namespace doesn’t mean

that namespace-qualifying them is pointless. Instead, it’s often more clear to do so,

especially when a namespace aggregates a specific functionality and its keywords are

meaningful in that context.

Separating the plumbing from the domain

Within a namespace named crypto, the keywords ::rsa and ::blowfish make

sense as being namespace-qualified. Likewise, should we create a namespace

aquarium, then using ::blowfish within is contextually meaningful. Likewise, when

adding metadata to structures, you should consider using qualified keywords as keys

and directives if their intention is domain-oriented. Observe the following code:

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70

CHAPTER 4 On scalars

(continued)

(defn do-blowfish [directive]

(case directive

:aquarium/blowfish (println "feed the fish")

:crypto/blowfish

(println "encode the message")

:blowfish

(println "not sure what to do")))

(ns crypto)

(user/do-blowfish :blowfish)

; not sure what to do

(user/do-blowfish ::blowfish)

; encode the message

(ns aquarium)

(user/do-blowfish :blowfish)

; not sure what to do

(user/do-blowfish ::blowfish)

; feed the fish

When switching to different namespaces using ns, you can use the namespace-qual-

ified keyword syntax to ensure that the correct domain-specific code path is executed.

Namespace qualification is especially important when you’re creating ad-hoc hierar-

chies and defining multimethods, both discussed in section 9.2.

4.4

Symbolic resolution

In the previous section, we covered the differences between symbols and keywords.

Whereas keywords were fairly straightforward, symbols abide by a slightly more com-

plicated system for lookup resolution.

Symbols in Clojure are roughly analogous to identifiers in many other languages—

words that refer to other things. In a nutshell, symbols are primarily used to provide a

name for a given value. But in Clojure, symbols can also be referred to directly, by

using the symbol or quote function or the ' special operator. Symbols tend to be dis-

crete entities from one lexical contour to another, and often even within a single con-

tour. Unlike keywords, symbols aren’t unique based solely on name alone, as you can

see in the following:

(identical? 'goat 'goat)

;=> false

The reason identical? returns false in this example is because each goat symbol is a

discrete object that only happens to share a name and therefore a symbolic represen-

tation. But that name is the basis for symbol equality:

(= 'goat 'goat)

;=> true

(name 'goat)

"goat"

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Symbolic resolution

71

The identical? function in Clojure only ever returns true when the symbols are in

fact the same object:

(let [x 'goat y x] (identical? x y))

;=> true

In the preceding example, x is also a symbol, but when evaluated in the (identical?

x x) form it returns the symbol goat, which is actually being stored on the runtime

call stack. The question arises: why not make two identically named symbols the same

object? The answer lies in metadata, which we discuss next.

4.4.1

Metadata

Clojure allows the attachment of metadata to various objects, but for now we’ll focus

on attaching metadata to symbols. The with-meta function takes an object and a map

and returns another object of the same type with the metadata attached. The reason

why equally named symbols are often not the same instance is because each can have

its own unique metadata:

(let [x (with-meta 'goat {:ornery true})

y (with-meta 'goat {:ornery false})]

[(= x y)

(identical? x y)

(meta x)

(meta y)])

;=> [true false {:ornery true} {:ornery false}]

The two locals x and y both hold an equal symbol 'goat, but they’re different

instances, each containing separate metadata maps obtained with the meta function.

The implications of this are that symbol equality isn’t dependent on metadata or iden-

tity. This equality semantic isn’t limited to symbols, but is pervasive in Clojure, as we’ll

demonstrate throughout this book. You’ll find that keywords can’t hold metadata6

because any equally named keyword is the same object.

4.4.2

Symbols and namespaces

Like keywords, symbols don’t belong to any specific namespace. Take, for example,

the following code:

(ns where-is)

(def a-symbol 'where-am-i)

a-symbol

;=> where-am-i

(resolve 'a-symbol)

;=> #'where-is/a-symbol

à-symbol

;=> where-is/a-symbol

6 Java class instances, including strings, can’t hold metadata either.

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72

CHAPTER 4 On scalars

The initial evaluation of a-symbol shows the expected value where-am-i. But attempt-

ing to resolve the symbol using resolve and using syntax-quote returns what looks

like (as printed at the REPL) a namespace-qualified symbol. This is because a symbol’s

qualification is a characteristic of evaluation and not inherent in the symbol at all.

This also applies to symbols qualified with class names. This evaluation behavior will

prove beneficial when we discuss macros in chapter 8, but for now we can summarize

the overarching idea known as Lisp-1 (Gabriel 2001).

4.4.3

Lisp-1

Clojure is what’s known as a Lisp-1, which in simple terms means it uses the same

name resolution for function and value bindings. In a Lisp-2 programming language

like Common Lisp, these name resolutions are performed differently depending on

the context of the symbol, be it in a function call position or a function argument

position. There are many arguments for and against both Lisp-1 and Lisp-2, but

against Lisp-1 one downside bears consideration. Because the same name-resolution

scheme is used for functions and their arguments, there’s a real possibility of

shadowing existing functions with other locals or Vars. Name shadowing isn’t neces-

sarily non-idiomatic if done thoughtfully, but if done accidentally it can lead to some

unexpected and obscure errors. You should take care when naming locals and defin-

ing new functions so that name-shadowing complications can be avoided.

Though name-shadowing errors tend to be rare, the benefit in a simplified mecha-

nism for calling and passing first-class functions far outweighs the negative. Clojure’s

adoption of a Lisp-1 resolution scheme makes for cleaner implementations and there-

fore highlights the solution rather than muddying the waters with the nuances of sym-

bolic lookup. For example, the best function highlights this perfectly in the way that

it takes the greater-than function > and calls it within its body as f:

(defn best [f xs]

(reduce #(if (f % %2) % %2) xs))

(best > [1 3 4 2 7 5 3])

;=> 7

A similar function body using a Lisp-2 language would require the intervention of

another function (in this case funcall) responsible for invoking the function explic-

itly. Likewise, passing any function would require the use of a qualifying tag marking it

as a function object, as seen here:

(defun best (f xs)

(reduce #'(lambda (l r)

(if (funcall f l r) l r))

xs))

(best #'> '(1 3 4 2 7 5 3))

;=> 7

This section isn’t intended to champion the cause of Lisp-1 over Lisp-2, only to high-

light the differences between the two. Many of the design decisions in Clojure provide

succinctness in implementation, and Lisp-1 is no exception. The preference for Lisp-1

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Regular expressions—the second problem

73

versus Lisp-2 typically boils down to matters of style and taste; by all practical mea-

sures, they’re equivalent.

Having covered the two symbolic scalar types, we now move into a type that you’re

(for better or worse) likely familiar with: the regular expression.

4.5

Regular expressions—the second problem

Some people, when confronted with a problem, think “I know, I’ll use regular expres-

sions.” Now they have two problems.

—Jamie Zawinski

Regular expressions are a powerful and compact way to find specific patterns in text

strings. Though we sympathize with Zawinski’s attitude and appreciate his wit, some-

times regular expressions are a useful tool to have on hand. Although the full capabil-

ities of regular expressions (or regexes) are well beyond the scope of this section

(Friedl 1997), we’ll look at some of the ways Clojure leverages Java’s regex capabilities.

Java’s regular expression engine is reasonably powerful, supporting Unicode and

features such as reluctant quantifiers and “look-around” clauses. Clojure doesn’t try to

reinvent the wheel and instead provides special syntax for literal Java regex patterns

plus a few functions to help Java’s regex capabilities fit better with the rest of Clojure.

4.5.1

Syntax

A literal regular expression in Clojure looks like this:

#"an example pattern"

This produces7 a compiled regex object that can be used either directly with Java

interop method calls or with any of the Clojure regex functions described later:

(class #"example")

;=> java.util.regex.Pattern

Though the pattern is surrounded with double quotes like string literals, the way

things are escaped within the quotes isn’t the same. This difference is easiest to see in

patterns that use backslash-delimited character classes. When compiled as a regex, a

string "\\d" will match a single digit and is identical to a literal regex without the dou-

ble backslash. Note that Clojure will even print the pattern back out using the literal

syntax:

(java.util.regex.Pattern/compile "\\d")

;=> #"\d"

In short, the only rules you need to know for embedding unusual literal characters or

predefined character classes are listed in the javadoc for Pattern.8

7

Literal regex patterns are compiled to java.util.regex.Pattern instances at read-time. This means, for example,

if you use a literal regex in a loop, it’s not recompiled each time through the loop, but just once when the

surrounding code is compiled.

8

See the online reference at http://java.sun.com/j2se/1.5.0/docs/api/java/util/regex/Pattern.html.

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CHAPTER 4 On scalars

Regular expressions accept option flags, shown in table 4.1, that can make a pat-

tern case-insensitive or enable multiline mode, and Clojure’s regex literals starting

with (?<flag>) set the mode for the rest of the pattern.

Table 4.1 Regex flags: these are the flags that can be used within Clojure regular expression patterns,

their long name, and a description of what they do. See Java's documentation for the java.util.

regex.Pattern class for more details.

Flag

Flag name

Description

d

UNIX_LINES

., ^, and $ match only the Unix line terminator '\n'.

i

CASE_INSENSITIVE

ASCII characters are matched without regard to upper or lower case.

x

COMMENTS

Whitespace and comments in the pattern are ignored.

m

MULTILINE

^ and $ match near line terminators instead of only at the beginning or

end of the entire input string.

s

DOTALL

. matches any character including the line terminator.

u

UNICODE_CASE

Causes the i flag to use Unicode case insensitivity instead of ASCII.

For example, the pattern #"(?i)yo" would match the strings “yo”, “yO”, “Yo”, and “YO”.

4.5.2

Functions

Java’s regex Pattern object has several methods that can be used directly, but only

split is used regularly to split a string into an array9 of Strings, breaking the original

where the pattern matches:

(seq (.split #"," "one,two,three"))

;=> ("one" "two" "three")

The re-seq function is Clojure’s regex workhorse. It returns a lazy seq of all matches

in a string, which means it can be used to efficiently test whether a string matches at

all or to find all matches in a string or a mapped file:

(re-seq #"\w+" "one-two/three")

;=> ("one" "two" "three")

The preceding regular expression has no capturing groups, so each match in the

returned seq is simply a string. A capturing group in the regex causes each returned

item to be a vector:

(re-seq #"\w*(\w)" "one-two/three")

;=> (["one" "e"] ["two" "o"] ["three" "e"])

9 Java arrays don’t print very pleasantly at the Clojure REPL, so we used seq in this example so you can see the

Strings inside.

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Summary

75

So where .split returns the text between regex matches, re-seq returns the matches

themselves.10 Now that we’ve looked at some nice functions you can use, we’ll talk

about one object you shouldn’t.

4.5.3

Beware of mutable matchers

Java’s regular expression engine includes a Matcher object that mutates in a non-

thread-safe way as it walks through a string finding matches. This object is exposed by

Clojure via the re-matcher function and can be used as an argument to re-groups

and the single-parameter form of re-find. We highly recommend avoiding all of

these unless you’re certain you know what you’re doing. These dangerous functions

are used internally by the implementations of some of the recommended functions

described earlier, but in each case they’re careful to disallow access to the Matcher

object they use. Use Matchers at your own risk, or better yet don’t use them directly11

at all.

4.6

Summary

Clojure’s scalar types generally work as expected, but its numerical types have a poten-

tial for frustration in certain situations. Though you may rarely encounter issues with

numerical precision, keeping in mind the circumstances under which they occur

might prove useful in the future. Given its inherent arbitrary-precision big decimal

and rational numerics, Clojure provides the tools for perfectly accurate calculations.

Keywords in Clojure serve many purposes and are ubiquitous in idiomatic code. When

dealing directly with symbols, Clojure’s nature as a Lisp-1 defines the nature of how

symbolic resolution occurs. Finally, Clojure provides regular expressions as first-class

data types, and their usage is encouraged where appropriate.

As you might’ve speculated, this chapter was nice and short due to the relative sim-

plicity of scalar types. In the following chapter, we’ll step it up a notch or 10 when cov-

ering Clojure’s composite data types. Though scalars are interesting and deeper than

expected, the next chapter will start you on your way to understanding Clojure’s true

goal: providing a sane approach to application state.

10 If you want both at the same time, you may want to look at the partition function in the clojure-contrib

library, found in the clojure.contrib.string namespace.

11 The clojure.contrib.string namespace has a bevy of functions useful for leveraging regular expres-

sions.

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Composite data types

It is better to have 100 functions

operate on one data structure than 10

functions on 10 data structures.

—Alan Perlis

This chapter covers

 Persistence, sequences, and complexity

 Vectors: creating and using them in all their varieties

 Lists: Clojure’s code form data structure

 How to use persistent queues

 Persistent sets

 Thinking in maps

 Putting it all together: finding the position of items in a

sequence

Clojure provides a rich set of composite data types and we’ll cover them all: vectors,

lists, queues, sets, and maps. In this chapter, we’ll dig into the strengths and weak-

nesses of each. We’ll spend more time on vectors and maps than on the other types,

because those two are used in a wider variety of circumstances and warrant the

extra discussion. Finally, we’ll discuss the design of a simple function to leverage

76

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Persistence, sequences, and complexity

77

many of the lessons learned in this chapter, and you’ll gain specific insight into the

preceding quote. By the way, we use the terms composite types and collections inter-

changeably, so please bear that in mind as we proceed.

Before we look at the primary collection types individually, we’ll discuss the things

they have in common. For example, you may have heard of Clojure’s sequence

abstraction —all the persistent collections use it, so we’ll examine that as well as some

algorithmic complexity concepts we’ll be referring to throughout the chapter.

5.1

Persistence, sequences, and complexity

Clojure’s composite data types have some unique properties compared to composites

in many mainstream languages. Terms such as persistent and sequence come up, and not

always in a way that makes their meaning clear. In this section we’ll define their mean-

ings carefully. We’ll also briefly examine the topic of algorithmic complexity and Big-O

notation as they apply to Clojure collections.

The term persistent is particularly problematic because it means something differ-

ent in other contexts. In the case of Clojure, we believe that a phrase immortalized by

Inigo Montoya from the novel and subsequent film The Princess Bride summarizes your

likely initial reaction...

5.1.1

“You keep using that word. I do not think

it means what you think it means.”

Although storage to disk may be the more common meaning of persistent today, Clo-

jure uses an older meaning of the word having to do with immutable in-memory col-

lections with specific properties. In particular, a persistent collection in Clojure allows

you to preserve historical versions (Okasaki 1999) of its state, and promises that all

versions will have the same update and lookup complexity guarantees. The specific

guarantees depend on the collection type, and we’ll cover those details along with

each kind of collection.

Here you can see the difference between a persistent data structure and one that’s

not by using a Java array:

(def ds (into-array [:willie :barnabas :adam]))

(seq ds)

;=> (:willie :barnabas :adam)

What we’ve done is create a three-element array of keywords and used seq to produce

an object that displays nicely in the REPL. Any change to the array ds happens in-

place, thus obliterating any historical version:

(aset ds 1 :quentin)

;=> :quentin

(seq ds)

;=> (:willie :quentin :adam)

But using one of Clojure’s persistent data structures paints a different picture:

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CHAPTER 5 Composite data types

(def ds [:willie :barnabas :adam])

ds

;=> [:willie :barnabas :adam]

(def ds1 (replace {:barnabas :quentin} ds))

ds

;=> [:willie :barnabas :adam]

ds1

;=> [:willie :quentin :adam]

The original vector ds did not change on the replacement of the keyword :barnabas

but instead created another vector with the changed value. A natural concern when

confronted with this picture of persistence is that a naive implementation would copy

the whole collection on each change, leading to slow operations and poor use of

memory. Clojure’s implementations (Bagwell 2001) are instead efficient by sharing

structural elements from one version of a persistent structure to another. This may

seem magical, but we’ll demystify it in the next chapter. For now it’s sufficient to

understand that each instance of a collection is immutable and efficient. This fact

opens numerous possibilities that wouldn’t work for standard mutable collections.

One of these is the sequence abstraction.

5.1.2

Sequence terms and what they mean

It is better to have 100 functions operate on one data abstraction than 10

functions on 10 data structures.

—Rich Hickey

The words sequential, sequence, and seq don’t sound very different from each other, but

they mean specific things in Clojure. We’ll start with specific definitions of each term

to help you tell them apart, and then go into a bit of detail about how they relate to

equality partitions and the sequence abstraction.

TERMS

A sequential collection is one that holds a series of values without reordering them. As

such it’s one of three broad categories of collection types, which we discuss in the next

subsection.

A sequence is a sequential collection that represents a series of values that may or

may not exist yet. They may be values from a concrete collection or values that are

computed as necessary. A sequence may also be empty.

Clojure has a simple API called seq for navigating collections. It consist of two func-

tions: first and rest. If the collection has anything in it, (first coll) returns the

first element; otherwise it returns nil. (rest coll) returns a sequence of the items

other than the first. If there are no other items, rest returns an empty sequence and

never nil. Functions that promise to return sequences, such as map and filter, work

the same way as rest. A seq is any object that implements the seq API, thereby support-

ing the functions first and rest. You might consider it an immutable variant of an

enumerator or iterator.

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Persistence, sequences, and complexity

79

There’s also a function called seq that accepts a wide variety of collection-like

objects. Some collections, such as lists, implement the seq API directly, so calling seq

on them returns the collection itself. More often, calling seq on a collection returns a

new seq object for navigating that collection. In either case, if the collection is empty,

seq returns nil and never an empty sequence. Functions that promise to return seqs

(not sequences), such as next, work the same way.

Clojure’s sequence library manipulates collections, strings, arrays, and so on as if

they were sequences, using the seq function and seq API.

BEWARE TYPE-BASED PREDICATES Clojure includes a few predicates with

names like the words just defined. Though they’re not frequently used, it

seems worth mentioning that they may not mean exactly what the definitions

here might suggest. For example, every object for which sequential? returns

true is a sequential collection, but it returns false for some that are also

sequential. This is because of implementation details that may be improved

sometime after Clojure 1.2.

EQUALITY PARTITIONS

Clojure classifies each composite data type into one of three logical categories or par-

titions: sequentials, maps, and sets. These divisions draw clear distinctions between

the types and help define equality semantics. Specifically, two objects will never be

equal if they belong to different partitions. Few composite types are actually sequences,

though several such as vectors are sequential.

If two sequentials have the same values in the same order, = will return true for

them, even if their concrete types are different, as shown:

(= [1 2 3] '(1 2 3))

;=> true

Conversely, even if two collections have the same values in the same order, if one is a

sequential collection and the other isn’t, = will return false, as shown here:

(= [1 2 3] #{1 2 3})

;=> false

Examples of things that are sequential include Clojure lists and vectors, and Java lists

such as java.util.ArrayList. In fact everything that implements java.util.List is

included in the sequential partition.

Generally things that fall into the other partitions include set or map in their name

and so are easy to identify.

THE SEQUENCE ABSTRACTION

Many Lisps build their data types (McCarthy 1962) on the cons-cell abstraction, an

elegant two-element structure illustrated in figure 5.1.

Clojure also has a couple of cons-cell-like structures that are covered in section 5.4,

but they’re not central to Clojure’s design. Instead, the conceptual interface fulfilled

by the cons-cell has been lifted off the concrete structure illustrated previously and

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CHAPTER 5 Composite data types

Figure 5.1 Each cons-cell is a simple pair, a car and a cdr. A. A list with two cells,

each of which has a value X and Y as the head (the car in Lisp terminology) and a list

as the tail (the cdr). This is very similar to first and rest in Clojure sequences. 

B. A cons-cell with a simple value for both the head and tail. This is called a dotted

pair but is not supported by any of Clojure's built in types.

been named sequence. All an object needs to do to be a sequence is to support the two

core functions: first and rest. This isn’t much, but it’s all that's required for the

bulk of Clojure’s powerful library of sequence functions and macros to be able to

operate on the collection: filter, map, for, doseq, take, partition, the list goes on.

At the same time, a wide variety of objects satisfy this interface. Every Clojure col-

lection provides at least one kind of seq object for walking through its contents,

exposed via the seq function. Some collections provide more than one; for example

vectors support rseq and maps support the functions keys and vals. All of these func-

tions return a seq, or if the collection is empty, nil.

You can see examples of this by looking at the types of objects returned by various

expressions. Here’s the map class:

(class (hash-map :a 1))

;=> clojure.lang.PersistentHashMap

Unsurprisingly, the hash-map function returns an object of type PersistentHashMap.

Passing that map object to seq returns an entirely new kind of object:

(seq (hash-map :a 1))

;=> ([:a 1])

(class (seq (hash-map :a 1)))

;=> clojure.lang.PersistentHashMap$NodeSeq

This class name suggests it’s a seq of nodes on a hash map. Similarly we can get a seq

of keys on the same map:

(seq (keys (hash-map :a 1)))

;=> (:a)

(class (keys (hash-map :a 1)))

;=> clojure.lang.APersistentMap$KeySeq

Note that these specific class names are an implementation detail that may change in

the future, but the concepts they embody are central to Clojure and unlikely to

change.

Having laid the foundation for a deeper dive into the sequence abstraction, we

now must quickly diverge into a simplified discussion of asymptotic complexity and

Big-O notation. If you’re already comfortable with these topics then by all means skip

forward to section 5.2. If you need a refresher or an overview, then the next section is

a minimalist introduction (Cormen 2009) to the topic.

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Persistence, sequences, and complexity

81

5.1.3

Big-O

This book isn’t heavily focused on asymptotic complexity but we do mention it a hand-

ful of times throughout, so here we’ll cover the minimum required for understanding

these few mentions. You may have gone your entire career without having to under-

stand Big-O notation, and you may likely go the remainder similarly. But that’s no rea-

son not to learn more, and a bit of understanding about Big-O and its implications

will go a long way toward helping you in choosing between Clojure collections, as well

as to design and analyze algorithms in general.

Algorithmic complexity is a system for describing the relative space and time costs

for algorithms. Typically the complexity of an algorithm is described using what’s

known as Big-O notation. For example, you may have heard that finding an element

in a linked list is O(n), which is read as “order n.” What this means is that if you have a

list (:a :b :c) of length 3, then to verify that the keyword :c is in that list requires

three comparisons. This highlights the worst case of list access because :c is at the end

of the list, but we don’t worry too much about the worst-case scenario unless that’s the

only difference between two algorithms. On the other hand, to verify that :a is in the

same list is O(1), which is read as constant time. Finding :a represents the best case for

list access because it’s at the front of the list. Rarely do your lists always look exactly

like our example, and therefore you shouldn’t build your hopes that elements will

always be at the front. Therefore, in analyzing algorithms you rarely care about the

best-case scenario because it’s too rare to matter much. What you really care about

when analyzing algorithms is the expected case, or what you’d likely see in practice.

When looking at a few million runs of verifying that some value is contained in a mil-

lion different lists, you’d inevitably see that the average number of comparisons

required approaches whatever the length of a list was, divided by two. But because

doubling the length of the list would also double the number of comparisons done in

both the expected and worst case, they’re all grouped into the same Big-O category:

O(n) also known as linear time.

Thus two algorithms that are in the same Big-O category may perform very differ-

ently, especially on small work loads. This makes the most difference when there’s a

large constant factor, work that the algorithm has to do up front regardless of the size of

the work load.

When the work load is small, an O(1) algorithm with a large constant factor may

be more costly than an O(n) algorithm that’s without extra costs. But as the work load

increases, an O(1) algorithm will always overtake the O(n) algorithm as shown in fig-

ure 5.2. Big-O doesn’t concern itself with these constant factors or small work loads.

When learning about Clojure’s persistent data structures, you’re likely to hear the

term O(log32 n) for those based on the persistent hash trie and O(log2 n) for the

sorted structures. Accessing an element in a Clojure persistent structure by index is

O(log n), or logarithmic. Logarithmic complexity describes a class of algorithms that

are effectively immune from large changes in the size of their data. In the case of Clo-

jure’s persistent structures, what this means is that there’s little difference in “hops”

(such as comparisons) between locating an element in a structure containing 100

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CHAPTER 5 Composite data types

Figure 5.2 Overtaking the smaller. In Big-O, regardless of the other ancillary costs, the higher order

of magnitude will always overtake the lower eventually.

elements or 1 million elements. In practice you may notice some difference because

for a billion objects O(log2 n) would require approximately 30 comparisons for a

lookup, whereas O(log32 n) would require only about 6. Given the smaller number of

operations required for the O(log32 n) data structures, they can be viewed as provid-

ing a nearly O(1) lookup and update.

We’ve covered the basic ideas behind persistence and the sequence abstraction,

and even touched on the basics of Big-O notation. Now we’ll discuss all of Clojure’s

primary collection types and how these concepts apply to each, starting with vectors.

5.2

Vectors: creating and using them in all their varieties

Vectors store zero or more values sequentially indexed by number, a bit like arrays,

but are immutable and persistent. They’re versatile and make efficient use of memory

and processor resources at both small and large sizes.

Vectors are probably the most frequently used collection type in Clojure code.

They’re used as literals for argument lists and let bindings, for holding large amounts

of application data, and as stacks and as map entries. We’ll also address the efficiency

considerations including growing on the right end, subvectors, and reversals, and

finally discuss where vectors aren’t an optimal solution.

5.2.1

Building vectors

The vector’s literal square-bracket syntax is one reason you might choose to use a vec-

tor over a list. For example, the let form would work perfectly well, and with a nearly

identical implementation, if it took a literal list of bindings instead of a literal vector.

But the square brackets are visually different from the round parentheses surround-

ing the let form itself as well as the likely function calls in the body of the let form,

and this is useful for humans (so we hear). Using vectors to indicate bindings for let,

with-open, fn, and such is idiomatic in Clojure and is a pattern you’re encouraged to

follow in any similar macros you write.

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Vectors: creating and using them in all their varieties

83

The most common way to create a vector is with the literal syntax described earlier.

But in many cases you’ll want to create a vector out of the contents of some other kind

of collection. For this there’s the function vec:

(vec (range 10))

;=> [0 1 2 3 4 5 6 7 8 9]

If you already have a vector but want to “pour” several values into it, then into is your

friend:

(let [my-vector [:a :b :c]]

(into my-vector (range 10)))

;=> [:a :b :c 0 1 2 3 4 5 6 7 8 9]

If you want it to return a vector, the first argument to into must be a vector. The sec-

ond arg can be any sequence, such as what range returns, or anything else that works

with seq function. You can view the operation of into as similar to a O(n) concatena-

tion based on the size of the second argument.1 Clojure also provides a vector func-

tion to build a vector from its arguments, which is handy for constructs like (map

vector a b).

PRIMITIVE VECTORS

Clojure can store primitive types inside of vectors using the vector-of function,

which takes any of :int, :long, :float, :double, :byte, :short, :boolean, or :char

as its argument and returns an empty vector. This returned vector will act just like any

other vector, except that it’ll store its contents as primitives internally. All of the nor-

mal vector operations still apply, and the new vector will attempt to coerce any addi-

tions into its internal type when being added:

(into (vector-of :int) [Math/PI 2 1.3])

;=> [3 2 1]

(into (vector-of :char) [100 101 102])

;=> [\d \e \f]

(into (vector-of :int) [1 2 623876371267813267326786327863])

; java.lang.IllegalArgumentException: Value out of range for int:

-8359803716404783817

In addition, all caveats mentioned in section 4.1 regarding overflow, underflow, and

so forth also apply to vectors of primitives.

Using vec and into, it’s easy to build vectors much larger than are conveniently

built using vector literals. But once you have a large vector like that, what are you

going to do with it?

5.2.2

Large vectors

When collections are small, the performance differences between vectors and lists

hardly matters at all. But as both get larger, each becomes dramatically slower at oper-

ations the other can still perform efficiently. Vectors are particularly efficient at three

things relative to lists: adding or removing things from the right end of the collection,

1 Vectors can’t be concatenated any more efficiently than O(n).

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CHAPTER 5 Composite data types

accessing or changing items in the interior of the collection by numeric index, and

walking in reverse order. Adding and removing from the end is done by treating the

vector as a stack—we’ll cover that later.

Any item in a vector can be accessed by its index number from 0 up to but not

including (count my-vector) in essentially constant time.2 You can do this using the

function nth; the function get, essentially treating the vector like a map; or by invoking

the vector itself as a function. Look at each of these as applied to this example vector:

(def a-to-j (vec (map char (range 65 75))))

a-to-j

;=> [\A \B \C \D \E \F \G \H \I \J]

All three of these do the same work and each returns \E:

(nth a-to-j 4)

(get a-to-j 4)

(a-to-j 4)

Which to use is a judgment call, but table 5.1 highlights some points you might con-

sider when choosing.

Table 5.1 Vector lookup options: the three ways to look up an item in a vector and how each responds

to different exceptional circumstances

nth

get

Vector as a function

If the vector is nil

Returns nil

Returns nil

Throws an exception

If the index is out 

Returns “not found” or

Returns nil

Throws an exception

of range

throws exception

Supports a 

Yes

Yes

No

“not found” arg

(nth [] 9 :whoops)

(get [] 9 :whoops)

Because vectors are indexed, they can be efficiently walked in either direction, left-to-

right or right-to-left. The seq and rseq functions return sequences that do exactly that:

(seq a-to-j)

;=> (\A \B \C \D \E \F \G \H \I \J)

(rseq a-to-j)

;=> (\J \I \H \G \F \E \D \C \B \A)

Any item in a vector can be “changed” using the assoc function. Clojure does this in

essentially constant time using structural sharing between the old and new vectors as

described at the beginning of this chapter:

(assoc a-to-j 4 "no longer E")

;=> [\A \B \C \D "no longer E" \F \G \H \I \J]

The assoc function for vectors only works on indices that already exist in the vector,

or as a special case, exactly one step past the end. In this case, the returned vector will

2 Several operations on Clojure’s persistent data structures are described in this book as “essentially constant

time.” In all cases these are O(log32 n).

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Vectors: creating and using them in all their varieties

85

be one item larger than the input vector. More frequently vectors are “grown” using

the conj function as you’ll see in the next section.

There are a few higher-powered functions provided that use assoc internally. For

example, the replace function works on both seqs and vectors, but when given a vec-

tor, it uses assoc to fix up and return a new vector:

(replace {2 :a, 4 :b} [1 2 3 2 3 4])

;=> [1 :a 3 :a 3 :b]

The functions assoc-in and update-in are for working with nested structures of vec-

tors and/or maps, like this one:3

(def matrix

[[1 2 3]

[4 5 6]

[7 8 9]])

All of assoc-in, get-in, and update-in take a series of indices to pick items from

each more deeply nested level. For a vector arranged like the earlier matrix example,

this amounts to row and column coordinates:

(get-in matrix [1 2])

;=> 6

(assoc-in matrix [1 2] 'x)

;=> [[1 2 3] [4 5 x] [7 8 9]]

The update-in function works the same way, but instead of taking a value to overwrite

an existing value, it takes a function to apply to an existing value. It’ll replace the value

at the given coordinates with the return value of the function given:

(update-in matrix [1 2] * 100)

;=> [[1 2 3] [4 5 600] [7 8 9]]

The coordinates refer to the value 6, and the function given here is * taking an argu-

ment 100, so the slot becomes the return value of (* 6 100). There’s also a function

get-in for retrieving a value in a nested vector. Before exploring its operation, we’ll

create a function neighbors in listing 5.1 that given a y-x location in an equilateral 2D

matrix, returns a sequence of the locations surrounding it.

Listing 5.1 A function for finding the neighbors of a spot on a 2D matrix

(defn neighbors

([size yx] (neighbors [[-1 0] [1 0] [0 -1] [0 1]] size yx))

([deltas size yx]

(filter (fn [new-yx]

(every? #(< -1 % size) new-yx))

(map #(map + yx %) deltas))))

3

Nested vectors are far from the most efficient way to store or process matrices, but they’re convenient to

manipulate in Clojure and so make a good example here. More efficient options include a single vector,

arrays, or a library for matrix processing such as Colt or Incanter at http://incanter.org.

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CHAPTER 5 Composite data types

The operation of neighbors is fairly straightforward. The deltas local describes that a

neighbor can be one spot away, but only along the x or y axis (not diagonal). The

function first walks through deltas and builds a vector of each added to the yx point

provided. This operation will of course generate illegal point coordinates, so those are

then removed using filter, which checks to ensure that the indices lie between -1

and the provided size. You can test this function using get-in as follows:

(map #(get-in matrix %) (neighbors 3 [0 0]))

;=> (4 2)

For each neighbor coordinate returned from neighbors, we use get-in to retrieve

the value at that point. Indeed the position [0 0] corresponding to the value 1 has

the neighboring values 4 and 2. We’ll use neighbors again before this book comes to

an end, but next we’ll look at growing and shrinking vectors—treating them like

stacks.

5.2.3

Vectors as stacks

Classic stacks have at least two operations, push and pop, and with respect to Clojure

vectors these operations are called conj and pop respectively. The conj function adds

elements to and pop removes elements from the right side of the stack. Because vec-

tors are immutable, pop returns a new vector with the rightmost item dropped—this is

different from many mutable stack APIs, which generally return the dropped item.

Consequently, peek becomes more important as the primary way to get an item from

the top of the stack:

(def my-stack [1 2 3])

(peek my-stack)

;=> 3

(pop my-stack)

;=> [1 2]

(conj my-stack 4)

;=> [1 2 3 4]

(+ (peek my-stack) (peek (pop my-stack)))

;=> 5

Each of these operations completes in essentially constant time. Most of the time, a

vector that’s used as a stack is used that way throughout its life. It’s helpful to future

readers of your code to keep this is mind and use the stack operations consistently,

even when other functions might work. For example, last on a vector returns the

same thing as peek, but besides being slower, it leads to unnecessary confusion about

how the collection is being used. If the algorithm involved calls for a stack, use conj

not assoc for growing the vector, peek not last, and pop not dissoc for shrinking it.

The functions conj, pop, and peek work on any object that implements clojure.

lang.IPersistentStack.4 Besides vectors, Clojure lists also implement this interface,

4 The conj function also works with all of Clojure’s other persistent collection types, even if they don’t imple-

ment clojure.lang.IPersistentStack.

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Vectors: creating and using them in all their varieties

87

but the functions operate on the left side of lists instead of the right side as with vec-

tors. When operating on either via the stack discipline, it’s best to ignore the ordering,

because it tends to just add confusion.

5.2.4

Using vectors instead of reverse

The ability of vectors to grow efficiently on the right side and then be walked left-to-

right produces a noteworthy emergent behavior: idiomatic Clojure code rarely uses

the reverse function. This is different from most Lisps and schemes. When process-

ing a list, it’s pretty common to want to produce a new list in the same order. But if all

you have are classic Lisp lists, often the most natural algorithm5 leaves you with a back-

ward list that needs to be reversed. Here’s an example of a function similar to Clo-

jure’s map

(defn strict-map1 [f coll]

(loop [coll coll, acc nil]

(if (empty? coll)

(reverse acc)

(recur (next coll) (cons (f (first coll)) acc)))))

(strict-map1 - (range 5))

;=> (0 -1 -2 -3 -4)

This is perfectly good, idiomatic Clojure code, except for that glaring reverse of the

final return value. After the entire list has been walked once to produce the desired

values, reverse walks it again to get them in the right order. This is both inefficient

and nonidiomatic. One way to get rid of the reverse is to use a vector instead of a list

as the accumulator:

(defn strict-map2 [f coll]

(loop [coll coll, acc []]

(if (empty? coll)

acc

(recur (next coll) (conj acc (f (first coll)))))))

(strict-map2 - (range 5))

;=> [0 -1 -2 -3 -4]

A small change, but the code is now a touch cleaner and a bit faster. It does return a

vector instead of a list, but this is rarely a problem, because any client code that wants

to treat this as a seq can usually do so automatically.6

The examples we’ve shown so far have all been plain vectors, but we’ll turn now to

the special features of some other vector types, starting with subvectors.

5

...the most natural tail-recursive algorithm anyway.

6

Another way to get rid of a reverse is to build a lazy sequence instead of a strict collection; this is how Clo-

jure’s own map function is implemented.

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CHAPTER 5 Composite data types

5.2.5

Subvectors

Although items can’t be removed efficiently from a vector (except the rightmost

item), subvectors provide a fast way to take a slice of an existing vector based on start

and end indices created using the subvec function:

(subvec a-to-j 3 6)

;=> [\D \E \F]

The first index given to subvec is inclusive (starts at index 3) but the second is exclu-

sive (ends before index 6). The new subvector internally hangs onto the entire original

a-to-j vector, making each lookup performed on the new vector cause the subvector

to do a little offset math and then look it up in the original. This makes creating a sub-

vector fast. You can use subvec on any kind of vector and it’ll work fine. But there’s

special logic for taking a subvec of a subvec, in which case the newest subvector keeps

a reference to the original vector, not the intermediate subvector. This prevents

subvectors-of-subvectors from stacking up needlessly, and keeps both the creation and

use of the sub-subvecs fast and efficient.

5.2.6

Vectors as MapEntries

Clojure’s hash map, just like hash tables or dictionaries in many other languages, has a

mechanism to iterate through the entire collection. Clojure’s solution for this iterator

is, unsurprisingly, a seq. Each item of this seq needs to include both the key and the

value, so they’re wrapped in a MapEntry. When printed, each entry looks like a vector:

(first {:width 10, :height 20, :depth 15})

;=> [:width 10]

But not only does a MapEntry look like a vector, it really is one:

(vector? (first {:width 10, :height 20, :depth 15}))

;=> true

This means you can use all the regular vector functions on it: conj, get, and so on. It

even supports destructuring, which can be handy. For example, the following locals

dimension and amount will take on the value of each key/value pair in turn:

(doseq [[dimension amount] {:width 10, :height 20, :depth 15}]

(println (str (name dimension) ":") amount "inches"))

; width: 10 inches

; height: 20 inches

; depth: 15 inches

;=> nil

A MapEntry is its own type and has two functions for retrieving its contents: key and

val, which do exactly the same thing as (nth my-map 0) and (nth my-map 1), respec-

tively. These are sometimes useful for the clarity they can bring to your code, but fre-

quently destructuring is used instead, because it’s so darned handy.

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Vectors: creating and using them in all their varieties

89

So now you know what vectors are, what specific kinds of vectors are included in Clo-

jure, and some of the things that they’re good at doing. To round out your understand-

ing of vectors, we’ll conclude with a brief look at things that vectors are bad at doing.

5.2.7

What vectors aren’t

Vectors are versatile, but there are some commonly desired patterns where they might

seem like a good solution but in fact aren’t. Though we prefer to focus on the positive,

we hope a few negative examples will help you escape from using the wrong tool for

the job.

VECTORS AREN’T SPARSE

If you have a vector of length n, the only position where you can insert a value is at

index n —appending to the far right end. You can’t skip some indices and insert at a

higher index number. If you want a collection indexed by nonsequential numbers,

consider a hash map or sorted map. Although you can replace values within a vector,

you can’t insert or delete items such that indices for the subsequent items would have

to be adjusted. Clojure doesn’t currently have a native persistent collection that sup-

ports this kind of operation, but a possible future addition, finger trees, may help for

these use cases.

VECTORS AREN’T QUEUES

Some people have tried to use vectors as queues. One approach would be to push

onto the right end of the vector using conj and then to pop items off the left using

rest or next. The problem with this is that rest and next return seqs, not vectors, so

subsequent conj operations wouldn’t behave as desired. Using into to convert the seq

back into a vector is O(n), which is less than ideal for every pop.

Another approach is to use subvec as a “pop,” leaving off the leftmost item.

Because subvec does return a vector, subsequent conj operations will push onto the

right end as desired. But as described earlier, subvec maintains a reference to the

entire underlying vector, so none of the items being popped this way will ever be gar-

bage collected. Also less than ideal.

So what would be the ideal way to do queue operations on a persistent collection?

Why, use a PersistentQueue, of course. See section 5.5 for details.

VECTORS AREN’T SETS

If you want to find out whether a vector contains a particular value, you might be

tempted to use the contains? function, but you’d be disappointed by the results. Clo-

jure’s contains? is for asking whether a particular key, not value, is in a collection,

which is rarely useful for a vector.

In this section we showed how to create vectors using literal syntax or by building

them up programmatically. We looked at how to push them, pop them, and slice

them. We also looked at some of the things vectors can’t do well. One of these was

adding and removing items from the left side; though vectors can’t do this, lists can,

which we’ll discuss next.

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CHAPTER 5 Composite data types

5.3

Lists: Clojure’s code form data structure

Clojure’s PersistentLists are by far the simplest of Clojure’s persistent collection types.

A PersistentList is a singly linked list where each node knows its distance from the end.

List elements can only be found by starting with the first element and walking each

prior node in order, and can only be added or removed from the left end.

In idiomatic Clojure code, lists are used almost exclusively to represent code

forms. They’re used literally in code to call functions, macros, and so forth as we’ll dis-

cuss shortly. Code forms are also built programmatically to then be evaled or used as

the return value for a macro. If the final usage of a collection isn’t as Clojure code,

lists rarely offer any value over vectors and are thus rarely used. But lists have rich her-

itage in Lisps so we’ll discuss when they should be used in Clojure, and also when they

shouldn’t—situations in which there are now better options.

5.3.1

Lists like Lisps like

All flavors of Lisp have lists that they like to use, and Clojure lists, already introduced

in chapter 2, are similar enough to be familiar. The functions have different names,

but what other Lisps call car is the same as first on a Clojure list. Similarly cdr is the

same as next. But there are substantial differences as well. Perhaps the most surpris-

ing is the behavior of cons. Both cons and conj add something to the front of a list,

but their arguments in a different order from each other:

(cons 1 '(2 3))

;=> (1 2 3)

(conj '(2 3) 1)

;=> (1 2 3)

In a departure from classic Lisps, the “right” way to add to the front of a list is with

conj. For each concrete type, conj will add elements in the most efficient way, and for

lists this means at the left side. Additionally, a list built using conj is homogeneous—

all the objects on its next chain are guaranteed to be lists, whereas sequences built

with cons only promise that the result will be some kind of seq. So you can use cons to

add to the front of a lazy seq, a range, or any other type of seq, but the only way to get

a bigger list is to use conj.7 Either way, the next part has to be some kind of sequence,

which points out another difference from other Lisps: Clojure has no “dotted pair.” If

you don’t know what that is, don’t worry about it. All you need to know is that if you

want a simple pair in a Clojure program, use a vector of two items.

All seqs print with rounded parentheses, but this does not mean they’re the same

type or will behave the same way. For example many of these seq types don’t know their

own size the way lists do, so calling count on them may be O(n) instead of O(1).8 An

7

Or to conj or cons onto nil. This is a special case, because nil isn’t the same as an empty collection of any

specific type. Clojure could have just left this unsupported, perhaps throwing an exception if you did (cons 1

nil), but instead it provides a reasonable default behavior: building a list one item long.

8

You can test for this property of being countable in constant time using the counted? function. For example

(counted? (range 10)) returns true in Clojure 1.0, but false in 1.1 because the implementation of

range changed between those versions and no longer provided O(1) counting.

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How to use persistent queues

91

unsurprising difference between lists in Clojure versus other Lisps is that they’re

immutable. At least that had better not be surprising anymore. Changing values within

a list is generally discouraged in other Lisps anyway, but in Clojure it’s impossible.

5.3.2

Lists as stacks

Lists in all Lisps can be used as stacks, but Clojure goes further by supporting the

IPersistentStack interface. This means you can use the functions peek and pop to do

roughly the same thing as first and next. Two details are worth noting. One is that

next and rest are legal on an empty list, but pop throws an exception. The other is that

next on a one-item list returns nil, whereas rest and pop both return an empty list.

When you want a stack, the choice between using a list versus a vector is a some-

what subtle decision. Their memory organization is quite different, so it may be worth

testing your usage to see which performs better. Also, the order of values returned by

seq on a list is backward compared to seq on a vector, and in rare cases this can point

to one or the other as the best solution. In the end, it may come down primarily to

personal taste.

5.3.3

What lists aren’t

Probably the most common misuse of lists is to hold items that will be looked up by

index. Though you can use nth to get the 42nd (or any other) item from a list, Clo-

jure will have to walk the list from the beginning to find it. Don’t do that. In fact, this

is a practical reason why lists can’t be used as functions, as in ((list :a) 0). Vectors

are good at looking things up by index, so use one of those instead.

Lists are also not sets. All the reasons we gave in the previous section for why it’s a

bad idea to frequently search a vector looking for a particular value apply to lists as

well. Even moreso since contains? will always return false for a list. See the section on

sets later in this chapter instead.

Finally, lists aren’t queues. You can add items to one end of a list, but you can’t

remove things from the other end. So what should you use when you need a queue?

Funny you should ask...

5.4

How to use persistent queues

We mentioned in section 5.2 that new Clojure developers often attempt to implement

simple queues using vectors. Though this is possible, such an implementation leaves

much to be desired. Instead, Clojure provides a persistent immutable queue that will

serve all your queueing needs. In this section we’ll touch on the usage of the

PersistentQueue class, where its first-in-first-out (FIFO) queueing discipline (Knuth

1997) is described by conj adding to the rear, pop removing from the front, and peek

returning the front element without removal.

Before going further, it’s important to point out that Clojure’s PersistentQueue is

a collection, not a workflow mechanism. Java has classes deriving from the

java.util.concurrent.BlockingQueue interface for workflow, which often are use-

ful in Clojure programs, and those aren’t these. If you find yourself wanting to

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CHAPTER 5 Composite data types

repeatedly check a work queue to see if there’s an item of work to be popped off, or if

you want to use a queue to send a task to another thread, you do not want the

PersistentQueue discussed in this section.

5.4.1

A queue about nothing

Search all you like, but the current implementation of Clojure doesn’t provide9 a core

construction function for creating persistent queues. That being the case, how would

you go about creating a queue? The answer is that there’s a readily available empty

queue instance to use, clojure.lang.PersistentQueue/EMPTY. The printed represen-

tation for Clojure’s queues isn’t incredibly informative, but you can change that by

providing a method for them on the print-method multimethod, as shown:

(defmethod print-method clojure.lang.PersistentQueue

[q, w]

(print-method '<- w) (print-method (seq q) w) (print-method '-< w))

clojure.lang.PersistentQueue/EMPTY

;=> <-nil-<

Using print-method in this way is a convenient mechanism for printing types in logi-

cal ways, as we did earlier with the queue-fish that’s not only fun, but indicates an

direction of flow for conj and pop.

You might think that popping an empty queue would raise an exception, but the

fact is that this action results in just another empty queue. Likewise, peeking an empty

queue will return nil. Not breathtaking for sure, but this behavior helps to ensure

that queues work in place of other sequences. In fact, the functions first, rest, and

next also work on queues and give the results that you might expect, though rest and

next return seqs not queues. Therefore, if you’re using a queue as a queue, it’s best to

use the functions designed for this purpose: peek, pop, and conj.

5.4.2

Putting things on

The mechanism for adding elements to a queue is conj:

(def schedule

(conj clojure.lang.PersistentQueue/EMPTY

:wake-up :shower :brush-teeth))

;=> <-(:wake-up :shower :brush-teeth)-<

Clojure’s persistent queue is implemented internally using two separate collections,

the front being a seq and the rear being a vector, as shown in figure 5.3.

All insertions occur in the rear vector and all removals occur in the front seq, tak-

ing advantage of each collection’s strength. When all the items from the front list have

9

The Clojure core language grows carefully, tending to incorporate only features that have proven useful.

Queues currently stand at the edge of this growth, meaning that there might be more support for them in the

future. Unlike the other collections in this chapter, the code you write with queues might be rendered non-

idiomatic by future improvements.

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How to use persistent queues

93

Figure 5.3 The two

collections used internally

in a single queue. peek

returns the front item of

the seq, pop returns a new

queue with the front of the

seq left off, and conj adds

a new item to the back of

the vector.

been popped, the back vector is wrapped in a seq to become the new front, and an

empty vector is used as the new back. Typically, an immutable queue such as this is

implemented with the rear as a list in reverse order, because insertion to the front of a

list is an efficient operation. But using a Clojure vector eliminates the need for a

reversed list.

5.4.3

Getting things

Clojure provides the peek function to get the front element in a queue:

(peek schedule)

;=> :wake-up

The fact that performing peek doesn’t modify the contents of a persistent queue

should be no surprise by now.

5.4.4

Taking things off

To “remove” elements from the front of a queue, use the pop function and not rest:

(pop schedule)

;=> <-(:shower :brush-teeth)-<

(rest schedule)

;=> (:shower :brush-teeth)

Although rest returns something with the same values and even prints the same as

what pop returns, the former is a seq not a queue. This is potentially the source of sub-

tle bugs, because subsequent attempts to use conj on it won’t preserve the speed guar-

antees of the queue type and the queue functions pop peek and conj won’t behave as

expected.

We’ve talked numerous times in this chapter about the sequence abstraction, and

though it’s an important consideration, it shouldn’t always be used. Instead, it’s

important to know your data structures, their sweet spots, and idiomatic operations.

By doing so, you can write code that’s specialized in ways that leverage the perfor-

mance characteristics you need for a given problem space. Clojure’s persistent queues

illustrate this fact perfectly. To further highlight this point, we’ll now explore Clojure’s

set type.

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CHAPTER 5 Composite data types

5.5

Persistent sets

Clojure sets work the same as mathematical sets, in that they’re collections of

unsorted unique elements. In this section we’ll cover sets by explaining their strong

points, weaknesses, and idioms. We’ll also cover some of the functions from the

clojure.set namespace.

5.5.1

Basic properties of Clojure sets

Sets are functions of their elements that

return the matched element or nil:

Finding items in a sequence 

using a set and some

(#{:a :b :c :d} :c)

This property of sets combines

;=> :c

with the some function to provide

(#{:a :b :c :d} :e)

an extremely useful idiom for

;=> nil

searching a seq for any of multiple

items. The

Set elements can be accessed via the

some function takes a

get func-

predicate and a sequence. It

tion, which will return the queried value if it

applies said predicate to each ele-

exists in the given set:

ment in turn, returning the first

truthy value returned by the predi-

(get #{:a 1 :b 2} :b)

cate or else nil:

;=> :b

(some #{:b} [:a 1 :b 2])

(get #{:a 1 :b 2} :nothing-doing)

;=> :b

;=> nil

(some #{1 :b} [:a 1 :b 2])

As a final point, sets, like all of Clojure’s col-

;=> 1

lections, support heterogeneous values.

Using a set as the predicate sup-

HOW CLOJURE POPULATES SETS

plied to some allows you to check

The key to understanding how Clojure sets

whether any of the truthy values in

the set are contained within the

determine which elements are discrete lies in

given sequence. This is a fre-

one simple statement. Given two elements

quently used Clojure idiom for

evaluating as equal, a set will contain only

searching for containment within a

one, independent of concrete types:

sequence.

#{[] ()}

;=> #{[]}

#{[1 2] (1 2)}

;=> #{[1 2]}

#{[] () #{} {}}

;=> #{#{} {} []}

From the first two examples, even though [] and () are of differing types, they’re con-

sidered equal because their elements are equal or in this case empty. But the last

example illustrates nicely that collections within an equality partition will always be

equal if their elements are equal, but never across partitions.

5.5.2

Keeping your sets in order with sorted-set

There’s not much to say about creating sorted sets with the sorted-set function. But

there’s a simple rule that you should bear in mind:

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Persistent sets

95

(sorted-set :b :c :a)

;=> #{:a :b :c}

(sorted-set [3 4] [1 2])

;=> #{[1 2] [3 4]}

(sorted-set :b 2 :c :a 3 1)

; java.lang.ClassCastException: clojure.lang.Keyword cannot be cast to

java.lang.Number

As long as the arguments to the sorted-set function are mutually comparable, you’ll

receive a sorted set; otherwise an exception is thrown. This can manifest itself when

dealing with sorted sets down stream from their point of creation, leading to potential

confusion:

(def my-set (sorted-set :a :b))

;; ... some time later

(conj my-set "a")

;=> java.lang.ClassCastException: clojure.lang.Keyword cannot be cast to

java.lang.String

The difficulty in finding the reason for this exception will increase as the distance

between the creation of my-set and the call to conj increases. You can adjust this rule

a bit by using sorted-set-by instead, and providing your own comparator. This works

exactly like the comparator for sorted-map-by, which we’ll cover in section 6.6.2.

Sorted maps and sorted sets are also similar in their support of subseq to allow effi-

ciently jumping to a particular key in the collection, and walking through it from

there. This is covered in section 5.6.

5.5.3

contains?

As we touched on in section 5.2, there’s sometimes confusion regarding the usage of

Clojure’s contains? function. Many newcomers to Clojure expect this function to

work the same as Java’s java.util.Collection#contains method; this assumption is

false, as shown:

(contains? #{1 2 4 3} 4)

;=> true

(contains? [1 2 4 3] 4)

;=> false

If you were to draw a false analogy between Java’s .contains methods and contains?,

then both of the function calls noted here should’ve returned true. The official

documentation for contains? describes it as a function that returns true if a given key

exists within a collection. When reading the word key, the notion of a map springs to

mind, but the fact that this function also works on sets hints at their implementation

details. Sets are implemented as maps with the same element as the key and value,10

but there’s an additional check for containment before insertion.

10 All implementation caveats apply.

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CHAPTER 5 Composite data types

5.5.4

clojure.set

Mathematical sets form the basis of much of modern mathematical thought, and Clo-

jure’s basic set functions in the clojure.set namespace are a clear reflection of the

classical set operations. In this subsection we’ll briefly cover each function and talk

about how, when applicable, they differ from the mathematical model. First, we’ll start

with a simple picture.

Figure 5.4 describes the nature of Clojure’s set functions, each of which will be

shown presently. Note that Clojure’s set functions take an arbitrary number of sets and

apply the operation incrementally.

INTERSECTION

Clojure’s clojure.set/intersection function works as you might expect. Given two

sets, intersection returns a set of the common elements. Given n sets, it’ll incremen-

tally return the intersection of resulting sets and the next set, as seen in the following

code:

(clojure.set/intersection #{:humans :fruit-bats :zombies}

#{:chupacabra :zombies :humans})

;=> #{:zombies :humans}

(clojure.set/intersection #{:pez :gum :dots :skor}

#{:pez :skor :pocky}

#{:pocky :gum :skor})

;=> #{:skor}

In the first example, the resulting set is simply the common elements between the

given sets. The second example is the result of the intersection of the first two sets

then intersected with the final set.

UNION

There’s also likely no surprise when using the clojure.set/union function:

(clojure.set/union #{:humans :fruit-bats :zombies}

#{:chupacabra :zombies :humans})

;=> #{:chupacabra :fruit-bats :zombies :humans}

(clojure.set/union #{:pez :gum :dots :skor}

#{:pez :skor :pocky}

#{:pocky :gum :skor})

;=> #{:pez :pocky :gum :skor :dots}

Figure 5.4 Basic set operations. The three Venn diagrams show a graphical

representation of Clojure’s set functions: intersection, union, and difference.

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Thinking in maps

97

Given two sets, the resulting set will contain all of the distinct elements from both. In

the first example this means :zombies and :humans only show up once each in the

return value. Note in the second example that more than two sets may be given to

union, but as expected each value given in any of the input sets is included exactly

once in the output set.

DIFFERENCE

The only set function that could potentially cause confusion on first glance is

clojure.set/difference, which by name implies some sort of opposition to a union

operation. Working under this false assumption you might assume that difference

would operate thusly:

(clojure.set/difference #{1 2 3 4} #{3 4 5 6})

;=> #{1 2 5 6}

But if you were to evaluate this expression in your REPL, you’d receive a very different

result:

(clojure.set/difference #{1 2 3 4} #{3 4 5 6})

;=> #{1 2}

The reason for this result is that Clojure’s difference function calculates what’s

known as a relative complement (Stewart 1995) between two sets. In other words,

difference can be viewed as a set subtraction function “removing” all elements in a

set A that are also in another set B.

5.6

Thinking in maps

It’s difficult to write a program of any significant size without the need for a map of

some sort. The use of maps is ubiquitous in writing software because frankly it’s diffi-

cult to imagine a more robust data structure. But we as programmers tend to view

maps as a special case structure outside of the normal realm of data objects and

classes. The object-oriented school of thought has relegated the map as a supporting

player in favor of the class. We’re not going to talk about the merits, or lack thereof,

for this relegation here, but in upcoming sections we’ll discuss moving away from

thinking in classes and instead thinking in the sequence abstraction, maps, protocols,

and types. Having said all of that, it need hardly be mentioned that maps should be

used to store named values. In this section we talk about the different types of maps

and the tradeoffs surrounding each.

5.6.1

Hash maps

Arguably, the most ubiquitous11 form of map found in Clojure programs is the hash

map, which provides an unsorted key/value associative structure. In addition to the

literal syntax touched on in chapter 2, hash maps can be created using the hash-map

function, which likewise takes alternating key/value pairs, with or without commas:

11 Although with the pervasiveness of the map literal, the ubiquity may instead fall to the array map.

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CHAPTER 5 Composite data types

(hash-map :a 1, :b 2, :c 3, :d 4, :e 5)

;=> {:a 1, :c 3, :b 2, :d 4, :e 5}

Clojure hash maps support heterogeneous keys, meaning that they can be of any type

and each key can be of a differing type, as this code shows:

(let [m {:a 1, 1 :b, [1 2 3] "4 5 6"}]

[(get m :a) (get m [1 2 3])])

;=> [1 "4 5 6"]

As we previously mentioned at the beginning of this chapter, many of Clojure’s com-

posite types can be used as functions, and in the case of maps they’re functions of

their keys. Using maps in this way will act the same as the use of the get function in

the previous code sample, as shown when building a vector of two elements:

(let [m {:a 1, 1 :b, [1 2 3] "4 5 6"}]

[(m :a) (m [1 2 3])])

;=> [1 "4 5 6"]

Providing a map to the seq function will return a sequence of map entries:

(seq {:a 1, :b 2})

;=> ([:a 1] [:b 2])

Of course, this sequence appears to be composed of the sets of key/value pairs con-

tained in vectors, and for all practical purposes should be treated as such. In fact, a

new hash map can be created idiomatically using this precise structure:

(into {} [[:a 1] [:b 2]])

;=> {:a 1, :b 2}

Even if your embedded pairs aren’t vectors, they can be made to be for building a new

map:

(into {} (map vec '[(:a 1) (:b 2)]))

;=> {:a 1, :b 2}

In fact, your pairs don’t have to be explicitly grouped, because you can use apply to cre-

ate a hash map given that the key/value pairs are laid out in a sequence consecutively:

(apply hash-map [:a 1 :b 2])

;=> {:a 1, :b 2}

You can also use apply in this way with sorted-map and array-map. Another idiomatic

way to build a map is to use zipmap to “zip” together two sequences, the first of which

contains the desired keys and the second their corresponding values:

(zipmap [:a :b] [1 2])

;=> {:b 2, :a 1}

The use of zipmap illustrates nicely the final property of map collections. Hash maps

in Clojure have no order guarantees. If you do require ordering, then you should use

sorted maps, discussed next.

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Thinking in maps

99

5.6.2

Keeping your keys in order with sorted maps

It’s impossible to rely on a specific ordering of the key/value pairs for a standard Clo-

jure map, because there are no order guarantees at all. Using the sorted-map and

sorted-map-by functions, you can construct maps with order assurances. By default,

the function sorted-map will build a map sorted by the comparison of its keys:

(sorted-map :thx 1138 :r2d 2)

;=> {:r2d 2, :thx 1138}

You may require an alternative key ordering, or perhaps an ordering for keys that isn’t

easily comparable. In these cases you must use sorted-map-by, which takes an addi-

tional comparison function:12

(sorted-map "bac" 2 "abc" 9)

;=> {"abc" 9, "bac" 2}

(sorted-map-by #(compare (subs %1 1) (subs %2 1)) "bac" 2 "abc" 9)

;=> {"bac" 2, "abc" 9}

This means that sorted maps don’t generally support heterogeneous keys the same as

hash maps, although it depends on the comparison function provided. For example,

the preceding one assumes all keys are strings. The default sorted-map comparison

function compare supports maps whose keys are all mutually comparable with each

other. Attempts to use keys that aren’t supported by whichever comparison function

you’re using will generally result in a cast exception:

(sorted-map :a 1, "b" 2)

;=> java.lang.ClassCastException: clojure.lang.Keyword cannot be cast to

java.lang.String

One remarkable feature supported by sorted maps (and also sorted sets) is the ability

to jump efficiently to a particular key and walk forward or backward from there

through the collection. This is done with the subseq and rsubseq functions for for-

ward and backward respectively. Even if you don’t know the exact key you want, these

functions can be used to “round up” the next closest key that exists.

Another way that sorted maps and hash maps differ is in their handling of numeric

keys. A number of a given magnitude can be represented by many different types; for

example 42 can be a long, int, float, and so on. Hash maps would treat each of these

different objects as different, whereas a sorted map would treat them as the same. You

can see the contrast in this example, where the hash map keeps both keys while the

sorted map keeps just one:

(assoc {1 :int} 1.0 :float)

;=> {1.0 :float, 1 :int}

(assoc (sorted-map 1 :int) 1.0 :float)

;=> {1 :float}

12 Note that simple boolean functions like > can be used as comparison functions.

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CHAPTER 5 Composite data types

This is because the comparison function used by the sorted map not only determines

order by equality, and if two keys compare as equal, only one will be kept. This applies

to comparison functions provided to sorted-map-by as well as the default comparator

shown previously.

Sorted maps will otherwise work just like hash maps and can be used interchange-

ably. You should use sorted maps if you need to specify or guarantee a specific key

ordering. On the other hand, if you need to maintain insertion ordering, then the use

of array maps is required as you’ll see.

5.6.3

Keeping your insertions in order with array maps

If you hope to perform an action under the assumption that a given map is insertion-

ordered, then you’re setting yourself up for disappointment. But you might already

know that Clojure provides a special map that ensures insertion ordering called an

array map:

(seq (hash-map :a 1, :b 2, :c 3))

;=> ([:a 1] [:c 3] [:b 2])

(seq (array-map :a 1, :b 2, :c 3))

;=> ([:a 1] [:b 2] [:c 3])

So when insertion order is important, you should explicitly use an array map. Array

maps can be populated quickly by ignoring the form of the key/value pairs and blindly

copying them into place. For structures sized below a certain count, the cost associated

with map lookup bridges the gap between a linear search through an equally sized

array or list. That’s not to say that the map will be slower; instead, it allows the map and

linear implementations to be comparable. Sometimes your best choice for a map is not

a map at all, and like most things in life there are tradeoffs. Thankfully, Clojure takes

care of these considerations for you by adjusting the concrete implementations behind

the scenes as the size of the map increases. The precise types in play aren’t important,

because Clojure is careful to document its promises and to leave undefined aspects

subject to change and/or improvement. It’s usually a bad idea to build your programs

around concrete types, and always bad to build around undocumented behaviors. Clo-

jure handles the underlying efficiency considerations so you don’t have to. But be aware that if

ordering is important, you should avoid operations that inadvertently change the

underlying map implementation from an array map.

We’ve covered the basics of Clojure maps in this section, including common usage

and construction techniques. Clojure maps, minus some implementation details,

shouldn’t be surprising to anyone. It’ll take a while to grow accustomed to dealing

with immutable maps, but in time even this nuance will become second nature.

Now that we’ve looked at Clojure’s primary collection types and their differences

in detail, we’ll take some time to work through a simple case study. This case study,

creating a function named pos, will illustrate the thought processes you might con-

sider on your way toward designing an API built on the principles of the sequence

abstraction.

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Putting it all together: finding the position of items in a sequence

101

5.7

Putting it all together: finding the position of items 

in a sequence

We sometimes underestimate the influence of little things.

—Charles W. Chesnutt

The case study for this chapter will be to design and implement a simple function to

locate the positional index13 of an element within a sequence. We’re going to pool

together much of the knowledge that you’ve gained in this chapter in order to illus-

trate the steps you might take in designing, writing, and ultimately optimizing a Clo-

jure collection function. Of course, we’re going to work against the sequence

abstraction and will therefore design the solution accordingly.

The function, named pos, must

 Work on any composite type returning indices corresponding to some value

 Return a numerical index for sequential collections or associated key for maps

and sets

 Otherwise return nil

5.7.1

Implementation

If we were to address each of the requirements for pos literally and directly, we might

come up with a function that looks like the following listing.

Listing 5.2 First cut at our position function

(defn pos [e coll]

(let [cmp (if (map? coll)

#(= (second %1) %2)

Map compare

#(= %1 %2))]

Default compare

(loop [s coll idx 0]

Start at beginning

(when (seq s)

(if (cmp (first s) e)

Compare

(if (map? coll)

(first (first s))

Map returns key

idx)

...Else index

(recur (next s) (inc idx)))))))

(pos 3 [:a 1 :b 2 :c 3 :d 4])

;=> 5

(pos :foo [:a 1 :b 2 :c 3 :d 4])

;=> nil

(pos 3 {:a 1 :b 2 :c 3 :d 4})

;=> :c

(pos 3 '(:a 1 :b 2 :c 3 :d 4))

;=> 5

(pos \3 ":a 1 :b 2 :c 3 :d 4")

;=> 13

13 Stuart Halloway describes a similar function index-of-any in his book Programming Clojure that views the

problem largely through the lens of reduced complexity. We like his example and this one because it’s simple

yet powerful and nicely illustrative of the way that Clojure functions should be written.

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CHAPTER 5 Composite data types

Pretty hideous right? We think so too. Apart from being overly complicated, it’d likely

be more useful if we instead returned a sequence of all the indices matching the item,

so we’ll add that to the requirements. But we’ve built a heavy load with the first cut at

pos and should probably step back a moment to reflect. First of all, it’s probably the

wrong approach to handle map types and other sequence types differently. The use of

the predicate map? to detect the type of the passed collection is incredibly constrain-

ing, in that it forces different collections to be processed differently. That’s not to say

that the use of type-based predicates is strictly prohibited, only that you should try to

favor more generic algorithms or at least to minimize their usage.

As chance has it, the exact nature of the problem demands that we view collections

as a set of values paired with a given index, be it explicit in the case of maps or implicit

in the case of other sequences’ positional information. Therefore, imagine how easy

this problem would be if all collections were laid out as a sequence of pairs ([index1

value1] [index2 value2] ... [indexn valuen]). Well, there’s no reason why they

couldn’t, as shown next.

Listing 5.3 An index function

(defn index [coll]

(cond

(map? coll) (seq coll)

(set? coll) (map vector coll coll)

:else (map vector (iterate inc 0) coll)))

This simple function14 can generate a uniform representation for indexed collections:

(index [:a 1 :b 2 :c 3 :d 4])

;=> ([0 :a] [1 1] [2 :b] [3 2] [4 :c] [5 3] [6 :d] [7 4])

(index {:a 1 :b 2 :c 3 :d 4})

;=> ([:a 1] [:b 2] [:c 3] [:d 4])

(index #{:a 1 :b 2 :c 3 :d 4})

;=> ([1 1] [2 2] [3 3] [4 4] [:a :a] [:c :c] [:b :b] [:d :d])

As shown, we’re still using type-based predicates, but we’ve raised the level of abstrac-

tion to the equality partitions in order to build contextually relevant indices. Now, the

function for finding the positional indices for the desired value is trivial:

(defn pos [e coll]

(for [[i v] (index coll) :when (= e v)] i))

(pos 3 [:a 1 :b 2 :c 3 :d 4])

;=> (5)

(pos 3 {:a 1, :b 2, :c 3, :d 4})

;=> (:c)

(pos 3 [:a 3 :b 3 :c 3 :d 4])

;=> (1 3 5)

(pos 3 {:a 3, :b 3, :c 3, :d 4})

;=> (:a :c :b)

14 Clojure has a core function keep-indexed that works similarly but doesn’t implicitly build indices along

equality partitions. For a vector, you could build the index as (keep-indexed #(-> [% %2]) [:a :b :c :d]).

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Summary

103

Much better! But there’s one more deficiency with the pos function from a Clojure

perspective. Typically in Clojure it’s more useful to pass a predicate function in cases

such as these, so that instead of pos determining raw equality, it can build its result

along any dimension, as shown:

(pos #{3 4} {:a 1 :b 2 :c 3 :d 4})

;=> (:c :d)

(pos even? [2 3 6 7])

;=> (0 2)

We can modify pos only slightly to achieve the ideal level of flexibility, as shown next.

Listing 5.4 Our final version of pos

(defn pos [pred coll]

(for [[i v] (index coll) :when (pred v)] i))

We’ve vastly simplified the original solution and generated two potentially useful func-

tions (Martin 2002) in the process. By following some simple Clojure principles, we

were able to solve the original problem statement in a concise and elegant manner.

5.8

Summary

Clojure favors simplicity in the face of growing software complexity. If problems are

easily solved by collection abstractions then those abstractions should be used. Most

problems can be modeled on such simple types, yet we continue to build monolithic

class hierarchies in a fruitless race toward mirroring the “real world”—whatever that

means. Perhaps it’s time to realize that we no longer need to layer self-imposed com-

plexities on top of software solutions that are already inherently complex. Not only

does Clojure provide the sequential, set, and map types useful for pulling ourselves

from the doldrums of software complexity, but it’s also optimized for dealing with

them.

Now that we’ve discussed each of these types in detail, we’re going to take a step

back and talk about three important properties of Clojure’s collection types that until

now we’ve only touch upon lightly: immutability, persistence, and laziness.

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Part 3

Functional

programming

In this part of the book, we’ll expose some of the underpinnings of Clojure’s

approach to functional programming, as well as some practical uses of it. Clo-

jure provides mechanisms for immutability, deferred execution, closures, and

recursion. We’ll show examples of how these can work together to let you create

data structures of your own, and find routes through a weighted graph.

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Being lazy and

set in your ways

This chapter covers

 Immutability

 Designing a persistent toy

 Laziness

 Putting it all together: a lazy quicksort

We’ve now reached the apex of imperative knowledge and stand at the precipice

leading toward functional programming. We mentioned in section 2.3 that the def-

initions of functional programming are widely disparate, and unfortunately this

book won’t work to unify them. Instead, we’ll start in this chapter to build a basis

for Clojure’s style of functional programming by digging into its core supporting

maxims. In addition, this chapter covers in greater depth the parts of Clojure’s

composite types that we only touched on.

6.1

On immutability

We’ve touched on immutability throughout this book, but we’ve avoided discussing

why Clojure has chosen it as a cornerstone principle. Though no panacea, fostering

107

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108

CHAPTER 6 Being lazy and set in your ways

immutability at the language level solves many difficult problems right out of the box,

while simplifying many others. Coming from a language background where mutability

interwoven with imperative programming methods reign, it often requires a signifi-

cant conceptual leap to twist your mind to accept and utilize immutability and func-

tional programming. In this section, we’ll build a conceptual basis for immutability as

it relates to Clojure’s underlying philosophy as well as why you should work to foster

immutability even when outside the warming confines of Clojure proper.

6.1.1

Defining immutability

In many cases, when talking specifically about Clojure’s immutable data structures, we

could be talking about the broader category of immutable objects without loss of

meaning. But we should probably set down some conditions defining just what’s

meant by immutability.

EVERY DAY IS LIKE SUNDAY

An entire branch of philosophy named predestination is devoted to exploring the

notion that there’s no such thing as free will, but instead, everything that we are or

ever will be is determined beforehand. Though this possibility for our own lives may

seem bleak, the notion does nicely encapsulate the first principle of immutability: all

of the possible properties of immutable objects are defined at the time of their con-

struction and can’t be changed thereafter.

IMMUTABILITY THROUGH CONVENTION

Computer systems are in many ways open systems, providing the keys to the vault if

you’re so inclined to grab them. But in order to foster an air of immutability in your

own systems, it’s important to create a facade of immutability. Creating immutable

classes in Java requires a few steps (Goetz 2006). First, a class itself and all of its fields

should be labeled as final. Next, in no way should an object’s this reference escape

during construction. And finally, any internal mutable objects should originate, either

whole-cloth or through a copy, within the class itself and thus never escape. Obviously

we’re simplifying, because there are finer details to this recipe for Java immutability,

but for now these simplified highlights serve to show that by observing convention,

even an inherently mutable language such as Java can be made to be immutable. Clo-

jure directly supports immutability as a language feature1 with its core data structures.

By providing immutable data structures as a primary language feature, Clojure sepa-

rates (Braithwaite 2007) the complexity of working with immutable structures from

the complexities of their implementation. By providing immutability either as a core

language feature or through convention, you can reap enormous benefit.

1 We’re intentionally glossing over Clojure’s features that support mutability such as reference types and tran-

sients in order to keep this section focused.

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On immutability

109

6.1.2

Being set in your ways—immutability

Clojure’s immutable data structures aren’t bolted onto the language as an after-

thought or as a choice in an a-la-carte menu. Instead, their inclusion in the language

runs deep to its philosophical core.

INVARIANTS

Invariant-based programming involves the definition of constraints on classes and

functions in order to provide assurances that if instances enter into certain states,

assertion errors will arise. Providing invariants within a mutable system requires a fair

amount of assertion weaving within the methods of any given class. But by observing a

practice of immutability, invariants are defined solely within the construction mecha-

nism and can never be violated thereafter.

REASONING

Because the life of an immutable object is one of predestiny, the matter of reasoning

about its possible states is simplified. It follows that the act of testing such a system is

simplified, in that the set of possible states and transitions is constrained.

EQUALITY HAS MEANING

Equality in the presence of mutability has no meaning. Equality in the face of mutabil-

ity and concurrency is utter lunacy. If any two objects resolve as being equal now, then

there’s no guarantee that they will a moment from now. And if two objects aren’t

equal forever, then they’re technically never equal (Baker 1993). Providing immuta-

ble objects once again assigns meaning to equality, in that if two objects are equal now,

then they’ll always be so.

SHARING IS CHEAP

If you’re certain that an object will never change, then sharing said object becomes a

simple matter of providing a reference to it. In Java, to do so often requires a lot of

defensive copying. Along this vein, because we can freely share references for immuta-

ble objects, we can likewise intern them for free.

FLATTENING THE LEVELS OF INDIRECTION

There’s a marked difference between a mutable object and a mutable reference. The

default in Java is that there are references that might point to mutable data. But in

Clojure, there are only mutable references. This may seem like a minor detail, but it

certainly works to reduce unnecessary complexities.

IMMUTABILITY FOSTERS CONCURRENT PROGRAMMING

Immutable objects are always thread safe.

—Brian Goetz, 

Java Concurrency in Practice

If an object can’t change, it can be shared freely between different threads of execu-

tion without fear of concurrent modification errors. There can be little debate about

this particular point, but that fact doesn’t answer the question of how mutation

occurs. Without delving into the specifics, you likely already know that Clojure

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110

CHAPTER 6 Being lazy and set in your ways

isolates mutation to its reference types while the data wrapped with them is left

unchanged. We’ll leave this alone for now, becuase we’ll devote chapter 11 to this

and related topics.

6.2

Designing a persistent toy

We won’t go into terrible detail about the internals of Clojure’s persistent data struc-

tures—we’ll leave that to others (Krukow 2009). But we do want to explore the notion

of structural sharing. Our example will be highly simplified compared to Clojure’s

implementations, but it should help clarify some of the techniques used.

The simplest shared-structure type is the list. Two different items can be added to

the front of the same list, producing two new lists that share their next parts. We’ll try

this out by creating a base list and then two new lists from that same base:

(def baselist (list :barnabas :adam))

(def lst1 (cons :willie baselist))

(def lst2 (cons :phoenix baselist))

lst1

;=> (:willie :barnabas :adam)

lst2

;=> (:phoenix :barnabas :adam)

You can think of baselist as a historical version of both lst1 and lst2. But it’s also

the shared part of both lists. More than being equal, the next parts of both lists are

identical —the same instance:

(= (next lst1) (next lst2))

;=> true

(identical? (next lst1) (next lst2))

;=> true

So that’s not too complicated, right? But the features supported by lists are also lim-

ited. Clojure’s vectors and maps also provide structural sharing, while allowing you to

change values anywhere in the collection, not just on one end. The key is the struc-

ture each of these datatypes uses internally. We’ll now build a simple tree to help dem-

onstrate how a tree can allow interior changes and maintain shared structure at the

same time.

Each node of our tree will have three fields: a value, a left branch, and a right

branch. We’ll put them in a map, like this:

{:val 5, :L nil, :R nil}

That’s the simplest possible tree—a single node holding the value 5, with empty left

and right branches. This is exactly the kind of tree we want to return when a single

item is added to an empty tree. To represent an empty tree, we’ll use nil. With the

structure decision made, we can write our own conj function xconj to build up our

tree, starting with just the code for this initial case:

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Designing a persistent toy

111

(defn xconj [t v]

(cond

(nil? t) {:val v, :L nil, :R nil}))

(xconj nil 5)

;=> {:val 5, :L nil, :R nil}

Hey, it works! Not too impressive yet though, so we need to handle the case where an

item is being added to a nonempty tree. We keep our tree in order by putting values

less than a node’s :val in the left branch, and other values in the right branch. That

means we need a test like this:

(< v (:val t))

When that’s true, we need the new value v to go into the left branch, (:L t). If this

were a mutable tree, we’d change the value of :L to be the new node. Instead, we

should build a new node, copying in the parts of the old node that don’t need to

change. Something like this:

{:val (:val t),

:L (insert-new-val-here),

:R (:R t)}

This will be the new root node. Now we just need to figure out what to put for insert-

new-val-here. If the old value of :L is nil, we simply need a new single-node tree—

we even have code for that already, so we could use (xconj nil v). But what if :L isn’t

nil? In that case, we want to insert v in its proper place within whatever tree :L is

pointing to—so (:L t) instead of nil:

(defn xconj [t v]

(cond

(nil? t) {:val v, :L nil, :R nil}

(< v (:val t)) {:val (:val t),

:L (xconj (:L t) v),

:R (:R t)}))

(def tree1 (xconj nil 5))

tree1

;=> {:val 5, :L nil, :R nil}

(def tree1 (xconj tree1 3))

tree1

;=> {:val 5, :L {:val 3, :L nil, :R nil}, :R nil}

(def tree1 (xconj tree1 2))

tree1

;=> {:val 5, :L {:val 3, :L {:val 2, :L nil, :R nil}, :R nil}, :R nil}

There, it’s working. At least it seems to be—there’s a lot of noise in that output, mak-

ing it difficult to read. Here’s a function to traverse the tree in sorted order, convert-

ing it to a seq that will print more succinctly:

(defn xseq [t]

(when t

(concat (xseq (:L t)) [(:val t)] (xseq (:R t)))))

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112

CHAPTER 6 Being lazy and set in your ways

(xseq tree1)

;=> (2 3 5)

Now we just need a final condition for handling the insertion of values that are not less

than the node value:

(defn xconj [t v]

(cond

(nil? t) {:val v, :L nil, :R nil}

(< v (:val t)) {:val (:val t),

:L (xconj (:L t) v),

:R (:R t)}

:else {:val (:val t),

:L (:L t),

:R (xconj (:R t) v)}))

Now that we have the thing built, we hope you understand well enough how it’s put

together that this demonstration of the shared structure will be unsurprising:

(def tree2 (xconj tree1 7))

(xseq tree2)

;=> (2 3 5 7)

(identical? (:L tree1) (:L tree2))

;=> true

Both tree1 and tree2 share a common structure, which is more easily visualized in

figure 6.1.

This example demonstrates several features that it has in common with all of Clo-

jure’s persistent collections:

 Every “change” creates at least a new root node, plus new nodes as needed in

the path through the tree to where the new value is being inserted.

 Values and unchanged branches are never copied, but references to them are

copied from nodes in the old tree to nodes in the new one.

 This implementation is completely thread-safe in a way that’s easy to check—no

object that existed before a call to xconj is changed in any way, and newly cre-

ated nodes are in their final state before being returned. There’s no way for any

other thread, or even any other functions in the same thread, to see anything in

an inconsistent state.

tree1

tree2

L

5 R

L

R

nil

5

L

3 R

L

R

Figure 6.1 Shared structure tree: no matter

nil

nil

7 nil

how big the left side of a tree’s root node is,

something can be inserted on the right side

without copying, changing, or even examining

L

the left side. All those values will be included

nil

2 Rnil

in the new tree, along with the inserted value.

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Laziness

113

Our example fails, though, when compared to Clojure’s production-quality code:

 It’s just a binary tree.2

 It can only store numbers.

 It’ll overflow the stack if the tree gets too deep.

 It produces (via xseq) a non-lazy seq that will contain a whole copy of the tree.

 It can create unbalanced trees that’ll have bad “worst case” algorithmic 

complexity.3

Though structural sharing as described using xconj as a basis example can reduce the

memory footprint of persistent data structures, it alone is insufficient. Instead, Clojure

leans heavily on the notion of lazy sequences to further reduce its memory footprint,

as we’ll explore further in the next section.

6.3

Laziness

Through all the windows I only see infinity.

— House of Leaves 

by Mark Z. Danielewski

Clojure is partially a lazy language. This isn’t to say that Clojure vectors lie around the

house every day after school playing video games and refusing to get a job. Instead,

Clojure is lazy in the way it handles its sequence types—but what does that mean?

First, we’ll start by defining what it means for a language to be eager, or in other words,

not lazy. Many programming languages are eager in that arguments to functions are

immediately evaluated when passed, and Clojure in most cases follows this pattern as

well. Observe the following:

(- 13 (+ 2 2))

;=> 9

The expression (+ 2 2) is eagerly evaluated, in that its result 4 is passed on to the sub-

traction function during the actual call, and not at the point of need. But a lazy pro-

gramming language such as Haskell (Hudak 2000) will evaluate a function argument

only if that argument is needed in an overarching computation.

In this section we’ll discuss how laziness can be used to avoid nontermination,

unnecessary calculations, and even combinatorially exploding computations. We’ll

also discuss the matter of utilizing infinite sequences, a surprisingly powerful tech-

nique. Finally, we’ll use Clojure’s delay and force to build a simple lazy structure. First,

we’ll start with a simple example of laziness that you may be familiar with from Java.

6.3.1

Familiar laziness with logical-and

Laziness isn’t limited to the case of the evaluation of function arguments; a common

example can be found even in eager programming languages. Take the case of Java’s

2

Clojure’s sorted collections are binary trees, but its hash maps, hash sets, and vectors all have up to 32

branches per node. This results in dramatically shallower trees, and therefore faster lookups and updates.

3

Clojure’s sorted map and sorted set do use a binary tree internally, but they implement red-black trees to keep

the left and right sides nicely balanced.

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CHAPTER 6 Being lazy and set in your ways

logical-and operator &&. Java implementations optimize this particular operator to

avoid performing unnecessary operations should an early subexpression evaluate to

false. This lazy evaluation in Java allows the following idiom:

if (obj != null && obj.isWhatiz()) {

...

}

For those of you unfamiliar with Java, the preceding code says: “if the object obj isn’t

null, then call the method isWhatiz.” Without a short-circuiting (or lazy, if you will)

&& operator, the preceding operation would always throw a java.lang.NullPointer-

Exception whenever obj was set to null. Though this simple example doesn’t qualify

Java as a lazy language, it does illustrate the first advantage of lazy evaluation— laziness

allows the avoidance of errors in the evaluation of compound structures.

Clojure’s and operator also works this way, as do a number of other operators, but

we won’t discuss this type of short-circuiting laziness too deeply. Listing 6.1 illustrates

what we mean using the case of a series of nested if expressions.

Listing 6.1 Short-circuiting if expression

(defn if-chain [x y z]

(if x

(if y

(if z

(do

(println "Made it!")

:all-truthy)))))

(if-chain () 42 true)

; Made it!

;=> :all-truthy

(if-chain true true false)

;=> nil

The call to println is evaluated only in the case of three truthy arguments. But we can

perform the equivalent action given only the and macro:

(defn and-chain [x y z]

(and x y z (do (println "Made it!") :all-truthy)))

(and-chain () 42 true)

; Made it!

;=> :all-truthy

(and-chain true false true)

;=> false

You may see tricks like this from time to time, but they’re not widespread in idiomatic

Clojure code. Regardless, we’ve presented them as a launching point for the rest of

the discussion in the section. We’ll now proceed to discussing how your own Clojure

programs can be made more generally lazy by following an important recipe.

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Laziness

115

6.3.2

Understanding the lazy-seq recipe

Here’s a seemingly simple function steps that takes a sequence and makes a deeply

nested structure from it:

(steps [1 2 3 4])

;=> [1 [2 [3 [4 []]]]]

Seems simple enough, no? Your first instinct might be to tackle this problem recur-

sively, as suggested by the form of the desired result:

(defn rec-step [[x & xs]]

(if x

[x (rec-step xs)]

[]))

(rec-step [1 2 3 4])

;=> [1 [2 [3 [4 []]]]]

Things look beautiful at this point; we’ve created a simple solution to a simple prob-

lem. But therein bugbears lurk. What would happen if we ran this same function on a

large set?

(rec-step (range 200000))

;=> java.lang.StackOverflowError

Observing the example, running the same function over a sequence of 200,000 ele-

ments4 causes a stack overflow. How can we fix this problem? Perhaps it’s fine to say

that you’ll never encounter such a large input set in your own programs; such

tradeoffs are made all of the time. But Clojure provides lazy sequences to help tackle

such problems without significantly complicating your source code. Additionally, idi-

omatic Clojure code will always strive to deal with, and produce, lazy sequences.

Stepping back a bit, we should examine the lazy-seq recipe for applying laziness to

your own functions:

1

Use the lazy-seq macro at the outermost level of your lazy sequence producing

expression(s).

2

If you happen to be consuming another sequence during your operations, then

use rest instead of next.

3

Prefer higher-order functions when processing sequences.

4

Don’t hold onto your head.

These rules of thumb are simple, but they take some practice to utilize to their fullest.

For example, #4 is especially subtle in that the trivial case is easy to conceptualize, but

it’s more complex to implement in large cases. For now we’ll gloss over #3, because

we’ll talk about that approach separately in section 7.1.

So how can you leverage these rules of thumb to ensure laziness?

4 On our machines, 200,000 elements is enough to cause a stack overflow, but your machine may require more

or fewer depending on your JVM configuration.

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CHAPTER 6 Being lazy and set in your ways

rest versus next

The difference between rest and next can be seen in the following example:

(def very-lazy (-> (iterate #(do (print \.) (inc %)) 1)

rest rest rest))

;=> ..#'user/very-lazy

(def less-lazy (-> (iterate #(do (print \.) (inc %)) 1)

next next next))

;=> ...#'user/less-lazy

When building a lazy seq from another, rest doesn’t realize any more elements than

it needs to; next does. In order to determine whether a seq is empty, next needs to

check whether there’s at least one thing in it, thus potentially causing one extra real-

ization. Here’s an example:

(println (first very-lazy)) ; .4

(println (first less-lazy)) ; 4

Grabbing the first element in a lazy seq built with rest causes a realization as

expected. But the same doesn’t happen for a seq built with next because it’s already

been previously realized. Using next causes a lazy seq to be one element less lazy,

which might not be desired if the cost of realization is expensive. In general, we rec-

ommend that you use next unless you’re specifically trying to write code to be as lazy

as possible.

UTILIZING LAZY-SEQ AND REST

In order to be a proper lazy citizen, you should produce lazy sequences using the

lazy-seq macro:

(defn lz-rec-step [s]

(lazy-seq

(if (seq s)

[(first s) (lz-rec-step (rest s))]

[])))

(lz-rec-step [1 2 3 4])

;=> (1 (2 (3 (4 ()))))

(class (lz-rec-step [1 2 3 4]))

;=> clojure.lang.LazySeq

(dorun (lz-rec-step (range 200000)))

;=> nil

There are a few points of note for our new implementation. First, we’ve eliminated

destructuring on the function arguments because the & arguments within are implic-

itly destructured via the nthnext function. As we mentioned in our rules of thumb,

when consuming a sequence within the body of a lazy-seq you’ll want to use rest,

which we did in lz-rec-step. Second, we’re no longer producing nested vectors as the

output of the function, but instead a lazy sequence LazySeq, which is the by-product of

the lazy-seq macro.

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Laziness

117

With only minor adjustments, we’ve created a lazy version of the step function

while also maintaining simplicity. The first two rules of the lazy sequence recipe can

be used in all cases when producing lazy sequences. You’ll see this pattern over and

over in idiomatic Clojure code.

If what’s going on here still doesn’t quite make sense to you, consider this even

simpler example:

(defn simple-range [i limit]

(lazy-seq

(when (< i limit)

(cons i (simple-range (inc i) limit)))))

This behaves similarly to Clojure’s built-in function range, but it’s simpler in that it

doesn’t accept a step argument and has no

support for producing chunked seqs:5

lazy-seq

lazy-seq

lazy-seq

cons

cons

thunk

(simple-range 0 9)

0

1

(simple-range 2 9)

;=> (0 1 2 3 4 5 6 7 8)

Note that it follows all the lazy-seq recipe

Figure 6.2 Each step of a lazy seq may be in

one of two states. If the step is unrealized, it’ll

rules you’ve seen so far. Figure 6.2 is a repre-

contain a function or closure of no arguments

sentation of what’s in memory when the

(a thunk) that can be called later to realize

REPL has printed the first two items in a

the step. When this happens, the thunk’s

return value is cached instead, and the thunk

simple-range seq but hasn’t yet printed any

itself is released as pictured in the first two

more than that.

lazy seq boxes, transitioning the step to the

One way in which complications may

realized state. Note that although not shown

here, a realized lazy seq may simply contain

arise is by accidentally holding onto the head

nothing at all, called nil, indicating the end

of a lazy sequence. This is addressed by the

of the seq.

third rule of lazy sequences.

6.3.3

Losing your head

The primary advantage of laziness in Clojure is that it prevents the full realization of

interim results during a calculation. If you manage to hold onto the head of a

sequence somewhere within a function, then that sequence will be prevented from

being garbage collected. The simplest way to retain the head of a sequence is to bind

it to a local. This condition can occur with any type of value bind, be it to a reference

type or through the usage of let or binding:

(let [r (range 1e9)] [(first r) (last r)])

;=> [0 999999999]

(let [r (range 1e9)] [(last r) (first r)])

; java.lang.OutOfMemoryError: GC overhead limit exceeded

Clojure’s compiler can deduce that in the first example, the retention of r is no longer

needed when the computation of (last r) occurs, and therefore aggressively clears it.

5 Chunked seqs are a technique for improving performance that we cover in chapter 12.

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CHAPTER 6 Being lazy and set in your ways

But in the second example, the head is needed later in the overall computation and

can no longer be safely cleared. Of course, the compiler could perform some rearrang-

ing with the order of operations for this case, but it doesn’t because in order to do so

safely it would have to guarantee that all of the composite functions were pure. It’s

okay if you’re not clear on what a pure function is right now—we’ll cover it in section

7.1. In a nutshell, take to heart that Clojure can’t rearrange operations, because there’s

no way to guarantee that order is unimportant. This is one area where a purely func-

tional lazy language such as Haskell (Thompson 1999) really shines by comparison.

6.3.4

Employing infinite sequences

Because Clojure’s sequences are lazy, they have the potential to be infinitely long. Clo-

jure provides a number of functions for generating and working with infinite

sequences:

; Run at your own risk

(iterate (fn [n] (/ n 2)) 1)

;=> (1 1/2 1/4 1/8 ...)

It sure is a nice trick (although you might not think so had you chosen to ignore our

warning), but what could you possibly use infinite sequences for? Working with infi-

nite sequences often fosters more declarative solutions. Take a simple example as a

start. Imagine that we have a function that calculates a triangle number for a given

integer:

(defn triangle [n]

(/ (* n (+ n 1)) 2))

(triangle 10)

;=> 55

The function triangle can then be used to build a sequence of the first 10 triangle

numbers:

(map triangle (range 1 11))

;=> (1 3 6 10 15 21 28 36 45 55)

There’s nothing wrong with the preceding solution, but it suffers from a lack of flexi-

bility in that it does what it does and that’s all. By defining a sequence of all of the tri-

angle numbers as in the following listing, you can perform more interesting “queries”

in order to retrieve the desired elements.

Listing 6.2 Infinite sequences foster declarative solutions.

(def tri-nums (map triangle (iterate inc 1)))

(take 10 tri-nums)

Get first 10

;=> (1 3 6 10 15 21 28 36 45 55)

(take 10 (filter even? tri-nums))

;=> (6 10 28 36 66 78 120 136 190 210)

Get first 10 even

(nth tri-nums 99)

What Gauss found

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Laziness

119

;=> 5050

(double (reduce + (take 1000 (map / tri-nums))))

Converge on 2

;=> 1.998001998001998

(take 2 (drop-while #(< % 10000) tri-nums))

First 2 greater than 10,000

;=> (10011 10153)

;; ...

The queries used three ubiquitous Clojure functions: map, reduce, and filter. The

map function applies a function to each element in a sequence and returns the result-

ing sequence. The reduce function applies a function to each value in the sequence

and the running result to accumulate a final value. Finally, the filter function applies

a function to each element in a sequence and returns a new sequence of those ele-

ments where said function returned a truthy value. All three of these functions retain

the laziness of a given sequence.

Defining the infinite sequence of triangle numbers allows you to take elements

from it as needed, only calculating those particular items.

6.3.5

The delay and force macros

Although Clojure sequences are largely lazy, Clojure itself isn’t. In most cases, expres-

sions in Clojure are evaluated once prior to their being passed into a function rather

than at the time of need. But Clojure does provide mechanisms for implementing

what are known as call-by-need semantics. The most obvious of these mechanisms is its

macro facilities, but we’ll defer that discussion until chapter 8. The other mechanism

for providing what we’ll call explicit laziness are Clojure’s delay and force. In short,

the delay macro is used to defer the evaluation of an expression until explicitly forced

using the force function. Using these laziness primitives, we can wrap an expression

in a call to delay and use it only if necessary on the callee’s side:

(defn defer-expensive [cheap expensive]

(if-let [good-enough (force cheap)]

good-enough

(force expensive)))

(defer-expensive (delay :cheap)

(delay (do (Thread/sleep 5000) :expensive)))

;=> :cheap

(defer-expensive (delay false)

(delay (do (Thread/sleep 5000) :expensive)))

;=> :expensive

You can simulate this behavior with the use of anonymous functions, where delay is

replaced by (fn [] expr) and force by (delayed-fn), but using delay/force allows

you to explicitly check for delayed computations using delay?. Additionally, delay

caches its calculation, therefore allowing its wrapped expression to be calculated only

once. Of course, you could simulate the same behavior using memoization,6 but why

would you in this case when delay and force solve the problem more succinctly?

6 We’ll cover memoization in section 12.4.

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CHAPTER 6 Being lazy and set in your ways

if-let and when-let

The if-let and when-let macros are useful when you’d like to bind the results of

an expression based on if it returns a truthy value. This helps to avoid the need to

nest if/when and let as shown:

(if :truthy-thing

(let [res :truthy-thing] (println res)))

; :truthy-thing

(if-let [res :truthy-thing] (println res))

; :truthy-thing

The latter is much more succinct.

There are more complicated usage patterns for delay and force besides the simple

scheme outlined previously. For example, we can implement a version of the lazy

sequence of triangular numbers from a few sections prior using delay and force:

(defn inf-triangles [n]

{:head (triangle n)

:tail (delay (inf-triangles (inc n)))})

(defn head [l] (:head l))

(defn tail [l] (force (:tail l)))

The function inf-triangles creates a lazy linked-list of nodes. Each node is a map

containing a value mapped to :head and a link to the remainder of the list keyed as

:tail. The head of the list is the result of applying the function triangle to the incre-

menting counter passed recursively within the body of delay. As you can imagine, the

head of a node is always calculated as we walk down the linked-list, even if it’s never

accessed. This type of lazy structure is known as head strict but differs from Clojure's

lazy-seq, which delays both the head and tail and then realizes them at the same time.

We can now create a structure similar to the original tri-nums and start getting at

its contained elements:

(def tri-nums (inf-triangles 1))

(head tri-nums)

;=> 1

(head (tail tri-nums))

;=> 3

(head (tail (tail tri-nums)))

;=> 6

One thing to note about the preceding code is that accessing the values 3 and 6 were

deferred calculations only occurring on demand. The structure of the example is

shown in figure 6.3.

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Putting it all together: a lazy quicksort

121

tri-nums

(inf-triangles 1)

:head 1

:tail

delay

(inf-triangles 2)

:head 3

:tail

delay

(inf-triangles 3)

Figure 6.3 Lazy linked-list example. Each node of

:head 6

this linked list contains a value (the head) and a delay

:tail

delay...

(the tail). The creation of the next part is forced by a

call to tail—it doesn't exist until then.

Though we can navigate the entire chain of triangular numbers using only head and

tail, it’s probably a better idea7 to use them as primitives for more complicated

functions:

(defn taker [n l]

(loop [t n, src l, ret []]

(if (zero? t)

ret

(recur (dec t) (tail src) (conj ret (head src))))))

(defn nthr [l n]

(if (zero? n)

(head l)

(recur (tail l) (dec n))))

(taker 10 tri-nums)

;=> [1 3 6 10 15 21 28 36 45 55]

(nthr tri-nums 99)

;=> 5050

Of course, writing programs using delay and force is an onerous way to go about the

problem of laziness, and you’d be better served by using Clojure’s lazy sequences to

full effect rather than building your own from these basic blocks. But the preceding

code, in addition to being simple to understand, harkens back to chapter 5 and the

entire sequence “protocol” being built entirely on the functions first and rest.

Pretty cool, right?

6.4

Putting it all together: a lazy quicksort

In a time when the landscape of programming languages is rife with new program-

ming languages and pregnant with more, it seems inconceivable that the world would

need another quicksort implementation. Inconceivable or not, we won’t be deterred

from adding yet another to the rich ecosystem of pet problems. Our implementation

7 And as we’ll cover in section 9.3, participating in the ISeq protocol is even better.

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CHAPTER 6 Being lazy and set in your ways

of quicksort differs from many in a few key ways. First, we’ll implement a lazy, tail-

recursive version. Second, we’ll split the problem such that it can be executed incre-

mentally. Only the calculations required to obtain the part of a sequence desired will

be calculated. This will illustrate the fundamental reason for laziness in Clojure: the

avoidance of full realization of interim results.

THE IMPLEMENTATION

Without further ado, we present our quicksort implementation.8

Listing 6.3 A lazy, tail-recursive quicksort implementation

(ns joy.q)

(defn nom [n] (take n (repeatedly #(rand-int n))))

(defn sort-parts

"Lazy, tail-recursive, incremental quicksort. Works against

and creates partitions based on the pivot, defined as 'work'."

[work]

(lazy-seq

Pull 

(loop [[part & parts] work]

apart work

(if-let [[pivot & xs] (seq part)]

Define pivot

(let [smaller? #(< % pivot)]

comparison fn

(recur (list*

(filter smaller? xs)

Work all < pivot

pivot

Work pivot itself

(remove smaller? xs)

Work all > pivot

parts)))

Concat parts

(when-let [[x & parts] parts]

(cons x (sort-parts parts)))))))

Sort rest 

(defn qsort [xs]

if more parts

(sort-parts (list xs)))

The key detail in the code above is that sort-parts works not on a plain sequence of

elements but on a carefully constructed list that alternates between lazy seqs and piv-

ots. Every element before each pivot is guaranteed to be less than the pivot and every-

thing after will be greater, but the sequences between the pivots are as yet unsorted.

When qsort is given an input sequence of numbers to sort, it creates a new work list

consisting of just that input sequence and passes this work to sort-parts. The loop

inside sort-parts pulls apart the work, always assuming that the first item, which it

binds to part, is an unsorted sequence. It also assumes that if there is a second item,

which will be at the head of parts, it is a pivot. It recurs on the sequence at the head

of work, splitting out pivots and lazy seqs until the sequence of items less than the

most recent pivot is empty, in which case the if-let test is false, and that most recent

pivot is returned as the first item in the sorted seq. The rest of the built up list of work

8

This listing uses the list* function, which for some reason is somewhat rarely seen. In cases like this, how-

ever, it is exactly what is needed. list* is like list except it expects its last argument to be a list on which to

prepend its other arguments. We’ll use it again in chapter 8.

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Putting it all together: a lazy quicksort

123

held by the returned lazy sequence to be passed into

greater than pivot

sort-parts again when subsequent sorted items are

pivot

needed.

(1 (2 4 3))

You can see a snapshot of the work list for the

partition

function call (qsort [2 1 4 3]) in figure 6.4, at an

less than pivot

intermediate point in its process.

The figure includes the characteristics of a stan-

Figure 6.4 The qsort function

dard quicksort implementation, and you can run it

shown earlier would use a structure

like this for its work list when

to see that the final sequence is sorted:

sorting the vector [2 1 4 3]. Note

that all the parts described by a

(qsort [2 1 4 3])

standard quicksort implementation

;=> (1 2 3 4)

are represented here.

(qsort (nom 20))

;=> (0 2 3 5 6 7 7 8 9 10 11 11 11 12 12 13 14 16 17 19)

The implementation of the sort-parts function works to provide an incremental

solution for lazy quicksort. This incremental approach stands in opposition to a

monolithic approach (Okasaki 1996) defined by its performance of the entire calcula-

tion when any segment of the sequence is accessed. For example, grabbing the first

element in a lazy sequence returned from qsort will perform only the necessary calcu-

lations required to get that first item:

(first (qsort (nom 100)))

;=> 1

Of course, the number returned here will likely be different in your REPL, but the

underlying structure of the lazy sequence used internally by sort-parts will be similar

to that shown in figure 6.5.

The lazy qsort will be able to gather the first element because it only takes some

small subset of comparisons to gather the numbers into left-side smaller and right-side

(5 3 1 7 4 2 8 6)

A

Figure 6.5 Internal structure of

qsort. Each filter and remove

(3 1 4 2) 5 (7 8 6)

lazily returns items from its parent

sequence only as required. So to

return the first two items of the seq

B

returned by qsort, no remove steps

are required from either level A or B.

(1 2) 3 (4) 5 (6) 7 (8)

To generate the sequence (4), a

single remove step at level B would

pivot

be needed to eliminate everything less

(filter #(< % 3)

(remove #(< % 7)

(filter #(< % 5)

(remove #(< % 5)

filter

than 3. As more items are forced from

xs)

xs)

the seq returned by qsort, more of

remove

the internal filter and remove

steps will be run.

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CHAPTER 6 Being lazy and set in your ways

larger partitions and sort those smaller pieces only. The characteristic of the quicksort

algorithm is especially conducive to laziness, because it’s fairly cheap to make and

shuffle partitions where those with a smaller magnitude can be shuffled first. What

then are the benefits of a lazy, tail-recursive, incremental quicksort? The answer is that

you can take sorted portions of a large sequence without having to pay the cost of sort-

ing its entirety, as the following command hints:

(take 10 (qsort (nom 10000)))

;=> (0 0 0 4 4 7 7 8 9 9)

On our machines, this command required roughly 11,000 comparisons, which for all

intents and purposes is an O(n) operation—an order of magnitude less than quick-

sorts’s best case. Bear in mind that as the take value gets closer to the number of

actual elements, this difference in asymptotic complexity will shrink. But it’s an

extremely efficient way to determine the smallest n values in a large unsorted (Knuth

1998) sequence.

6.5

Summary

We’ve covered the topics of immutability, persistence, and laziness in this chapter. Clo-

jure’s core composite data types are all immutable and persistent by default, and

though this fact might presuppose fundamental inefficiencies, we’ve shown how Clo-

jure addresses them. The implementation of a persistent sorted binary tree demon-

strated how structural sharing eliminated the need for full copy-on-write. But

structural sharing isn’t enough to guarantee memory efficiency, and that’s where the

benefits of laziness come into the fold. The implementation of a lazy, tail-recursive

quicksort demonstrated that laziness guarantees that sequences won’t be fully realized

in memory at any given step.

In the next chapter, we’ll dive into Clojure’s notion of functional programming.

Along the way, you’ll notice that much of the shape of functional implementations in

Clojure will be influenced by the topics discussed in this chapter.

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Functional

programming

This chapter covers

 Functions in all their forms

 Closures

 Thinking recursively

 Putting it all together: A* pathfinding

At the core of functional programming is a formal system of computation known as

the lambda calculus (Pierce 2002). Clojure functions, in adherence with the lambda

calculus, are first-class—they can be both passed as arguments and returned as

results from other functions. This book isn’t about the lambda calculus. Instead

we’ll explore Clojure’s particular flavor of functional programming. We’ll cover a

vast array of useful topics, including function composition, partial evaluation,

recursion, lexical closures, pure functions, function constraints, higher-order func-

tions, and first-class functions. We’ll use that last item as our starting point.

125

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126

CHAPTER 7 Functional programming

7.1

Functions in all their forms

In chapter 5, we mentioned that most of Clojure’s composite types can be used as

functions of their elements. As a refresher, recall that vectors are functions of their

indices, so executing ([:a :b] 0) will return :a. But this can be used to greater effect

by passing the vector as a function argument:

(map [:chthon :phthor :beowulf :grendel] #{0 3})

;=> (:chthon :grendel)

In the example, we’ve used the vector as the function to map over a set of indices,

indicating its first and fourth elements by index. Clojure collections offer an interest-

ing juxtaposition, in that not only can Clojure collections act as functions, but Clojure

functions can also act as data—an idea known as first-class functions.

7.1.1

First-class functions

In a programming language such as Java, there’s no notion of a standalone function.1

Instead, every problem solvable by Java must be performed with the fundamental phi-

losophy that everything is an object. This view on writing programs is therefore rooted

in the idea that behaviors within a program must be either modeled as class instances

or attached to them—wise or not. Clojure, on the other hand, is a functional pro-

gramming language and views the problem of software development as the applica-

tion of functions to data. Likewise, functions in Clojure enjoy equal standing with

data—functions are first-class citizens. Before we start, we should define what makes

something first-class:

 It can be created on demand.

 It can be stored in a data structure.

 It can be passed as an argument to a function.

 It can be returned as the value of a function.

Those of you coming from a background in Java might find the idea of creating func-

tions on demand analogous to the practice of creating anonymous inner classes to

handle Swing events (to name only one use case). Though similar enough to start on

the way toward understanding functional programming, it’s not a concept likely to

bear fruit, so don’t draw conclusions from this analogy.

CREATING FUNCTIONS ON DEMAND USING COMPOSITION

Even a cursory glance at Clojure is enough to confirm that its primary unit of compu-

tation is the function, be it created or composed of other functions:

(def fifth (comp first rest rest rest rest))

(fifth [1 2 3 4 5])

;=> 5

1

Although the likely inclusion of closures in some future version of Java should go a long way toward invalidat-

ing this. Additionally, for those of you coming from a language such as Python, Scala, or another Lisp, the

notion of a first-class function is likely not as foreign as we make it out to be.

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The function fifth wasn’t defined with fn or defn forms shown before, but instead

built from existing parts using the comp (compose) function. But it may be more inter-

esting to take the idea one step further by instead proving a way to build arbitrary nth

functions2 as shown here:

(defn fnth [n]

(apply comp

(cons first

(take (dec n) (repeat rest)))))

((fnth 5) '[a b c d e])

;=> e

The function fnth builds a list of the function rest of the appropriate length with a

final first consed onto the front. This list is then fed into the comp function via

apply, which takes a function and a sequence of things and effectively calls said func-

tion with the list elements as its arguments. At this point, there’s no longer any doubt

that the function fnth builds new functions on the fly based on its arguments. Creat-

ing new functions in this way is a powerful technique, but it takes some practice to

think in a compositional way. It’s relatively rare to see more than one open-parenthe-

sis in a row like this in Clojure, but when you see it, it’s almost always because a func-

tion (such as fnth) is creating and returning a function that’s called immediately. A

general rule of thumb is that if you need a function that applies a number of functions

serially to the return of the former, then composition is a good fit:

(map (comp keyword #(.toLowerCase %) name) '(a B C))

;=> (:a :b :c)

Splitting functions into smaller, well-defined pieces fosters composability and, as a

result, reuse.

CREATING FUNCTIONS ON DEMAND USING PARTIAL FUNCTIONS

There may be times when instead of building a new function from chains of other

functions as comp allows, you need to build a function from the partial application of

another:

((partial + 5) 100 200)

;=> 305

The function partial builds (Tarver 2008) a new function that consists of the partial

application of the single argument 5 to the addition function. When the returned par-

tial function is passed the arguments 100 and 200, the result is their summation plus

that of the value 5 captured by partial.

PARTIAL APPLICATION ISN’T CURRYING The use of partial differs from the

notion of currying in a fundamental way. A function built with partial will

attempt to evaluate whenever it’s given another argument. A curried function

2 We know that Clojure provides an nth function that works slightly differently, but in this case please indulge

our obtuseness.

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CHAPTER 7 Functional programming

on the other hand will return another curried function until it receives a pre-

determined number of arguments—only then will it evaluate. Because Clojure

allows functions of variable number of arguments, currying makes little sense.

We’ll discuss more about utilizing partial later in this section, but as a final point

observe that ((partial + 5) 100 200) is equivalent to (#(apply + 5 %&) 100 200).

REVERSING TRUTH WITH COMPLEMENT

One final function builder discussed here is the complement function. Simply put, this

function takes a function that returns a truthy value and returns the opposite truthy

value:

(let [truthiness (fn [v] v)]

[((complement truthiness) true)

((complement truthiness) 42)

((complement truthiness) false)

((complement truthiness) nil)])

;=> [false false true true]

((complement even?) 2)

;=> false

Note that (complement even?) is equivalent to (comp not even?).

USING FUNCTIONS AS DATA

First-class functions can not only be treated as data; they are data. Because a function is

first-class, it can be stored in a container expecting a piece of data, be it a local, a refer-

ence, collections, or anything able to store a java.lang.Object. This is a significant

departure from Java, where methods are part of a class but don’t stand alone at run-

time (Forman 2004). One particularly useful method for treating functions as data is

the way that Clojure’s testing framework clojure.test stores and validates unit tests

in the metadata of a Var holding a function. These unit tests are keyed with the :test

keyword, laid out as

(defn join

{:test (fn []

(assert

(= (join "," [1 2 3]) "1,3,3")))}

[sep s]

(apply str (interpose sep s)))

We’ve modified our old friend join by attaching some metadata containing a faulty

unit test. Of course, by that we mean that the attached unit test is meant to fail in this

case. The clojure.test/run-tests function is useful for running attached unit tests

in the current namespace:

(use '[clojure.test :as t])

(t/run-tests)

; Testing user

;

; ERROR in (join) (test.clj:646)

; ...

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; actual: java.lang.AssertionError:

; Assert failed: (= (join "," [1 2 3]) "1,3,3")

; ...

As expected, the faulty unit test for join failed. Unit tests in Clojure only scratch the

surface of the boundless spectrum of examples using functions as data, but for now

they’ll do, as we move into the notion of higher-order functions.

7.1.2

Higher-order functions

A higher-order function is a function that does at least one of the following:

 Takes one or more functions as arguments

 Returns a function as a result

A Java programmer might be familiar with the practices of subscriber patterns or

schemes using more general-purpose callback objects. There are scenarios such as

these where Java treats objects like functions, but as with anything in Java, you’re really

dealing with objects containing privileged methods.

FUNCTIONS AS ARGUMENTS

In this book, we’ve used and advocated the use of the sequence functions map, reduce,

and filter—all of which expect a function argument that’s applied to the elements

of the sequence arguments. The use of functions in this way is ubiquitous in Clojure

and can make for truly elegant solutions. Let’s look at a simple example of a function

that takes a sequence of maps and a function working on each, and returns a

sequence sorted by the results of the function. The implementation in Clojure is

straightforward and clean:

(def plays [{:band "Burial", :plays 979, :loved 9}

{:band "Eno", :plays 2333, :loved 15}

{:band "Bill Evans", :plays 979, :loved 9}

{:band "Magma", :plays 2665, :loved 31}])

(def sort-by-loved-ratio (partial sort-by #(/ (:plays %) (:loved %))))

The function with the overly descriptive name sort-by-loved-ratio is built from the

partial application of the function sort-by and an anonymous function dividing the

:plays field by the :loved field. This is a simple solution to the problem presented,

and its usage is equally so:

(sort-by-loved-ratio plays)

;=> ({:band "Magma", :plays 2665, :loved 31}

{:band "Burial", :plays 979, :loved 9}

{:band "Bill Evans", :plays 979, :loved 9}

{:band "Eno", :plays 2333, :loved 15})

We intentionally used the additional higher-order function sort-by to avoid reimple-

menting core functions and instead build our program from existing parts. You should

strive for the same whenever possible.

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CHAPTER 7 Functional programming

FUNCTIONS AS RETURN VALUES

We’ve already used functions returning functions in this chapter with comp, partial,

and complement, but you could build functions that do the same. We’ll extend the ear-

lier example to provide a function that sorts rows based on some number of column

values. This is similar to the way that spreadsheets operate, in that you can sort on a

primary column while falling back on a secondary column to provide the sort order

on matching results in the primary. This behavior is typically performed along any

number of columns, cascading down from the primary column to the last; each sub-

group is sorted appropriately, as the expected result illustrates:

(sort-by (columns [:plays :loved :band]) plays)

;=> ({:band "Bill Evans", :plays 979, :loved 9}

{:band "Burial", :plays 979, :loved 9}

{:band "Eno", :plays 2333, :loved 15}

{:band "Magma", :plays 2665, :loved 31})

This kind of behavior sounds complex on the surface but is shockingly simple3 in its

Clojure implementation:

(defn columns [column-names]

(fn [row]

(vec (map row column-names))))

Running the preceding expression shows that the rows for Burial and Bill Evans

have a tertiary column sorting. The function columns returns another function

expecting a map. This return function is then supplied to sort-by to provide the

value on which the plays vector would be sorted. Perhaps you see a familiar pattern:

we apply the column-names vector as a function across a set of indices, building a

sequence of its elements at those indices. This action will return a sequence of the val-

ues of that row for the supplied column names, which is then turned into a vector so

that it can then be used as the sorting function,4 as structured here:

(vec (map (plays 0) [:plays :loved :band]))

;=> [979 9 "Burial"]

This resulting vector is then used by sort-by to provide the final ordering.

Building your programs using first-class functions in concert with higher-order

functions will reduce complexities and make your codebase more robust and exten-

sible. In the next subsection, we’ll explore pure functions, which all prior functions

in this section have been, and explain why your own applications should strive

toward purity.

3

Strictly speaking, the implementation of columns should use #(% row) instead of just row, because we can’t

always assume that the row is implemented as a map (a record might be used instead) and therefore directly

usable as a function. Records will be discussed further in chapter 8.

4

Because sort-by is higher-order, it naturally expects a function argument. As mentioned, vectors can also

be used as functions. However, as we will discuss in detail in section 10.4, all closure functions implement the

java.util.Comparator interface, which in this case is the driving force behind the sorting logic behind

sort-by!

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Prefer higher-order functions when processing sequences

We mentioned in section 6.3 that one way to ensure that lazy sequences are never

fully realized in memory is to prefer (Hutton 1999) higher-order functions for process-

ing. Most collection processing can be performed with some combination of the fol-

lowing functions:

map, reduce, filter, for, some, repeatedly, sort-by, keep

take-while, and drop-while

But higher-order functions aren’t a panacea for every solution. Therefore, we’ll cover

the topic of recursive solutions deeper in section 7.3 for those cases when higher-

order functions fail or are less than clear.

7.1.3

Pure functions

Simply put, pure functions are regular functions that, through convention, conform to

the following simple guidelines:

 The function always returns the same result, given some expected arguments.

 The function doesn’t cause any observable side-effects.

Though Clojure is designed to minimize and isolate side-effects, it’s by no means a

purely functional language. But there are a number of reasons why you’d want to

build as much of your system as possible from pure functions, and we’ll enumerate a

few presently.

REFERENTIAL TRANSPARENCY

If a function of some arguments always results in the same value and changes no other

values within the greater system, then it’s essentially a constant, or referentially trans-

parent (the reference to the function is transparent to time). Take a look at pure func-

tion keys-apply:

(defn keys-apply [f ks m]

"Takes a function, a set of keys, and a map and applies the function

to the map on the given keys. A new map of the results of the function

applied to the keyed entries is returned."

(let [only (select-keys m ks)]

(zipmap (keys only) (map f (vals only)))))

(keys-apply #(.toUpperCase %) #{:band} (plays 0))

;=> {:band "BURIAL"}

Using another pure function manip-map, we can then manipulate a set of keys based

on a given function:

(defn manip-map [f ks m]

"Takes a function, a set of keys, and a map and applies

the function to the map on the given keys. A modified

version of the original map is returned with the results

of the function applied to each keyed entry."

(conj m (keys-apply f ks m)))

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CHAPTER 7 Functional programming

(manip-map #(int (/ % 2)) #{:plays :loved} (plays 0))

;=> {:band "Burial", :plays 489, :loved 4}

The functions keys-apply and manip-map are both5 pure functions, illustrated by the

fact that you can replace them in the context of a larger program with their expected

return values and not change the outcome. Pure functions exist outside the bounds of

time. But if you make either keys-apply or manip-map reliant on anything but its

arguments or generate a side-effect within, then referential transparency dissolves.

We’ll add one more function to illustrate this:

(defn halve! [ks]

(map (partial manip-map #(int (/ % 2)) ks) plays))

(halve! [:plays])

;=> ({:band "Burial", :plays 489, :loved 9}

{:band "Eno", :plays 1166, :loved 15}

{:band "Bill Evans", :plays 489, :loved 9}

{:band "Magma", :plays 1332, :loved 31})

The function halve! works against the global plays and is no longer limited to gener-

ating results solely from its arguments. Because plays could change at any moment,

there’s no guarantee that halve! would return the same value given any particular

argument.

OPTIMIZATION

If a function is referentially transparent, then it can more easily be optimized using

techniques such as memoization (discussed in chapter 12) and algebraic manipula-

tions (Wadler 1989).

TESTABILITY

If a function is referentially transparent, then it’s easier to reason about and therefore

more straightforward to test. Building halve! as an impure function forces the need

to test against the possibility that plays could change at any time, complicating mat-

ters substantially. Imagine the confusion should you add further impure functions

based on further external transient values.

7.1.4

Named arguments

Some programming languages allow functions to take named arguments; Python is

one such language, as seen here:

def slope(p1=(0,0), p2=(1,1)):

return (float(p2[1] - p1[1])) / (p2[0] - p1[0])

slope((4,15), (3,21))

#=> -6.0

slope(p2=(2,1))

#=> 0.5

slope()

#=> 1.0

5 These functions are based on a similar implementation created by Steven Gilardi.

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133

The Python function slope calculates the slope of a line given two tuples defining

points on a line. The tuples p1 and p2 are defined as named parameters, allowing

either or both to be omitted in favor of default values, or passed in any order as a

named parameter. Clojure provides a similar feature using its destructuring mecha-

nism coupled with the optional arguments flag &. The same function would be written

using Clojure’s named arguments as in the following listing.

Listing 7.1 Named arguments in Clojure functions

(defn slope

[& {:keys [p1 p2] :or {p1 [0 0] p2 [1 1]}}]

(float (/ (- (p2 1) (p1 1))

(- (p2 0) (p1 0)))))

(slope :p1 [4 15] :p2 [3 21])

;=> -6.0

(slope :p2 [2 1])

;=> 0.5

(slope)

;=> 1.0

Clojure’s named arguments are built on the destructuring mechanism outlined in sec-

tion 3.3, allowing much richer ways to declare them.

7.1.5

Constraining functions with pre- and postconditions

Every function in Clojure can potentially be constrained on its inputs, its output, and

some arbitrary relationship between them. These constraints take the form of pre-

and postcondition vectors contained in a map defined in the function body. We can

simplify the slope function to the base case to more clearly illustrate the matter of

constraints:

(defn slope [p1 p2]

{:pre [(not= p1 p2) (vector? p1) (vector? p2)]

:post [(float? %)]}

(/ (- (p2 1) (p1 1))

(- (p2 0) (p1 0))))

The constraint map defines two entries: :pre constraining the input parameters and

:post the return value. The function calls in the constraint vectors are all expected to

return true for the constraints to pass (via logical and). In the case of the revised

slope function, the input constraints are that the points must not be equal, and they

must both be vectors. In the postcondition, the constraint is that the return result

must be a floating-point value. We run through a few scenarios in the following listing

to see how the new implementation works.

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CHAPTER 7 Functional programming

Listing 7.2 Testing the slope function constraints

(slope [10 10] [10 10])

Same points

; java.lang.AssertionError: Assert failed: (not= p1 p2)

(slope [10 1] '(1 20))

p2 is List

; java.lang.AssertionError: Assert failed: (vector? p2)

(slope [10 1] [1 20])

Float not returned

; java.lang.AssertionError: Assert failed: (float? %)

(slope [10.0 1] [1 20])

Any/all as floating point

;=> -2.111111111111111

Clojure also provides a simple assertion macro that can be used to emulate some pre-

and postconditions. Using assert instead of :pre is typically fairly straightforward.

But using assert instead of :post is cumbersome and awkward. On the contrary,

restricting yourself to constraint maps will cover most of the expected cases covered by

assert, which can be used to fill in the remaining holes (such as loop invariants). In

any case, constraint maps provide standard hooks into the assertion machinery of Clo-

jure, while using assert is by its nature ad hoc. Yet another advantage for :pre and

:post is that they allow the assertions to come from a different source than the body

of the function, which we’ll address next.

DECOUPLING ASSERTIONS FROM FUNCTIONS

The implementation of slope corresponds to a well-established mathematic property.

As a result, it makes perfect sense to tightly couple the constraints and the work to be

done to perform the calculation. But not all functions are as well-defined as slope,

and therefore could benefit from some flexibility in their constraints. Imagine a func-

tion that takes a map, puts some keys into it, and returns the new map, defined as

(defn put-things [m]

(into m {:meat "beef" :veggie "broccoli"}))

(put-things {})

;=> {:meat "beef", :veggie "broccoli"}

How would you add constraints to put-things? You could add them directly to the

function definition, but the consumers of the map might have differing requirements

for the entries added. Instead, observe how we can abstract the constraints into

another function:

(defn vegan-constraints [f m]

{:pre [(:veggie m)]

:post [(:veggie %) (nil? (:meat %))]}

(f m))

(vegan-constraints put-things {:veggie "carrot"})

; java.lang.AssertionError: Assert failed: (nil? (:meat %))

The vegan-constraints function applies specific constraints to an incoming function,

stating that the map coming in and going out should have some kind of veggie and

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135

should never have meat in the result. The beauty of this scheme is that you can create

contextual constraints based on the appropriate expected results, as shown next.

Listing 7.3 Menu constraints

(defn balanced-diet [f m]

{:post [(:meat %) (:veggie %)]}

Make sure there’s

(f m))

meat and veggie

(balanced-diet put-things {})

;=> {:veggie "broccoli", :meat "beef"}

(defn finicky [f m]

Never change

{:post [(= (:meat %) (:meat m))]}

the meat

(f m))

(finicky put-things {:meat "chicken"})

; java.lang.AssertionError: Assert failed: (= (:meat %) (:meat m))

By pulling out the assertions into a wrapper function, we’ve detached some domain-

specific requirements from a potentially globally useful function and isolated them in

aspects (Laddad 2003). By detaching pre- and postconditions from the functions them-

selves, you can mix in any implementation that you please, knowing that as long as it

fulfills the contract (Meyer 1991), its interposition is transparent. This is only the

beginning of the power of Clojure’s pre- and postconditions, and we’ll come back to it

a few times more to see how it can be extended and utilized.

Now that we’ve covered some of the powerful features available via Clojure’s func-

tions, we’ll take a step further by exploring lexical closures.

7.2

Closures

On his next walk with Qc Na, Anton attempted to impress his master by saying

“Master, I have diligently studied the matter, and now understand that objects are

truly a poor man’s closures.” Qc Na responded by hitting Anton with his stick,

saying “When will you learn? Closures are a poor man’s object.” At that moment,

Anton became enlightened.

—Part of a parable by Anton van Straaten

It took only 30 years, but closures (Sussman 1975) are now a key feature of main-

stream programming languages—Perl and Ruby support them, and JavaScript derives

much of what power it has from closures. So what’s a closure? In a sentence, a closure is

a function that has access to locals from a larger scope, namely the context in which it

was defined:

(def times-two

(let [x 2]

(fn [y] (* y x))))

The fn form defines a function and uses def to store it in a Var named times-two.

The let forms a lexical scope in which the function was defined, so the function gains

access to all the locals in that lexical context. That’s what makes this function a clo-

sure: it uses the local x that was defined outside the body of the function, and so the

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CHAPTER 7 Functional programming

local and its value become a property of the function itself. The function is said to close

over the local6 x, as in the following example:

(times-two 5)

;=> 10

This isn’t terribly interesting, but one way to make a more exciting closure is to have it

close over something mutable:

(def add-and-get

(let [ai (java.util.concurrent.atomic.AtomicInteger.)]

(fn [y] (.addAndGet ai y))))

(add-and-get 2)

;=> 2

(add-and-get 2)

;=> 4

(add-and-get 7)

;=> 11

The java.util.concurrent.atomic.AtomicInteger class simply holds an integer

value, and its .addAndGet method adds to its value, stores the result, and also returns

the result. The function add-and-get is holding onto the same instance of Atomic-

Integer, and each time it’s called, the value of that instance is modified. Unlike the

earlier times-two function, this one can’t be rewritten with the local ai defined inside

the function. If you tried, each time the function was called, it would create a new

instance with a default value of 0 to be created and stored in ai—clearly not what

should happen. A point of note about this technique is that when closing over some-

thing mutable, you run the risk of making your functions impure and thus more diffi-

cult to test and reason about, especially if the mutable local is shared.

FUNCTIONS RETURNING CLOSURES

Each of the previous examples created a single closure, but by wrapping similar code

in another function definition, you can create more closures on demand. For exam-

ple, we could take the earlier times-two example and generalize it to take an argu-

ment instead of using 2 directly:

(defn times-n [n]

(let [x n]

(fn [y] (* y x))))

We’ve covered functions returning functions before, but if you’re not already familiar

with closures, this may be a stretch. We now have an outer function stored in a Var

named times-n—note we’ve used defn instead of def. When times-n is called with an

argument, it’ll return a new closure created by the fn form and closing over the local

x. The value of x for this closure will be whatever was passed in to times-n. Thus when

we call this returned closure with an argument of its own, it’ll return the value of y

times x, as shown:

6 Locals like x in this example are sometimes called free variables. We don’t use the term because Clojure locals

are immutable.

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Closures

137

(times-n 4)

;=> #<user$times_n$fn__39 user$times_n$fn__39@427be8c2>

Viewing the function form for this closure isn’t too useful, so instead we can store it in

a Var, allowing us to call it by a friendlier name such as times-four:

(def times-four (times-n 4))

Here we’re using def again simply to store what times-n returns—a closure over the

number 4:

(times-four 10)

;=> 40

Note that when calling the closure stored in times-four, it used the local it had closed

over as well as the argument in the call.

CLOSING OVER PARAMETERS

In our definition of times-n, we created a local x using let and closed over that

instead of closing over the argument n directly. But this was only to help focus the dis-

cussion on other parts of the function. In fact, closures close over parameters of outer

functions in exactly the same way as they do over let locals. Thus times-n could be

defined without any let at all:

(defn times-n [n]

(fn [y] (* y n)))

All of the preceding examples would work exactly the same. Here’s another function

that creates and returns a closure in a similar way. Note again that the inner function

maintains access to the outer parameter even after the outer function has returned:

(defn divisible [denom]

(fn [num]

(zero? (rem num denom))))

We don’t have to store a closure in a Var, but can instead create one and call it

immediately:

((divisible 3) 6)

;=> true

((divisible 3) 7)

;=> false

Instead of storing or calling a closure, a particular need is best served by passing a clo-

sure along to another function that will use it.

PASSING CLOSURES AS FUNCTIONS

We’ve shown many examples in previous chapters of higher-order functions built in to

Clojure’s core libraries. What we’ve glossed over so far is that anywhere a function is

expected, a closure can be used instead. This has dramatic consequences for how

powerful these functions can be.

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CHAPTER 7 Functional programming

For example, filter takes a function (called a predicate in this case) and a sequence,

applies the predicate to each value of the sequence,7 and returns a sequence of the just

the values for which the predicate returned something truthy. A simple example of its

use would be to return only the even numbers from a sequence of numbers:

(filter even? (range 10))

;=> (0 2 4 6 8)

Note that filter only ever passes a single argument to the predicate given it. Without

closures, this might be restrictive, but with them we can simply close over the values

needed:

(filter (divisible 4) (range 10))

;=> (0 4 8)

It’s common to define a closure right on the spot where it’s used, closing over what-

ever local-context is needed, as shown:

(defn filter-divisible [denom s]

(filter (fn [num] (zero? (rem num denom))) s))

(filter-divisible 4 (range 10))

;=> (0 4 8)

This kind of on-the-spot anonymous function definition is desired frequently enough

that Clojure spends a little of its small syntax budget on the reader feature to make

such cases more succinct. This #() form was first introduced in chapter 2, and in this

case could be used to write the definition of filter-divisible as

(defn filter-divisible [denom s]

(filter #(zero? (rem % denom)) s))

(filter-divisible 5 (range 20))

;=> (0 5 10 15)

Though certainly more succinct than the extended anonymous function form and the

earlier example using a separate divisible function with filter, there’s a fine line to

balance between reuse8 and clarity. Thankfully, in any case the performance differ-

ences among the three choices are nominal.

SHARING CLOSURE CONTEXT

So far, the closures we’ve shown have stood alone, but it’s sometimes useful to have

multiple closures closing over the same values. This may take the form of an ad hoc

set of closures in a complex lexical environment, such as event callbacks or timer han-

dlers in a nested GUI builder. Or it may be a tidy, specifically designed bundle of val-

ues and related functions—something that can be thought of as an object.

7

Please don’t construe from this wording that filter always iterates through the whole input sequence. Like

most of the seq library, it’s lazy and only consumes as much of the input sequence as needed to produce the

values demanded of it.

8

By hiding divisible as an anonymous function inside filter-divisible, we reduce the reusability of this

code with no real benefit. Anonymous functions are best reserved for when the lexical context being closed

over is more complex or the body of the function too narrow in use to warrant being its own named function.

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Closures

139

To demonstrate this, we’ll build a robot object that has functions for moving it

around a grid based on its current position and bearing. For this we need a list of

coordinate deltas for compass bearings, starting with north and going clockwise:

(def bearings [{:x 0, :y 1} ; north

{:x 1, :y 0} ; east

{:x 0, :y -1} ; south

{:x -1, :y 0}]) ; west

Note that this is on a grid where y increases as you go north and x increases as you go

east—mathematical coordinate style rather than spreadsheet cells.

With this in place, it’s easy to write a function forward that takes a coordinate and

a bearing, and returns a new coordinate having moved forward one step in the direc-

tion of the bearing:

(defn forward [x y bearing-num]

[(+ x (:x (bearings bearing-num)))

(+ y (:y (bearings bearing-num)))])

Starting with a bearing of 0 (north) at 5,5 and going one step brings the bot to 5,6:

(forward 5 5 0)

;=> [5 6]

We can also try starting at 5,5 and with bearing 1 (east) or bearing 2 (south) and see

the desired results:

(forward 5 5 1)

;=> [6 5]

(forward 5 5 2)

;=> [5 4]

But we have no closures yet, so we’ll build a bot object that keeps not just its coordi-

nates, but also its bearing. In the process, we’ll move this standalone forward function

into the bot object itself. By making this a closure, we’ll also open up possibilities for

polymorphism later. So here’s a bot that knows how to move itself forward:

(defn bot [x y bearing-num]

{:coords [x y]

:bearing ([:north :east :south :west] bearing-num)

:forward (fn [] (bot (+ x (:x (bearings bearing-num)))

(+ y (:y (bearings bearing-num)))

bearing-num))})

We can create an instance of this bot and query it for its coordinates or its bearing:

(:coords (bot 5 5 0))

;=> [5 5]

(:bearing (bot 5 5 0))

;=> :north

But now that we’ve moved the forward function inside, we no longer pass in parame-

ters, because it gets everything it needs to know from the state of the bot that it closes

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140

CHAPTER 7 Functional programming

over. Instead, we use :forward to fetch the closure from inside the bot object and

then use an extra set of parentheses to invoke it with no arguments:

(:coords ((:forward (bot 5 5 0))))

;=> [5 6]

So now we have a somewhat complicated beastie but still only a single closure in the

mix. To make things more interesting, we’ll add turn-left and turn-right9 func-

tions, and store them right there in the object with :forward:

(defn bot [x y bearing-num]

{:coords [x y]

:bearing ([:north :east :south :west] bearing-num)

:forward (fn [] (bot (+ x (:x (bearings bearing-num)))

(+ y (:y (bearings bearing-num)))

bearing-num))

:turn-right (fn [] (bot x y (mod (+ 1 bearing-num) 4)))

:turn-left (fn [] (bot x y (mod (- 1 bearing-num) 4)))})

(:bearing ((:forward ((:forward ((:turn-right (bot 5 5 0))))))))

;=> :east

(:coords ((:forward ((:forward ((:turn-right (bot 5 5 0))))))))

;=> [7 5]

We won’t talk about the verbosity of using the bot object yet, and instead focus on

the features leveraged in the definition of bot itself. We’re freely mixing values com-

puted when a bot is created (such as the :bearing) and functions that create values

when called later. The functions are in fact closures, and each has full access to the

lexical environment. The fact that there are multiple closures sharing the same envi-

ronment isn’t awkward or unnatural and flows easily from the properties of closures

already shown.

We’d like to demonstrate one final feature of this pattern for building objects:

polymorphism. For example, here’s the definition of a bot that supports all of the

same usage as earlier, but this one has its wires crossed or perhaps is designed to work

sensibly in Alice’s Wonderland. When told to go forward it instead reverses, and it

turns left instead of right and vice versa:

(defn mirror-bot [x y bearing-num]

{:coords [x y]

:bearing ([:north :east :south :west] bearing-num)

:forward (fn [] (mirror-bot (- x (:x (bearings bearing-num)))

(- y (:y (bearings bearing-num)))

bearing-num))

:turn-right (fn [] (mirror-bot x y (mod (- 1 bearing-num) 4)))

:turn-left (fn [] (mirror-bot x y (mod (+ 1 bearing-num) 4)))})

9

The :turn-right function uses (+ 1 foo), even though in general (inc foo) would be more idiomatic.

Here it helps highlight to anyone reading the symmetry between turn-right and turn-left. In this case,

using + is more readable than using inc and so is preferred.

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Thinking recursively

141

By bundling the functions that operate on data inside the same structure as the data

itself, simple polymorphism is possible. Because each function is a closure, no object

state needs to be explicitly passed; instead, each function uses any locals required to

do its job.

It’s likely you cringed at the number of parentheses required to call these particu-

lar object closures, and rightfully so. We encourage you to extrapolate from the clo-

sure examples when dealing with your own applications, and see how they can solve a

variety of tricky and unusual problems. Although this kind of structure is simple and

powerful10 and may be warranted in some situations, Clojure provides other ways of

associating functions with data objects that are more flexible. In fact, the desire to

avoid a widespread need for this type of ad hoc implementation has motivated Clo-

jure’s reify macro, which we’ll cover in section 9.3.

COMPILE-TIME VERSUS RUN-TIME

When looking at code that includes a closure, it’s not immediately obvious how the

work is distributed between compile-time and run-time. In particular, when you see a

lot of code or processor-intensive work being done in a closure, you might wonder

about the cost of calling the function that creates the closure:

(defn do-thing-builder [x y z]

(fn do-thing [a b]

...

(massive-calculation x y z)

...))

But you don’t need to worry. When this whole expression is compiled, bytecode for

the bodies of do-thing and do-thing-builder are generated and stored in memory.11

In current versions of Clojure, each function definition gets its own class. But when

do-thing-builder is called, it doesn’t matter how large or slow the body of do-thing

is—all that’s done at run-time is the creation of an instance of do-thing’s class. This is

lightweight and fast. Not until the closure returned by do-thing-builder is called does

the complexity or speed of the body of that inner function matter at all.

In this section, you learned that closures are functions that close over lexical locals,

how to create them from inside other functions, how to pass them around and call

them, and even how to build lightweight objects using them. Next, we’ll take a look at

how functions and closures behave when they call themselves, a pattern lovingly

known as recursion.

7.3

Thinking recursively

You’re likely already familiar with the basics of recursion, and as a result can take heart

that we won’t force you to read a beginner’s tutorial again. But because recursive

10 ...a fact any sufficiently experienced JavaScript programmer would be able to confirm.

11 If the code is being compiled ahead of time by the compile function, the generated bytecode is also written

to disk in .class files.

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CHAPTER 7 Functional programming

solutions are prevalent in Clojure code, it’s important that we cover it well enough that

you can fully understand Clojure’s recursive offerings.

Recursion is often viewed as a low-level operation reserved for times when solu-

tions involving higher-order functions either fail or lead to obfuscation. Granted, it’s

fun to solve problems recursively because even for those of us who’ve attained some

level of acumen with functional programming, finding a recursive solution still injects

a bit of magic into our day. Recursion is a perfect building block for creating higher-

level looping constructs and functions, which we’ll show in this section.

7.3.1

Mundane recursion

A classically recursive algorithm is that of calculating some base number raised to an

exponent, or the pow function. A straightforward12 way to solve this problem recur-

sively is to multiply the base by each successively smaller value of the exponent, as

implemented in the following listing.

Listing 7.4 A version of pow using mundane recursion

(defn pow [base exp]

(if (zero? exp)

1

(* base (pow base (dec exp)))))

(pow 2 10)

;=> 1024

(pow 1.01 925)

;=> 9937.353723241924

We say that the recursive call is mundane 13 because it’s named explicitly rather than

through mutual recursion or implicitly with the recur special form. Why is this a prob-

lem? The answer lies in what happens when we try to call pow with a large value:

(pow 2 10000)

; java.lang.StackOverflowError

The implementation of pow is doomed to throw java.lang.StackOverflowError

because the recursive call is trapped by the multiplication operation. The ideal solu-

tion would be a tail-recursive version that uses the explicit recur form, thus avoiding

stack consumption and the resulting exception. One way to remove the mundane

recursive call is to perform the multiplication at a different point, thus freeing the

recursive call to occur in the tail position, as shown in the next listing.

Listing 7.5 A version of pow using tail recursion, accumulator, and helper function

(defn pow [base exp]

(letfn [(kapow [base exp acc]

12 Yes, we’re aware of Math/pow.

13 Typically mundane recursion is referred to as linear, or the case where the space requirements needed to per-

form the recursion is proportional to the magnitude of the input.

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Thinking recursively

143

(if (zero? exp)

acc

(recur base (dec exp) (* base acc))))]

(kapow base exp 1)))

(pow 2 10000)

;=> ... A very big number

This new version of pow uses two common techniques for converting mundane recur-

sion to tail recursion. First, it uses a helper function kapow that does the majority of

the work. Second, kapow itself uses an accumulator acc that holds the result of the

multiplication. The exponent exp is no longer used as a multiplicative value but

instead functions as a decrementing counter, eliminating a stack explosion.

REGULAR RECURSION IS FUN AGAIN WITH LAZY-SEQ

As mentioned in section 6.3, the lazy-seq recipe rule of thumb #1 states that you

should wrap your outer layer function bodies with the lazy-seq macro when generat-

ing lazy seqs. The implementation of lz-rec-step used mundane recursion but man-

aged to avoid stack overflow exceptions thanks to the use of lazy-seq. For functions

generating sequences, the use of lazy-seq might be a better choice than tail recur-

sion, because often the regular (mundane) recursive definition is the most natural

and understandable.

7.3.2

Tail calls and recur

In a language such as Clojure, where function locals are immutable, the benefit of tail

recursion is especially important for implementing algorithms that require the con-

sumption of a value or the accumulation of a result. Before we get deeper into imple-

menting tail recursion, we’ll take a moment to appreciate the historical

underpinnings of tail-call recursion and expound on its further role within Clojure.

GENERALIZED TAIL-CALL OPTIMIZATION

In the Lambda Papers, Guy L. Steele and Gerald Sussman describe their experiences

with the research and implementation of the early versions of the Scheme program-

ming language. The first versions of the interpreter served as a model for Carl

Hewitt’s Actor model (Hewitt 1973) of concurrent computation, implementing both

actors and functions. One day, while eating Ho-Hos,14 Steele and Sussman noticed

that the implementation of control flow within Scheme, implemented using actors,

always ended with one actor calling another in its tail position with the return to the

callee being deferred. Armed with their intimate knowledge of the Scheme compiler,

Steele and Sussman were able to infer that because the underlying architecture deal-

ing with actors and functions was the same, retaining both was redundant. Therefore,

actors were removed from the language and functions remained as the more general

construct. Thus, generalized tail-call optimization was thrust (Steele 1977) into the

world of computer science.

14 This isn’t true, but wouldn’t it be great if it were?

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CHAPTER 7 Functional programming

Generalized tail-call optimization as found in Scheme (Abelson 1996) can be

viewed as analogous to object delegation. Hewitt’s original actor model was rooted

heavily in message delegation of arbitrary depth, with data manipulation occurring at

any and all levels along the chain. This is similar to an adapter, except that there’s an

implicit resource management element involved. In Scheme, any tail call15 from a

function A to a function B results in the deallocation of all of A local resources and the

full delegation of execution to B. As a result of this generalized tail-call optimization,

the return to the original caller of A is directly from B instead of back down the call

chain through A again, as shown in figure 7.1.

Unfortunately for Clojure, neither the Java Virtual Machine nor its bytecode pro-

vide generalized tail-call optimization facilities. Clojure does provide a tail call special

form recur, but it only optimizes the case of a tail-recursive self-call and not the gener-

alized tail call. In the general case, there’s currently no way to reliably optimize

(Clinger 1998) tail calls.

TAIL RECURSION

The following function calculates the greatest common denominator of two numbers:

(defn gcd [x y]

(cond

(> x y) (gcd (- x y) y)

(< x y) (gcd x (- y x))

:else x))

no TCO

general TCO

caller

caller

(A

( )

A

(A)

(let [a 9]

(let [a 9]

(B))

(B))

dealloc A's

resources

(B)

(B)

; do stuff

; do stuff

; done

; done

de

d a

e ll

l o

l c

o

c B'

B s

'

dealloc B's

re

r s

e ou

o r

u c

r e

c s

resources

(A)

caller

; done

de

d al

a l

l o

l c

o A

'

A s

'

re

r so

s u

o r

u c

r es

e

caller

Figure 7.1 Generalized tail-call optimization: if you know that A calls B in the tail

position, then you also know that A’s resources are no longer needed, allowing

Scheme to deallocate them and defer to B for the return call instead.

15 Bear in mind that in this scenario, A and B can be different functions or the same function.

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Thinking recursively

145

The implementation of gcd is straightforward, but you’ll notice that we used mun-

dane recursion instead of tail recursion via recur. In a language such as Scheme con-

taining generalized tail-call optimization, the recursive calls will be optimized

automatically. On the other hand, because of the JVM’s lack of tail-call optimization,

the recur would be needed in order to avoid stack overflow errors.

Using the information in table 7.1, you can replace the mundane recursive calls

with the recur form, causing gcd to be optimized by Clojure’s compiler.

Table 7.1 Tail positions and recur targets

Form(s)

Tail position

recur target?

fn, defn

(fn [args] expressions tail)

Yes

loop

(loop [bindings] expressions tail)

Yes

let, letfn, binding

(let [bindings] expressions tail)

No

do

(do expressions tail)

No

if, if-not

(if test then tailelse tail)

No

when, when-not

(when test expressions tail)

No

cond

(cond test test tail ... :else else tail)

No

or, and

(or test test ... tail)

No

case

(case const const tail ... default tail)

No

WHY RECUR?

If you think that you understand why Clojure provides an explicit tail-call optimization

form rather than an implicit one, then go ahead and skip to the next section.

There’s no technical reason why Clojure couldn’t automatically detect and opti-

mize recursive tail calls—Scala does this—but there are valid reasons why Clojure

doesn’t.

First, because there’s no generalized TCO in the JVM, Clojure can only provide a

subset of tail-call optimizations: the recursive case and the mutually recursive case (see

the next section). By making recur an explicit optimization, Clojure doesn’t give the

pretense of providing full TCO.

Second, having recur as an explicit form allows the Clojure compiler to detect

errors caused by an expected tail call being pushed out of the tail position. If we

change gcd to always return an integer, then an exception is thrown because the

recur call is pushed out of the tail position:

(defn gcd [x y]

(int

(cond

(> x y) (recur (- x y) y)

(< x y) (recur x (- y x))

:else x)))

; java.lang.UnsupportedOperationException: Can only recur from tail position

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146

CHAPTER 7 Functional programming

With automatic recursive tail-call optimization, the addition of an outer int call

wouldn’t necessarily trigger (Wampler 2009)16 an error condition. But Clojure

enforces that a call to recur be in the tail position. This benefit will likely cause recur

to live on, even should the JVM acquire TCO.

The final benefit of recur is that it allows the forms fn and loop to act as anony-

mous recursion points.

Why recur indeed.

7.3.3

Don’t forget your trampoline

We touched briefly on the fact that Clojure can also optimize a mutually recursive

function relationship, but like the tail-recursive case, it’s done explicitly. Mutually

recursive functions are nice for implementing finite state machines (FSA), and in this

section we’ll show an example of a simple state machine modeling the operation of an

elevator (Mozgovoy 2009) for a two-story building. There are only four states that the

elevator FSA allows: on the first floor with the doors open or closed and on the second

floor with the door open or closed. The elevator can also take four distinct com-

mands: open doors, close doors, go up, and go down. Each command is only valid in a

certain context; for example, the close command is only valid when the elevator door

is open. Likewise, the elevator can only go up when on the first floor and only down

when on the second floor, and the door must be shut in both instances.

We can directly translate these states and transitions into a set of mutually recursive

functions by associating the states as a set of functions ff-open, ff-closed, sf-

closed, and sf-open, and the transitions :open, :close, :up, and :down, as conditions

for calling the next function. We’d like to create a function elevator that starts in the

ff-open state, takes a sequence of commands, and returns true or false if they corre-

spond to a legal schedule according to the FSA. For example, the sequence [:close

:open :done] would be legal, if not pointless, whereas [:open :open :done] wouldn’t

be legal, because an open door can’t be reopened. The function elevator could be

implemented as shown next.

Listing 7.6 Using mutually recursive functions to implement a finite state machine

(defn elevator [commands]

(letfn

Local functions

[(ff-open [[cmd & r]]

1st floor open

"When the elevator is open on the 1st floor

it can either close or be done."

#(case cmd

:close (ff-closed r)

:done true

false))

(ff-closed [[cmd & r]]

1st floor closed

"When the elevator is closed on the 1st floor

it can either open or go up."

16 The Scala 2.8 compiler recognizes a @tailrec annotation and triggers an error whenever a marked function

or method can’t be optimized.

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Thinking recursively

147

#(case cmd

:open (ff-open r)

:up (sf-closed r)

false))

(sf-closed [[cmd & r]]

2nd floor closed

"When the elevator is closed on the 2nd floor

it can either go down or open."

#(case cmd

:down (ff-closed r)

:open (sf-open r)

false))

(sf-open [[cmd & r]]

2nd floor open

"When the elevator is open on the 2nd floor

it can either close or be done"

#(case cmd

:close (sf-closed r)

:done true

false))]

(trampoline ff-open commands)))

Trampoline call

Using letfn in this way allows you to create local functions that reference each other,

whereas (let [ff-open #(...)] ...) wouldn’t, because it executes its bindings seri-

ally. Each state function contains a case macro that dispatches to the next state based

on a contextually valid command. For example, the sf-open state will transition to the

sf-closed state given a :close command, will return true on a :done command (cor-

responding to a legal schedule), or will otherwise return false. Each state is similar in

that the default case command is to return false indicating an illegal schedule. One

other point of note is that each state function returns a function returning a value

rather than directly returning the value. This is done so that the trampoline function

can manage the stack on the mutually recursive calls, thus avoiding cases where a long

schedule would blow the stack. Here’s the operation of elevator given a few example

schedules:

(elevator [:close :open :close :up :open :open :done])

;=> false

(elevator [:close :up :open :close :down :open :done])

;=> true

;; run at your own risk!

(elevator (cycle [:close :open]))

; ... runs forever

Like the recur special form, the trampoline for mutual recursion has a definitive syntactic

and semantic cost on the structure of your code. But whereas the call to recur could be

replaced by mundane recursion without too much effect, save for at the edges, the rules

for mutual recursion aren’t general. Having said that, the actual rules are simple:

1

Make all of the functions participating in the mutual recursion return a func-

tion instead of their normal result. Normally this is as simple as tacking a # onto

the front of the outer level of the function body.

2

Invoke the first function in the mutual chain via the trampoline function.

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CHAPTER 7 Functional programming

(elevator)

true

:done

:close

:up

:open

e

n

:close

ff-open

ff-closed

s S

f-closed

sf-open

s S

f-closed

... ff-open

2

2

Figure 7.2 Elevator trampoline: the trampoline function explicitly bounces between

mutually recursive calls.

The final example won’t cause a stack overflow because the trampoline function han-

dles the process of the self calls through the placement of the functions within a list

where each function is “bounced” back and forth explicitly, as seen in figure 7.2.

The typical use case for mutually recursive functions are state machines, of which

the elevator FSA is only a simple case.

7.3.4

Continuation-passing style

Before wrapping up this chapter, we’re going to take time to talk about a style of pro-

gramming not necessarily prevalent in Clojure, but moreso in the functional tradi-

tion: continuation-passing style. Continuation-passing style (CPS) is a hybrid between

recursion and mutual recursion, but with its own set of idioms. We won’t give you a

deep survey of CPS, but this subsection should provide a reasonable overview for

deeper exploration, should you be so inclined.

The nutshell version of CPS is that it’s a way of generalizing a computation (Fried-

man 2001) by viewing it in terms of up to three functions:

 An accept function that decides when a computation should terminate

 A return continuation that’s used to wrap the return values

 A continuation function used to provide the next step in the computation

There’s a reason why many sources on CPS will use the factorial function as a base

example: because it’s exceptionally illustrative, as we show next.

Listing 7.7 Factorial function using continuation-passing style

(defn fac-cps [n k]

(letfn [(cont [v] (k (* v n)))]

Next continuation

(if (zero? n)

Accept function

(k 1)

Return continuation

(recur (dec n) cont))))

(defn fac [n]

(fac-cps n identity))

(fac 10)

;=> 3628800

Though this approach is definitely different than the normal functional structure, it’s

not exactly interesting in and of itself. The power of CPS is that you can extract more

generic function builders using CPS. One such builder, shown in the following listing,

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Putting it all together: A* pathfinding

149

can be used to make a range of functions that happen to fall into the same mold of a

mathematical folding function.

Listing 7.8 Continuation-passing style function generator

(defn mk-cps [accept? end-value kend kont]

(fn [n]

((fn [n k]

(let [cont (fn [v] (k (kont v n)))]

Next continuation

(if (accept? n)

Accept continuation

(k end-value)

Return continuation

(recur (dec n) cont))))

n kend)))

(def fac (mk-cps zero? 1 identity #(* %1 %2)))

Factorial fn

(fac 10)

;=> 3628800

(def tri (mk-cps zero? 1 dec #(+ %1 %2)))

Triangular fn

(tri 10)

;=> 55

Though this is potentially a powerful technique, there are a number of reasons pre-

venting its widespread adoption in Clojure:

 Without generalized tail-call optimization, the number of continuation calls is

bounded by the size of the stack. If your own applications can guarantee a

bounded execution path for the CPS calls, then this may not be a problem in

practice.

 In the case of exception handling, CPS can cause the point of failure to bubble

out, especially on deferred computations such as in using delay, future, or

promise.17 In the abstract this may not seem to be a problem, but if your contin-

uation function is supposed to throw the error but an outer layer function is

doing so instead, then bugs might be difficult to track down.

 In a language such as Haskell that has ubiquitous lazy evaluation and pure func-

tions, it’s often not necessary to impose a strict order of execution. One way to

impose a strict order of execution is to design your programs along the

continuation-passing style. Though Clojure isn’t entirely lazy, the matter of out-

of-order execution isn’t a factor against CPS. But CPS isn’t conducive to paral-

lelization, which is antithetical to Clojure’s very nature.

7.4

Putting it all together: A* pathfinding

A* is a best-first pathfinding algorithm that maintains a set of candidate paths through

a “world” with the purpose of finding the least difficult (Bratko 2000) path to some

goal. The difficulty (or cost) of a path is garnered by the A* algorithm through the

use of a function, typically named f, that builds an estimate of the total cost from a

17 Clojure’s future and promise will be discussed in detail in chapter 11.

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CHAPTER 7 Functional programming

start point to the goal. The application of this cost-estimate function f is used to sort

the candidate paths (Hart 1968) in the order most likely to prove least costly.

THE WORLD

To represent the world, we’ll again use a simple 2D matrix representation:

(def world [[ 1 1 1 1 1]

[999 999 999 999 1]

[ 1 1 1 1 1]

[ 1 999 999 999 999]

[ 1 1 1 1 1]])

The world structure is made from the values 1 and 999 respectively, corresponding to

flat ground and cyclopean mountains. What would you assume is the optimal path

from the upper-left corner [0 0] to the lower-right [4 4]? Clearly the optimal (and

only) option is the Z-shaped path around the walls. Implementing an A* algorithm

should fit the bill, but first, we’ll talk a little bit about how to do so.

NEIGHBORS

For any given spot in the world, we need a way to calculate possible next steps. We can

do this brute force for small worlds, but we’d like a more general function. It turns out

if we restrict the possible moves to north, south, east, and west, then any given move is

+/-1 along the x or y axis. Taking advantage of this fact, we can use the neighbors

function from listing 5.1 as shown here:

(neighbors 5 [0 0])

;=> ([1 0] [0 1])

From the upper-left point, the only next steps are y=0, x=1 or y=1, x=0. So now that we

have that, think about how we might estimate the path cost from any given point. A

simple cost estimate turns out to be described as, “from the current point, calculate

the expected cost by assuming we can travel to the right edge, then down to the lower-

right.” An implementation of the h function estimate-cost that estimates the

remaining path cost is shown next.

Listing 7.9 A straight-line h function to estimate remaining path cost

(defn estimate-cost [step-cost-est size y x]

(* step-cost-est

(- (+ size size) y x 2)))

(estimate-cost 900 5 0 0)

;=> 7200

(estimate-cost 900 5 4 4)

;=> 0

From the y-x point [0 0] the cost of travelling 5 right and 5 down given an estimated

single-step cost step-cost-est is 9000. This is a pretty straightforward estimate based

on a straight-line path. Likewise, starting at the goal state [4 4] would cost nothing.

Still needed is the g function used to calculate the cost of the path so far, named

path-cost, which is provided in the following listing.

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Putting it all together: A* pathfinding

151

Listing 7.10 A g function used to calculate the cost of the path traversed so far

(defn path-cost [node-cost cheapest-nbr]

Add cheapest

(+ node-cost

neighbor cost,

(:cost cheapest-nbr 0)))

else 0

(path-cost 900 {:cost 1})

;=> 901

Now that we’ve created an estimated cost function and a current cost function, we can

implement a simple total-cost function for f.

Listing 7.11 f function to calculate the estimated cost of the path (+ (g ...) (h ...)) (defn total-cost [newcost step-cost-est size y x]

(+ newcost

(estimate-cost step-cost-est size y x)))

(total-cost 0 900 5 0 0)

;=> 7200

(total-cost 1000 900 5 3 4)

;=> 1900

The second example shows that if we’re one step away with a current cost of 1000,

then the total estimated cost will be 1900, which is expected. So now we have all of the

heuristic pieces in place. You may think that we’ve simplified the heuristic needs of

A*, but in fact this is all that there is to it. The actual implementation is complex,

which we’ll tackle next.

7.4.1

The A* implementation

Before we show the implementation of A*, we need one more auxiliary function min-

by, used to retrieve from a collection the minimum value dictated by some function.

The implementation of min-by would naturally be a straightforward higher-order

function, as shown:

(defn min-by [f coll]

(when (seq coll)

(reduce (fn [min this]

(if (> (f min) (f this)) this min))

coll)))

(min-by :cost [{:cost 100} {:cost 36} {:cost 9}])

;=> {:cost 9}

This function will come in handy when we want to grab the cheapest path deter-

mined by the cost heuristic. We’ve delayed enough! We’ll finally implement the A*

algorithm so that we navigate around the world. The following listing shows a tail-

recursive solution.

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CHAPTER 7 Functional programming

Listing 7.12 The main A* algorithm

(defn astar [start-yx step-est cell-costs]

(let [size (count cell-costs)]

(loop [steps 0

routes (vec (replicate size (vec (replicate size nil))))

work-todo (sorted-set [0 start-yx])]

Grab

(if (empty? work-todo)

Check done

first

[(peek (peek routes)) :steps steps]

route

(let [[_ yx :as work-item] (first work-todo)

Get next work item

rest-work-todo (disj work-todo work-item)

nbr-yxs (neighbors size yx)

Clear from todo

cheapest-nbr (min-by :cost

Calc least-cost

(keep #(get-in routes %)

nbr-yxs))

Calc path

newcost (path-cost (get-in cell-costs yx)

so far

cheapest-nbr)

oldcost (:cost (get-in routes yx))]

Check if new

(if (and oldcost (>= newcost oldcost))

is worse

(recur (inc steps) routes rest-work-todo)

(recur (inc steps)

Place new path

(assoc-in routes yx

in routes

{:cost newcost

:yxs (conj (:yxs cheapest-nbr [])

yx)})

(into rest-work-todo

Add estimated path

(map

to todo and recur

(fn [w]

(let [[y x] w]

[(total-cost newcost step-est size y x) w]))

nbr-yxs)))))))))

The main thrust of the astar function occurs at the check that (>= newcost oldcost).

Once we’ve calculated the newcost (the cost so far for the cheapest neighbor) and a

cost-so-far oldcost, we perform one of two actions. The first action occurs when the

newcost is greater than or equal to the oldcost and is to throw away this new path,

because it’s clearly a worse alternative. The other action is the core functionality corre-

sponding to the constant sorting of the work-todo, based on the cost of the path as

determined by the heuristic function total-cost. The soul of A* is based on the fact

that the potential paths stored in work-todo are always sorted and distinct (through

the use of a sorted set), based on the estimated path cost function. Each recursive

loop through the astar function maintains the sorted routes based on the current

cost knowledge of the path, added to the estimated total cost.

The results given by the astar function for the Z-shaped world are shown in the

next listing.

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153

Listing 7.13 Running the A* algorithm on the Z World

(astar [0 0]

900

world)

;=> [{:cost 17,

:yxs [[0 0] [0 1] [0 2] [0 3] [0 4] [1 4] [2 4]

[2 3] [2 2] [2 1] [2 0] [3 0] [4 0] [4 1]

[4 2] [4 3] [4 4]]}

:steps 94]

By following the y-x indices, you’ll notice that the

astar function traverses the Z World along the path

where cost is 1, as seen in figure 7.3.

We can also build another world, as shown next,

called Shrubbery World that contains a single weakling

Figure 7.3 A graphical

shrubbery at position [0 3], represented by the num-

representation of Z World clearly

ber 2, and see how astar navigates it.

shows the optimal/only path.

Listing 7.14 The Shrubbery World

(astar [0 0]

900

[[ 1 1 1 2 1]

The shrubbery is 2

[ 1 1 1 999 1]

[ 1 1 1 999 1]

[ 1 1 1 999 1]

[ 1 1 1 1 1]])

The clear path

;=> [{:cost 9,

:yxs [[0 0] [0 1] [0 2] [1 2] [2 2] [3 2]

[4 2] [4 3] [4 4]]}

:steps 134]

When tracing the best path, you will see that the astar function prefers the nonshrub-

bery path. But what would happen if we placed a man-eating bunny along the previ-

ously safe path, represented by an ominously large number, as shown next?

Listing 7.15 The bunny world

(astar [0 0]

900

The shrubbery

[[ 1 1 1 2 1]

looks inviting

[ 1 1 1 999 1]

[ 1 1 1 999 1]

[ 1 1 1 999 1]

The bunny

[ 1 1 1 666 1]])

lies in wait

;=> [{:cost 10,

:yxs [[0 0] [0 1] [0 2] [0 3] [0 4] [1 4]

[2 4] [3 4] [4 4]]}

:steps 132]

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CHAPTER 7 Functional programming

As expected, the astar function picks the shrubbery path (2) path instead of the evil

bunny path to reach the final destination.

7.4.2

Notes about the A* implementation

The A* algorithm was implemented as idiomatic Clojure source code. Each of the

data structures, from the sorted set to the tail-recursive astar function, to the higher-

order function min-by, was functional in nature and therefore extensible as a result.

We encourage you to explore the vast array of possible worlds traversable by our A*

implementation and see what happens should you change the heuristic (Dijkstra

1959) functions along the way. Clojure encourages experimentation, and by partition-

ing the solution this way, we’ve enabled you to explore different heuristics.

7.5

Summary

We’ve covered a lot about Clojure’s flavor of functional programming in this chapter,

and you may have noticed that it looks like many others. Clojure favors an approach

where immutable data is transformed through the application of functions. Addition-

ally, Clojure prefers that functions be free of side-effects and referentially transparent

(pure) in order to reduce the complexities inherent in widespread data mutation.

Lexical closures provide a simple yet powerful mechanism for defining functions that

carry around with them the value context in which they were created. This allows cer-

tain information to exist beyond their lexical context, much like a poor-man’s object.

Finally, Clojure is built with this in mind, in that its primary form of iteration is

through tail recursion as a natural result of its focus on immutability.

In the next chapter, we’ll explore the feature most identified with Lisp: macros.

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Part 4

Large-scale design

Clojure is a practical language, not an academic one; and in the real

world, programs grow large, change over time, and are confronted with shifting

requirements. In this part, we’ll show how Clojure’s Lisp heritage of “code is

data” can help address these problems. We’ll demonstrate the use of macros,

how to create a fluent builder, the benefits of a language that embraces the Java

platform, and how Clojure addresses the mutability of the real world.

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Macros

If you give someone Fortran, he has

Fortran. If you give someone Lisp, he

has any language he pleases.

—Guy Steele

This chapter covers

 Data is code is data

 Defining control structures

 Macros combining forms

 Using macros to control symbolic resolution time

 Using macros to manage resources

 Putting it all together: macros returning functions

Macros are where the rubber of “code is data” meets the road of making programs

simpler and cleaner. To fully understand macros, you need to understand the dif-

ferent times of Clojure, of which macros perform the bulk of their work at compile

time. We’ll start by looking at what it means for code to be data and data to be used

as code. This is the background you’ll need to understand that control structures in

Clojure are built out of macros, and how you can build your own. The mechanics of

macros are relatively simple, and before you’re halfway through this chapter you’ll

157

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CHAPTER 8 Macros

have learned all you technically need to write your own. Where macros get compli-

cated is when you try to bring theoretical knowledge of them into the real world, so to

help you combat that we’ll lead you on a tour of practical applications of macros.

What kinds of problems do macros solve? To start answering that question, con-

sider Clojure’s -> and ->> macros that return the result of a number of threaded

forms. To understand both versions of the arrow macros, we find it useful to think of

them as an arrow indicating the flow of data from one function to another—the form

(-> 25 Math/sqrt int list) can be read as

(-> 25

1

Take the value 25.

(Math/sqrt)

2

Feed it into the method Math/sqrt.

(i n

i t

n )

t

3

Feed that result into the function int.

l

( i

l s

i t

)

t )

4

Feed that result into the function list.

Graphically, this can be viewed as shown in

Figure 8.1 Arrow macro: each expression

figure 8.1.

is inserted into the following one at compile

time, allowing you to write the whole

It expands into the following expression:

expression inside-out when that feels 

more natural.

(list (int (Math/sqrt 25)))

When viewed this way, the -> macro can be said to thread a sequence of forms into

each in turn. This threading can be done within any form and is always stitched in as

the first argument to the outermost expression. On the other hand, the ->> macro

will thread the form as the last argument. Observe how the placement of commas1

works as visual markers for the stitch point:

(-> (/ 144 12) (/ ,,, 2 3) str keyword list)

;=> (:2)

(-> (/ 144 12) (* ,,, 4 (/ 2 3)) str keyword (list ,,, :33))

;=> (:32 :33)

(->> a (+ 5 ,,,) (let [a 5] ,,,))

;=> 10

Using the arrows macro is useful when many sequential operations need to be applied

to a single object. So this is one potential use case for macros: taking one form of an

expression and transforming it into another form. In this chapter, we’ll also look at

using macros to combine forms, change forms, control evaluation and resolution of

arguments, manage resources, and build functions. But first, what does it mean that

Clojure code is data, and why should you care?

8.1

Data is code is data

You’re already familiar with textual representations of data in your programs, at least

with strings, lists, vectors, maps, and so on. Clojure, like other Lisps, takes this one

step further by having programs be made entirely out of data. Function definitions in

Clojure programs are also represented using an aggregation of the various data

1 Because commas are considered whitespace. The use here is instructive and not meant as idiomatic.

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Data is code is data

159

structures mentioned in the previous chapters. Likewise, the expressions representing

the execution of functions and the use of control structures are also data structures!

These data representations of functions and their executions represent a concept dif-

ferent from the way other programming languages operate. Typically, there’s a sharp

distinction between data structures and functions of the language. In fact, most pro-

gramming languages don’t even remotely describe the form of functions in their tex-

tual representations. With Clojure, there’s no distinction between the textual form

and the actual form of a program. When a program is the data that composes the pro-

gram, then you can write programs to write programs. This may seem like nonsense

now, but as you’ll see throughout this chapter, it’s powerful.

To start with, look at the built-in Clojure function eval, whose purpose is to take a

data structure representing a Clojure expression, evaluate it, and return the result.

This behavior can be seen in the following examples:

(eval 42)

;=> 42

(eval '(list 1 2))

;=> (1 2)

(eval (list 1 2))

; java.lang.ClassCastException: java.lang.Integer cannot be cast to clojure.

lang.IFn

Why did we get an exception for the last example? The answer to that lies in the previ-

ous example. The quote in '(list 1 2) causes eval to view it as (list 1 2), which is

the function call to create the resulting list. Likewise, for the final example eval

received a list of (1 2) and attempted to use 1 as a function, thus failing. Not very

exciting, is it? The excitement inherent in eval stems from something that we men-

tioned2 earlier—if you provide eval a list in the form expected of a function call, then

something else should happen. This something else would be the evaluation of a function

call and not of the data structure itself. Look at what happens when we try evaluating

something more complicated:

(eval (list (symbol "+") 1 2))

;=> 3

In words, the steps involved were as follows:

1

The function symbol received a string + and returned a symbol data type of +.

2

The function list received three arguments: a symbol +, the integer 1, and the

integer 2, and returned a list of these elements.

3

The eval function received a list data type of (+ 1 2), recognized it as the func-

tion call form, and executed the + function with the arguments 1 and 2, return-

ing the integer 3.

2 All the way back in section 2.5.

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CHAPTER 8 Macros

8.1.1

Syntax-quote, unquote, and splicing

In section 1.5.6, we mentioned quoting and its effects on evaluation, and in this chapter

we’ll expand on that theme fully as it relates to Clojure’s macro facility. But the func-

tionality of the quoting forms is orthogonal to macros, and they can be used indepen-

dently. As we show3 in listing 8.1, using quoting and unquoting in a function allows us

to create an evaluation function, contextual-eval, that takes an explicit context map.

Listing 8.1 An implementation of eval taking a local context

(defn contextual-eval [ctx expr]

(eval

Build let bindings

`(let [~@(mapcat (fn [[k v]] [k `'~v]) ctx)]

at compile-time

~expr)))

(contextual-eval {'a 1, 'b 2} '(+ a b))

;=> 3

(contextual-eval {'a 1, 'b 2} '(let [b 1000] (+ a b)))

;=> 1001

Handling nested syntax-quotes

Dealing with nested syntax-quotes can at times be complicated. But you can visualize

the way in which unquoting affects the nested structures as result of repeated eval-

uations (Steele 1990) relative to its nesting level:

(let [x 9, y '(- x)]

(println `y)

(println ``y)

(println ``~y)

(println ``~~y)

(contextual-eval {'x 36} ``~~y))

; user/y

; (quote user/y)

; user/y

; (- x)

;=> -36

The nesting of the syntax-quotes in the first two println calls takes the value of y

further up the abstraction ladder. But by including a single unquote in the third

println, we again bring it back down. Finally, by unquoting a second time, we’ve cre-

ated a structure that can then be evaluated—and doing so yields the result -36. We

had to use contextual-eval in the tail because core eval doesn’t have access to

local bindings—only Var bindings. One final note is that had we attempted to unquote

one extra time, we’d have seen the exception java.lang.IllegalStateExcep-

tion: Var clojure.core/unquote is unbound. The reason for this error is that

unquote is the way to “jump” out of a syntax-quote, and to do so more than nesting

allows will cause an error. We won’t use this technique in this chapter, and in most

cases you won’t need to utilize it unless you’re planning to create macro-defining

macros—something we won’t do until section 13.1.

3 Thanks to George Jahad for the implementation on which contextual-eval is based.

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Defining control structures

161

Rarely will you see the use of syntax-quote outside the body of a macro, but there’s

nothing preventing it from being used this way—and doing so is powerful. But the

maximum power of quoting forms is fully realized when used with macros.

Working from a model where code is data, Clojure is able to manipulate structures

into different executable forms at both runtime and compile time. We’ve already

shown how this can be done at runtime using eval and contextual-eval, but this

doesn’t serve the purpose of compile-time manipulation. It probably doesn’t need say-

ing, but because this is a book about Clojure, we will: macros are the way to achieve

this effect.

8.1.2

Macro rules of thumb

Before we begin, we should list a few rules of thumb to observe when writing macros:

 Don’t write a macro if a function will do. Reserve macros to provide syntactic

abstractions or create binding forms.

 Write an example usage.

 Expand your example usage by hand.

 Use macroexpand, macroexpand-1, and clojure.walk/macroexpand-all4 liber-

ally to understand how your implementation works.

 Experiment at the REPL.

 Break complicated macros into smaller functions whenever possible.

Throughout this chapter, you’ll see all of these rules to varying degrees. Obviously,

we’re trying to balance best practices, teaching, and page counts, so we may not always

adhere entirely. Even so, we’ll try to highlight those times when we do break from the

recommended heuristics. Having said that, we’ll talk first about the most ubiquitous

use of macros: creating custom control structures.

8.2

Defining control structures

Most control structures in Clojure are implemented via macros, so they provide a nice

starting point for learning how macros can be useful. Macros can be built with or with-

out using syntax-quote, so we’ll show examples of each.

In languages lacking macros, such as Haskell5 for example, the definition of control

structures relies on the use of higher-order functions such as we showed in section

7.1.2. Though this fact in no way limits the ability to create control structures in

Haskell, the approach that Lisps take to the problem is different. The most obvious

advantage of macros over higher-order functions is that the former manipulate

compile-time forms, transforming them into runtime forms. This allows your programs

4

The macroexpand-all function is a useful debugging aid, as we’ll demonstrate in this chapter. But it’s worth

knowing that unlike the other macroexpand functions, it doesn’t use exactly the same logic as the Clojure

compiler itself, and thus may in some unusual circumstances produced misleading results.

5

Although there’s a GHC extension named Template Haskell that provides a macro-like capability, this isn’t

the norm.

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CHAPTER 8 Macros

to be written in ways natural to your problem domain, while still maintaining runtime

efficiency. Clojure already provides a rich set of control structures, including but not

limited to doseq, while, if, if-let, and do, but in this section we’ll write a few others.

8.2.1

Defining control structures without syntax-quote

Because the arguments to defmacro aren’t evaluated before being passed to the

macro, they can be viewed as pure data structures, and manipulated and analyzed as

such. Because of this, amazing things can be done on the raw forms supplied to mac-

ros even in the absence of unquoting.

Imagine a macro named do-until that will execute all of its clauses evaluating as

true until it gets one that is falsey:

(do-until

(even? 2) (println "Even")

(odd? 3) (println "Odd")

(zero? 1) (println "You never see me")

:lollipop (println "Truthy thing"))

; Even

; Odd

;=> nil

A good example of this type of macro is Clojure’s core macro cond, which with some

minor modifications can be made to behave differently:

(defmacro do-until [& clauses]

(when clauses

(list `when (first clauses)

(if (next clauses)

(second clauses)

(throw (IllegalArgumentException.

"do-until requires an even number of forms")))

(cons 'do-until (nnext clauses)))))

The first expansion of do-until illustrates how this macro operates:

(macroexpand-1 '(do-until true (prn 1) false (prn 2)))

;=> (when true (prn 1) (do-until false (prn 2)))

do-until recursively expands into a series of when calls, which themselves expand into

a series of if expressions:

(require '[clojure.walk :as walk])

(walk/macroexpand-all '(do-until true (prn 1) false (prn 2)))

;=> (if true (do (prn 1) (if false (do (prn 2) nil))))

(do-until true (prn 1) false (prn 2))

; 1

;=> nil

Now you could write out the nested if structure manually and achieve the same

result, but the beauty of macros lies in the fact that they can do so on your behalf

while presenting a lightweight and intuitive form. In cases where do-until can be

used, it removes the need to write and maintain superfluous boilerplate code. This

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Defining control structures

163

idea can be extended to macros in general and their propensity to reduce unneeded

boilerplate for a large category of circumstances, as the programmer desires. One

thing to note about do-until is that it’s meant to be used only for side effects, because

it’s designed to always return nil. Macros starting with do tend to act the same.

8.2.2

Defining control structures using syntax-quote and unquoting

Not all control structures will be as simple as do-until. Instead, there will be times

when you’ll want to selectively evaluate macro arguments, structures, or substructures.

In this section, we’ll explore one such macro named unless, implemented using

unquote and unquote-splice.

Ruby provides a control structure named unless that reverses the sense (Olsen

2007) of a when statement, executing the body of a block when a given condition eval-

uates to false:

(unless (even? 3) "Now we see it...")

;=> "Now we see it..."

(unless (even? 2) "Now we don't.")

;=> nil

The maverick implementation6 of unless as demonstrated previously and as shown in

the following listing is straightforward.

Listing 8.2 A Clojure Implementation of unless

(defmacro unless [condition & body]

`(if (not ~condition)

Unquote condition

(do ~@body)))

Splice body

(defn from-end [s n]

Use our unless

(let [delta (dec (- (count s) n))]

(unless (neg? delta)

Return nil

(nth s delta))))

if negative

(from-end (range 1 101) 10)

;=> 90

The body of the unless implementation uses three features first shown in section

1.5.6: syntax-quote (written as a single back-quote), unquote (written as ~), and

unquote-splice (written as ~@). Syntax-quote allows the if form to act as a template for

the expression that any use of the macro becomes when expanded. The unquote and

splicing-unquote provide the “blanks” where the values for the parameters condition

and body will be inserted.

Because unless relies on the result of a condition for its operation, it’s imperative

that it evaluate the condition part using unquote. If we didn’t use unquote in this

instance, then instead of evaluating a function (even? 3), it would instead attempt to

resolve a namespace Var named condition that may not exist —and if it does exist, it

6 The proper way to define unless is either (defmacro unless [& args] `(when-not ~@args)) or even

(clojure.contrib.def/defalias unless when-not)—or just use when-not from the start.

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CHAPTER 8 Macros

might be arbitrarily truthy at the time of the macro call. Some of the unintended con-

sequences of this mistake are shown in the next listing.

Listing 8.3 Name capture in unless

(macroexpand `(if (not condition) "got it"))

Missing ~

;=> (if (clojure.core/not user/condition) "got it")

resolves to Var

(eval `(if (not condition) "got it"))

; java.lang.Exception: No such var: user/condition

(def condition false)

Undesired result

(eval `(if (not condition) "got it"))

when bound

;=> "got it"

Clearly this isn’t the desired behavior. Instead, by unquoting the condition local, we

ensure that the function call is used instead. It’s easy to forget to add an unquote to

the body of a macro, and depending on the condition of your runtime environment,

the problem may not be immediately obvious.

8.3

Macros combining forms

Macros are often used for combining a number of forms and actions into one consis-

tent view. This behavior could be seen in the previous section with the do-until

macro, but it’s more general. In this section, we’ll show how macros can be used to

combine a number of tasks in order to simplify an API. Clojure’s defn macro is an

instance of this type of macro because it aggregates the processes of creating a func-

tion, including

 Creating the corresponding function object using fn

 Checking for and attaching a documentation string

 Building the :arglists metadata

 Binding the function name to a Var

 Attaching the collected metadata

You could perform all of these steps over and over again every time you wanted to cre-

ate a new function, but thanks to macros you can instead use the more convenient

defn form. Regardless of your application domain and its implementation, program-

ming language boilerplate code inevitably occurs. But identifying these repetitive tasks

and writing macros to simplify and reduce or eliminate the tedious copy-paste-tweak

cycle can work to reduce the incidental complexities inherent in a project. Where mac-

ros differ from techniques familiar to proponents of Java’s object-oriented style—

including hierarchies, frameworks, inversion of control, and the like—is that they’re

treated no differently by the language itself. Clojure macros work to mold the language

into the problem space rather than forcing you to mold the problem space into the

constructs of the language. There’s a specific term for this, domain-specific language, but

in Lisp the distinction between DSL and API is thin to the point of transparency.

Envision a scenario where you want to be able to define Vars that call a function

whenever their root bindings change. You could do this using the add-watch function

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Using macros to change forms

165

that allows for the attachment of a watcher to a reference type that’s called whenever a

change occurs within. The add-watch function itself takes three arguments: a refer-

ence, a watch function key, and a watch function called whenever a change occurs.

You could enforce that every time someone wants to define a new Var, they must fol-

low these steps:

1

Define the Var.

2

Define a function (maybe inline to save a step) that will be the watcher.

3

Call add-watch with the proper values.

A meager three steps isn’t too cumbersome a task to remember in a handful of uses,

but over the course of a large project it’s easy to forget and/or morph one of these

steps when the need to perform them many times occurs. Therefore, perhaps a better

approach is to define a macro to perform all of these steps for you, as the following

definition does:

(defmacro def-watched [name & value]

`(do

(def ~name ~@value)

(add-watch (var ~name)

:re-bind

(fn [~'key ~'r old# new#]

(println old# " -> " new#)))))

Ignoring symbol resolution and auto-gensym, which we’ll cover in upcoming sections,

the macro called as (def-watched x 2) expands into roughly the following:

(do (def x 2)

(add-watch (var x)

:re-bind

(fn [key r old new]

(println old " -> " new))))

The results of def-watched are thus

(def-watched x (* 12 12))

x

;=> 144

(def x 0)

; 144 -> 0

Lisp programs in general (and Clojure programs specifically) use macros of this sort

to vastly reduce the boilerplate needed to perform common tasks. Throughout this

chapter, you’ll see macros that combine forms, so there’s no need to dwell on the mat-

ter here. Instead, we’ll move on to a macro domain that does just that, with the added

bonus of performing some interesting transformations in the process.

8.4

Using macros to change forms

One way to design macros is to start by writing out example code that you wish

worked—code that has the minimal distance between what you must specify and the

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CHAPTER 8 Macros

specific application domain in which you’re working. Then, with the goal of making

this code work, you begin writing macros and functions to fill in the missing pieces.

For example, when designing software systems, it’s often useful to identify the

“things” comprising your given application domain, including their logical groupings.

The level of abstraction at this point in the design is best kept high (Rosenberg 2005)

and shouldn’t include details about implementation. Imagine that you want to

describe a simple domain of the ongoing struggle between humans and monsters:

 Man versus monster

 People

 Men (humans)

 Name

 Have beards?

 Monsters

 Chupacabra

 Eats goats?

Though this is a simple format, it needs work to be programmatically useful. There-

fore, the goal of this section is to write macros performing the steps to get from this

simple representation to the one more conducive to processing. One such structure is

a tree composed of individual generic nodes, each taking a form similar to that shown

in the next listing.

Listing 8.4 Domain DSL’s underlying form

{:tag <node form>,

Domain, grouping

:attrs {},

Name people

:content [<nodes>]}

Properties

You’d never say this is a beautiful format, but it does present practical advantages over

the original format—it’s a tree, it’s composed of simple types, it’s regular, and it’s rec-

ognizable to some existing libraries.

CLOJURE APHORISM Clojure is a design language where the conceptual

model is also Clojure.

We’ll start with the outer-level element, domain:

(defmacro domain [name & body]

`{:tag :domain,

:attrs {:name (str '~name)},

:content [~@body]})

The body of domain is fairly straightforward in that it sets the domain-level tree node

and splices the body of the macro into the :content slot. After domain expands, you’d

expect its body to be composed of a number of grouping forms, which are then han-

dled by the aptly named macro:

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Using macros to change forms

167

(declare handle-things)

(defmacro grouping [name & body]

`{:tag :grouping,

:attrs {:name (str '~name)},

:content [~@(handle-things body)]})

Similarly to domain, grouping expands into a node with its body spliced into the :con-

tent slot. But grouping differs from domain in that it splices in the result of the call to

a function handle-things:

(declare grok-attrs grok-props)

(defn handle-things [things]

(for [t things]

{:tag :thing,

:attrs (grok-attrs (take-while (comp not vector?) t))

:content (if-let [c (grok-props (drop-while (comp not vector?) t))]

[c]

[])})))

Because the body of a thing is fairly simple and regular, we can simplify the implemen-

tation of handle-things by again splitting it into two functions. The first function

grok-attrs handles everything within the body of a thing that’s not a vector, and the

other grok-props handles properties that are. In both cases, these leaf-level functions

return specifically formed maps:

(defn grok-attrs [attrs]

(into {:name (str (first attrs))}

(for [a (rest attrs)]

(cond

(list? a) [:isa (str (second a))]

(string? a) [:comment a]))))

The implementation of grok-attrs may seem overly complex, especially given that

the example domain model DSL only allows for a comment attribute and an optional

isa specification. But by laying out this way, we can easily expand the function to han-

dle a richer set of attributes later. Likewise with grok-props, we provide a more com-

plicated function to pull apart the vector representing a property so that it’s more

conducive to expansion:

(defn grok-props [props]

(when props

{:tag :properties, :attrs nil,

:content (apply vector (for [p props]

{:tag :property,

:attrs {:name (str (first p))},

:content nil}))}))

Now that we’ve created the pieces, take a look at the new DSL in action in the follow-

ing listing.

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CHAPTER 8 Macros

Listing 8.5 Exploring the domain DSL results

(def d

(domain man-vs-monster

(grouping people

Group of people

(Human "A stock human")

One kind of person

(Man (isa Human)

Another kind of person

"A man, baby"

[name]

[has-beard?]))

(grouping monsters

Group of monsters

(Chupacabra

One kind of monster

"A fierce, yet elusive creature"

[eats-goats?]))))

(:tag d)

;=> :domain

(:tag (first (:content d)))

;=> :grouping

Maybe that’s enough to prove to you that we’ve constructed the promised tree, but

probably not. Therefore, we can pass a tree into a function that expects one of that

form7 and see what comes out on the other end:

(use '[clojure.xml :as xml])

(xml/emit d)

Performing this function call will print out the corresponding XML representation,

minus the pretty printing, shown here.

Listing 8.6 An XML transformation of the domain DSL structure

<?xml version='1.0' encoding='UTF-8'?>

<domain name='man-vs-monster'>

(Domain ...)

<grouping name='people'>

(Grouping ...)

<thing name='Human' comment='A stock human'>

(Human ...)

<properties></properties>

</thing>

<thing name='Man' isa='Human' comment='A man, baby'>

(Man ...)

<properties>

<property name='name'/>

<property name='has-beard?'/>

</properties>

</thing>

</grouping>

(Grouping ...)

<grouping name='monsters'>

<thing name='Chupacabra' comment='A fierce, yet elusive creature'>

<properties>

<property name='eats-goats?'/>

</properties>

7

The namespace clojure.contrib.json in the Clojure contrib library also contains some functions that

would be able to handle the domain DSL structure seamlessly. Additionally, Enlive (http://mng.bz/8Hh6)

should also recognize the resultant structure.

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Using macros to control symbolic resolution time

169

</thing>

</grouping>

</domain>

Our approach was to define a single macro entry point domain, intended to build the

top-level layers of the output data structure and instead pass the remainder on to aux-

iliary functions for further processing. In this way, the body of the macro expands into

a series of function calls, each taking some subset of the remaining structure and

returning some result that’s spliced into the final result. This functional composition

approach is fairly common when defining macros. The entirety of the domain descrip-

tion could’ve been written within one monolithic macro, but by splitting the responsi-

bilities, you can more easily extend the representations for the constituent parts.

Macros take data and return data, always. It so happens that in Clojure, code is

data and data is code.

8.5

Using macros to control symbolic resolution time

Whereas functions accept and return values that are meaningful to your application

at runtime, macros accept and return code forms that are meaningful at compile

time. Any symbol has some subtleties depending on whether it’s fully qualified or not,

its resolution time, and its lexical context. These factors can be controlled in any par-

ticular case by the appropriate use of quoting and unquoting, which we explore in

this section.

Clojure macros are mostly safe from name capture, in that the use of syntax-quote

in macros is encouraged and idiomatic, and it’ll resolve symbols at macro-expansion

time. This strategy reduces complexity by ensuring that symbols refer to those avail-

able at a known instance rather than to those unknown in the execution context.

For example, consider one of the simplest possible macros:

(defmacro resolution [] `x)

Viewing the expansion of this macro is illuminating in understanding how Clojure

macros resolve symbols:

(macroexpand '(resolution))

;=> user/x

The expansion of the macro resolves the namespace of the syntax-quoted symbol x.

This behavior is useful in Clojure by helping to avoid free name capturing problems

that are possible in a macro system such as that found in Common Lisp.8 Here’s an

example that would trip up a lesser implementation of syntax-quote, but which does

just what we want in Clojure:

(def x 9)

(let [x 109] (resolution))

;=> 9

8

Among one of the ways that Common Lisp works to alleviate this kind of problem is the use of gensym. The

key difference is that in Common Lisp, you have to be careful to avoid name capturing, whereas Clojure avoids

it by default.

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CHAPTER 8 Macros

The x defined in the let isn’t the same as the namespace-qualified user/x referred to

by the macro resolution. As you might expect, the macro would’ve thrown an

unbound Var exception had we not first executed the call to def.

Clojure does provide a way to defer symbolic resolution for those instances where

it may be useful to resolve it within the execution context, which we’ll show now.

8.5.1

Anaphora

Anaphora9 in spoken language is a term used in a sentence referring back to a previ-

ously identified subject or object. It helps to reduce repetition in a phrase by replac-

ing “Jim bought 6,000 Christmas lights and hung all of the Christmas lights,” with “Jim

bought 6,000 Christmas lights and hung them all.” In this case, the word them is the

anaphora. Some programming languages use anaphora, or allow for their simple defi-

nition. Scala has a rich set of anaphoric (Odersky 2008) patterns primarily focused

around its _ operator:

Array(1, 2, 3, 4, 5).map(2 * _)

//=> res0: Array[Int] = Array(2, 4, 6, 8, 10)

In this Scala example, the underscore serves to refer back to an implicitly passed

argument to the map function, which in this case would be each element of the array

in succession. The same expression could be written with (x) => 2 * x—the syntax for

an anonymous function—in the body of the map call, but that would be unnecessarily

verbose.

Anaphora don’t nest, and as a result are generally not employed in Clojure. Within

a nested structure of anaphoric macros, you can only refer to the most immediate ana-

phoric binding, and never those from outer lexical contours, as demonstrated in list-

ing 8.7. For example, the Arc programming language (Graham Arc) contains a macro

named awhen similar to Clojure’s when, save that it implicitly defines a local named it

used within its body to refer to the value of the checked expression.

Listing 8.7 An example of anaphora and its weakness

(defmacro awhen [expr & body]

`(let [~'it ~expr]

Define anaphora

(when ~'it

Check its truth

(do ~@body))))

Inline body

(awhen [:a :b :c] (second it)

Use it in body

;=> :b

(awhen nil (println "Will never get here"))

;=> nil

(awhen :outer (awhen :inner [it]))

Fail to nest

;=> [:inner]

9 Anaphora is pronounced un-NAF-er-uh.

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Using macros to manage resources

171

Clojure provides similar macros that do nest and replace the need for anaphora: if-

let and when-let. When designing your own macros, it’s preferred that you build

them along these lines so that the macro itself takes the name to be bound. But just

because typical anaphorics are limited, that’s not to say that they’re entirely useless.

Instead, for your own libraries you may find that their usage is intuitive. You’ll see the

pattern ~'symbol at times in Clojure macros, because this is the idiomatic way to selec-

tively capture a symbolic name within the body of a macro. The reason for this bit of

awkwardness10 is because Clojure’s syntax-quote attempts to resolve symbols in the

current context, resulting in fully qualified symbols. Therefore, ~' avoids that resolu-

tion by unquoting a quote.

8.5.2

(Arguably) useful selective name capturing

We contend that there’s only one case to be made for selective name capturing in Clo-

jure macros—the case when you’re forced to embed third-party macros and functions

in your macros that rely on the existence of anaphora. One such macro is the proxy

macro in Clojure’s core libraries, which provides an anaphoric symbol named this

within its body for use therein. We’ll cover the proxy macro in depth in section 9.1, so

there’s no need to discuss it here. But bear in mind that should this macro ever be

embedded within your own macros, you may be forced to use the ~'this pattern.

HYGIENE A hygienic macro is one that doesn’t cause name capturing at macro

expansion time. Clojure macros help to ensure hygiene by namespace-

resolving symbols within the body of syntax-quote at macro-definition time.

As you saw, symbols are expanded into the form user/a-symbol within the

body of syntax-quote. To close this hygienic loop, Clojure also disallows the

definition of qualified locals within the body of a macro. In order to selec-

tively capture names within Clojure macros, you must explicitly do so via the

~'a-symbol pattern.

Clojure prefers that symbols be either declared or bound at macro-definition time.

But using the resolution deferment strategy outlined earlier, you can relax this

requirement for those instances where doing so would be useful.

8.6

Using macros to manage resources

Managing scarce resources or those with a finite lifetime is often viewed as a sweet

spot for macro usage. In Java, such activities are almost always performed using the

try/catch/finally idiom (Bloch 2008), as shown:

try {

// open the resource

}

catch (Exception e) {

// handle any errors

10 Awkwardness is good since it’s a strong signal to make the user aware he is drifting away from the true path to clojure enlightenment. —Christophe Grand

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CHAPTER 8 Macros

}

finally {

// in any case, release the resource

}

We showed in section 1.5.8 that Clojure also has a try/catch/finally form that can

be used in the same way, but like the Java idiom, you must remember to explicitly

close the resource within the finally block. Clojure provides a generic with-open

macro, demonstrated in listing 8.8, that when given a “closeable” object bound to a

name, will automatically call its .close method (assuming that one exists) within a

finally block.

Listing 8.8 An example of with-open

(import [java.io BufferedReader InputStreamReader]

[java.net URL])

(defn joc-www []

(-> "http://joyofclojure.com/hello" URL.

.openStream InputStreamReader. BufferedReader.))

(let [stream (joc-www)]

(with-open [page stream]

Begin IO block

(println (.readLine page))

(print "The stream will now close... "))

Close implicitly

(println "but let's read from it anyway.")

(.readLine stream))

Use illegally after close

; Hello Cleveland

; The stream will now close... but let's read from it anyway.

; java.io.IOException: Stream closed

Not all instances of resources in your own programs will be closeable. In these

instances, we present a generic template for resource allocating macros that can be

used for many cases, shown in the following listing.

Listing 8.9 A more general template for with-open-like macros

(defmacro with-resource [binding close-fn & body]

`(let ~binding

(try

(do ~@body)

(finally

(~close-fn ~(binding 0))))))

(let [stream (joc-www)]

(with-resource [page stream]

#(.close %)

(.readLine page)))

The macro with-resource is generic enough and so generally ubiquitous across dif-

fering flavors (Symbolics Inc.11) to almost be considered a Lisp design pattern. The

11 The spirit of this section was inspired by a similar discussion of “Writing Macros to Surround Code.” If you

can get your hands on the original Symbolics manuals, do so—they contain a wealth of information.

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Putting it all together: macros returning functions

173

macro with-resource differs from with-open in that it does not assume that its

resource is closeable but instead delegates the task of closing the resource to a close-

fn function taken as an argument. One final point is that with-resource avoids the

nesting problem of anaphoric macros because it requires that the resource be named

explicitly a la [stream (joc-www)]. This approach allows for the proper nesting of

with-resource macros; and in fact, the use of named bindings marked by vectors is

ubiquitous and idiomatic in Clojure.

8.7

Putting it all together: macros returning functions

In section 7.1, we introduced Clojure’s constraint facility that uses pre- and post-

condition checks on function arguments and return values respectively to ensure

some assertions about said function. In that section, we talked briefly about how sepa-

rating the constraints from the functions they’re constraining allows you to more flex-

ibly apply different assertion templates based on need and context.

CLOJURE APHORISM Clojure programmers don’t write their apps in Clojure.

They write the language that they use to write their apps in Clojure.

In this section, we’re going to take this idea one step further by introducing a macro

named contract that implements a simple DSL to describe function constraints. For

example, a proposed DSL should be nameable and describe its pre- and post-

conditions in an intuitive way, building a higher-order function that will be used to

apply its constraints later. The following sketches a contract specifying that a function

should take only a positive number and return its value multiplied by 2:

(contract doubler

[x]

(:require

(pos? x))

(:ensure

(= (* 2 x) %)))

The contract’s :require list (Meyer 2000) refers to preconditions, and the :ensure

list the postconditions. Given this description, how would you start to implement a

macro to realize this sketch? If you haven’t already gathered from the section title and

the initial problem statement, the macro must return a function, so we’ll start there

with the following listing.

Listing 8.10 The contract top-level macro

(declare collect-bodies)

(defmacro contract [name & forms]

(list* `fn name (collect-bodies forms)))

The contract macro calls a function collect-bodies that hasn’t been written yet, so

we had to use declare to avoid a compilation error. Hold fast, because we’re going to

implement that necessary function soon. But first, imagine what the form of the

returned function will be when it finally comes out of contract:

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CHAPTER 8 Macros

(fn doubler

([f x]

{:post [(= (* 2 x) %)],

:pre [(pos? x)]}

(f x)))

We also want to allow for the multi-arity function definition form so that the con-

tract can take more than one specification per arity function, each separated by a

vector of symbols. The first step down that path starts with an implementation of

collect-bodies:

(declare build-contract)

(defn collect-bodies [forms]

(for [form (partition 3 forms)]

(build-contract form)))

The primary task of collect-bodies is to build a list of the body portion of the

contract, each partitioned into three segments. These partitions represent the arg-

list, requires, and ensures of the contract, which we’ll then pass along to another

function named build-contract, that will build the arity bodies and corresponding

constraint maps. This is shown next.

Listing 8.11 The contract auxiliary function build-contract

(defn build-contract [c]

(let [args (first c)]

Grab args

(list

Build list...

(into '[f] args)

Include HOF + args

(apply merge

(for [con (rest c)]

(cond (= (first con) :require)

Process requires

(assoc {} :pre (vec (rest con)))

(= (first con) :ensure)

Process ensures

(assoc {} :post (vec (rest con)))

:else (throw (Exception. (str "Unknown tag " (first con)))))))

(list* 'f args))))

Build call to f

The function build-contract is where the heart of contract construction lies, build-

ing the arity bodies that contain constraint maps. The difference is that each body is a

higher-order function that takes an additional function as an argument, which the

arguments are then delegated to. This allows us to compose the contract function

with a constrained function, as shown in the next listing.

Listing 8.12 Composition of contract function and constrained function

(def doubler-contract

Define contract

(contract doubler

[x]

(:require

(pos? x))

(:ensure

(= (* 2 x) %))))

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Putting it all together: macros returning functions

175

(def times2 (partial doubler-contract #(* 2 %)))

Test correct fn

(times2 9)

;=> 18

(def times3 (partial doubler-contract #(* 3 %)))

Test incorrect fn

(times3 9)

; java.lang.AssertionError: Assert failed: (= (* 2 x) %)

As you might expect, times2 fulfills the contract, whereas times3 doesn’t. We could

extend doubler-contract to handle extended arities, as shown here.

Listing 8.13 Contract for multiple-arity functions

(def doubler-contract

(contract doubler

[x]

Define 1-arg contract

(:require

(pos? x))

(:ensure

(= (* 2 x) %))

[x y]

Define 2-arg contract

(:require

(pos? x)

(pos? y))

(:ensure

(= (* 2 (+ x y)) %))))

((partial doubler-

contract #(* 2 (+ %1 %2))) 2 3)

Test a correct fn

;=> 10

((partial doubler-

contract #(+ %1 %1 %2 %2)) 2 3)

Test another correct fn

;=> 10

((partial doubler-contract #(* 3 (+ %1 %2))) 2 3)

Test an incorrect fn

; java.lang.AssertionError:

; Assert failed: (= (* 2 (+ x y)) %)

We could extend the contract to cover any number of expected function arities using

contract, independent of the functions themselves. This provides a nice separation

of the work to be done from the expected work to be done. By using the contract

macro, we’ve provided a way to describe the expectations of a function, including but

not limited to

 The possible types of its inputs and output

 The relationship of the function output to its inputs

 The expected function arities

 The “shape” of the inputs and output

The contract macro could be extended in many complementary ways. For example,

Clojure’s function constraints are verified using logical and—the implications being

that any additional pre- or postcondition works to tighten the requirements. But there

may be times when loosening the constraints on the inputs and tightening them on the

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CHAPTER 8 Macros

output makes more sense. In any case, this section isn’t about the nuances of contracts

programming, and to dig deeper would elude the point that using macros to return

functions is an extremely powerful way to extend the capabilities of Clojure itself.

8.8

Summary

We’ve explored various use cases for macros and given examples of each. Though

instructive to the point under discussion, we also tried to show how macros can be

used to mold Clojure into the language that shortens the gap between your problem

space and solution space. In your own unique programs, you should try to do the

same. But the most important skill that you can learn on your path toward macro mas-

tery is the ability to recognize when to avoid using them. The general answer of course

is whenever, and as often as you can.

In the next chapter, we’ll cover various powerful way to organize and categorize

data types and functions using Clojure’s namespaces, multimethods, types, and

protocols.

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Combining

data and code

This chapter covers

 Namespaces

 Exploring Clojure multimethods with the Universal 

Design Pattern

 Types, protocols, and records

 Putting it all together: a fluent builder for chess moves

Clojure provides powerful features for grouping and partitioning logical units of

code and data. Most logical groupings occur within namespaces, Clojure’s ana-

logue to Java packages. We explore how to build, manipulate, and reason about

them. Also, in this chapter we’ll play with Clojure’s powerful multimethods that

provide polymorphism based on arbitrary dispatch functions. We then uncover

recent additions to Clojure supporting abstraction-oriented programming —types, pro-

tocols, and records. Finally, the chapter concludes with the creation of a fluent

chess-move facility, comparing a Java approach to solving the problem with a Clo-

jure approach.

177

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178

CHAPTER 9 Combining data and code

9.1

Namespaces

Newcomers to Clojure have a propensity to hack away at namespace declarations until

they appear to work. This may work sometimes, but it delays the process of learning

how to leverage namespaces more effectively.

From a high-level perspective, namespaces can be likened to a two-level mapping,

where the first level is a symbol to a namespace containing mappings of symbols to

Vars, as shown in figure 9.1. This conceptual model1 is slightly complicated by the fact

that namespaces can be aliased, but even in these circumstances the model holds true.

In the simplest possible terms, qualified symbols of the form joy.ns/authors

cause a two-level lookup: a symbol joy.ns used to lookup a namespace map and a

symbol authors used to retrieve a Var, as shown in the following listing.

Listing 9.1 Namespace navigation

(in-ns 'joy.ns)

(def authors ["Chouser"])

(in-ns 'your.ns)

(clojure.core/refer 'joy.ns)

joy.ns/authors

;=> ["Chouser"]

(in-ns 'joy.ns)

(def authors ["Chouser" "Fogus"])

(in-ns 'your.ns)

joy.ns/authors

;=> ["Chouser" "Fogus"]

Because a symbolic name refers to a Var in the current namespace or another, it fol-

lows that any referred Var always evaluates to the current value and not the value pres-

ent at referral time.

joy.nc/authors

pre

r fix

fi e

x s

s

jo

j y

o .

y n

. s

n

:use

'authors

'joy.ns

:as

'j

:import

'Date

Figure 9.1 The logical layout of

(var authors)

namespaces. The process to resolve

a Var joy.ns/authors includes a

j/authors

symbolic resolution of the namespace

and the Var name. The result is the

Var itself. Aliases created with :use

java.util.Date

work as expected.

1 As always, we’re trying to keep the level of discussion limited to abstractions rather than implementation

details.

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Namespaces

179

9.1.1

Creating namespaces

There are a number of ways to create a new namespace; each has its advantages and

use cases. The choice of one namespace-creation mechanism over another amounts

to choosing the level of control over the default symbolic mappings.

NS

In idiomatic Clojure source code, you’ll see the ns macro used almost exclusively. By

using the ns macro, you automatically get two sets of symbolic mappings—all classes

in the java.lang package and all of the functions, macros, and special forms in the

clojure.core namespace:

(ns chimp)

(reduce + [1 2 (Integer. 3)])

;=> 6

Using the ns macro creates a namespace if it doesn’t already exist, and switches to that

namespace. The ns macro is intended for use in source code files and not in the REPL,

although there’s nothing preventing it.

IN-NS

Using the in-ns function also imports the java.lang package like ns; but it doesn’t

create any mappings for functions or macros in clojure.core. The in-ns function

also takes an explicit symbol used as the namespace qualifier, as in

(in-ns 'gibbon)

(reduce + [1 2 (Integer. 3)])

; java.lang.Exception: Unable to resolve symbol: reduce in this context

(clojure.core/refer 'clojure.core)

(reduce + [1 2 (Integer. 3)])

;=> 6

The in-ns function is more amenable to REPL experimentation when dealing with

namespaces than ns.

CREATE-NS

The finest level of control for creating namespaces is provided through the create-ns

function, which when called takes a symbol and returns a namespace object:

(def b (create-ns 'bonobo))

b

;=> #<Namespace bonobo>

((ns-map b) 'String)

;=> java.lang.String

The call to create-ns doesn’t switch to the named namespace, but it does create Java

class mappings automatically. When given a namespace object (retrieved using the

find-ns function also), you can manipulate its bindings programmatically using the

functions intern and ns-unmap:

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CHAPTER 9 Combining data and code

(intern b 'x 9)

;=> #'bonobo/x

bonobo/x

;=> 9

In the preceding code, we bound the symbol x to the value 9 in the namespace

bonobo, and then referenced it directly using its qualified name bonobo/x. We can do

the same thing for any type of Var binding:

(intern b 'reduce clojure.core/reduce)

;=> #'bonobo/reduce

(intern b '+ clojure.core/+)

;=> #'bonobo/+

(in-ns 'bonobo)

(reduce + [1 2 3 4 5])

;=> 15

Because only Java class mappings are created by create-ns, you’ll have to intern any

Clojure core functions, as we did with + and reduce. You can even inspect the map-

pings within a namespace programmatically, and likewise remove specific mappings:

(in-ns 'user)

(get (ns-map 'bonobo) 'reduce)

;=> #'bonobo/reduce

(ns-unmap 'bonobo 'reduce) ;=> nil

(get (ns-map 'bonobo) 'reduce)

;=> nil

Finally, you can wipe a namespace using remove-ns:

(remove-ns 'bonobo)

;=> #<Namespace bonobo>

(all-ns)

;=> (#<Namespace clojure.set> #<Namespace clojure.main>

#<Namespace clojure.core> #<Namespace clojure.zip>

#<Namespace chimp> #<Namespace gibbon>

#<Namespace clojure.xml>)

You should be careful when populating namespaces

using create-ns and intern, because they cause

potentially confusing side-effects to occur. Their use is

intended only for advanced techniques, and even then

they should be used cautiously.

9.1.2

Expose only what’s needed

Knowing that namespaces operate as a two-level map-

ping will only get you so far in creating and using them

effectively. You must understand other practical matters

to use namespaces to their fullest. For example, for a

Figure 9.2 Namespace private

directories: the directories layout

given namespace joy.contracts, your directory struc-

for an illustrative joy.contracts

ture could look like that in figure 9.2.

namespace

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181

This directory structure is fairly

straightforward, but there are a couple

items to note. First, though the

namespace is named joy.contracts,

the corresponding Clojure source file is

located in the contracts-lib/src/joy

directory. This is a common technique

in organizing Java and Clojure projects,

where the actual source directories and

files are located in a common src subdi-

Figure 9.3 Namespace private source: the top of

rectory in the main project directory.

the source file for the joy.contracts namespace

The additional files build.xml, pom.xml,

and project.clj correspond to the build scripts for Apache Ant, Maven, and Leiningen,

respectively. These build scripts will know, through either configuration or conven-

tion, that the src directory contains the directories and source files for Clojure

namespaces and not part of the namespace logical layout. If you were to open the con-

tracts.clj file located in contracts-lib/src/joy in your favorite editor, then you might

see something like that shown in figure 9.3.

The file contracts.clj defines the namespace joy.contracts and defines the func-

tion build-contract using the defn- macro. The use of defn- in this way indicates to

Clojure that the build-contract function is private to the joy.contracts

namespace. The defn- macro is provided for convenience and simply attaches privi-

leged metadata to the Var containing the function. You could attach the same

namespace privacy metadata yourself, as shown:

(ns hider.ns)

(defn ^{:private true} answer [] 42)

(ns seeker.ns

(:refer hider.ns))

(answer)

; java.lang.Exception: Unable to resolve symbol: answer in this context

The use of ^{:private true} in this way will also work within a def and a defmacro,

and for these cases it’s required, because there’s no corresponding def- and def-

macro- in Clojure’s core.

HYPHENS/UNDERSCORES If you decide to name your namespaces with

hyphens, à la my-cool-lib, then the corresponding source file must be

named with underscores in place of the hyphens (my_cool_lib.clj).

Because Clojure namespace names are tied to the directory in which they reside, you

can also create a certain directory structure conducive to hiding implementation

details, as seen in figure 9.4.

By creating another subdirectory to contracts-lib/src/joy named impl, you can

effectively hide implementation details for your code. The public-facing API would be

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CHAPTER 9 Combining data and code

located in contracts.clj and the “hidden” implemen-

tation details in impl.clj. Your clients would be

expected to refer only to the elements in con-

tracts.clj, whereas your library could refer to ele-

ments in impl.clj, as shown in figure 9.5.

Of course, nothing’s stopping you from also ref-

erencing the joy.contracts.impl namespace, but

you do so at their own peril. There are never any

guarantees that implementation details will remain

Figure 9.4 Private API directories:

the same shape from one release to the next.

using the folder layout to hide

namespace implementation details

9.1.3

Declarative inclusions and exclusions

When defining namespaces, it’s important to include only the references that are

likely to be used. Clojure prefers a fine-grained Var mapping via a set of directives on

the ns macro: :exclude, :only, :as, :refer-clojure, :import, :use, :load, and

:require.

We’ll describe a namespace named joy.ns-ex first in prose and then using ns and

its directives. In this namespace, we want to exclude the defstruct macro from

Figure 9.5 Private API source: the client-facing API is located in

contracts.clj and the private API in impl.clj.

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183

clojure.core. Next, we want to use everything in clojure.set and clojure.xml

without namespace qualification. Likewise, we wish to use only the functions are and

is from the clojure.test namespace without qualification. We then want to load the

clojure.zip namespace and alias it as z. Finally, we want to import the Java classes

java.util.Date and java.io.File. By providing directives, the problem of

namespace inclusions and exclusions become a declarative matter, as shown:

(ns joy.ns-ex

(:refer-clojure :exclude [defstruct])

(:use (clojure set xml))

(:use [clojure.test :only (are is)])

(:require (clojure [zip :as z]))

(:import (java.util Date)

(java.io File)))

We’ll touch on further uses of namespaces throughout the rest of the book, with an

extensive example explaining their use as JVM class specifications in section 10.3.

AVOID NAKED :USE One point of note that we should mention is that the

(:use (clojure set xml)) statement is considered a promiscuous operation

and therefore discouraged. The :use directive without the :only option pulls

in all of the public Vars in clojure.set and clojure.xml indiscriminately.

Though this practice is useful when incrementally building your code, it

shouldn’t endure into the production environment. When organizing your

code along namespaces, it’s good practice to export and import only those

elements needed.

We now turn our focus to Clojure’s multimethods, a way of defining polymorphic

functions based on the results of arbitrary functions, which will get you halfway toward

a system of polymorphic types.

9.2

Exploring Clojure multimethods 

with the Universal Design Pattern

The most specific event can serve as a general example of a class of events.

—Douglas R. Hofstadter

In Douglas Hofstadter’s Pulitzer prize winning work Gödel, Escher, Bach: An Eternal

Golden Braid, he describes a notion of the Prototype Principle —the tendency of the

human mind to use specific events as models for similar but different events or things.

He presents the idea “that there is generality in the specific” (Hofstadter 1979). Build-

ing on this idea, programmer Steve Yegge coined the term The Universal Design Pattern

(UDP), extrapolating on Hofstadter’s idea (Yegge 2008) and presenting it in terms of

prototypal inheritance (Ungar 1987).

The UDP is built on the notion of a map or map-like object. Though not ground-

breaking, the flexibility in the UDP derives from the fact that each map contains a

reference to a prototype map used as a parent link to inherited fields. You might won-

der how anyone could model a software problem in this way, but we assure you that

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CHAPTER 9 Combining data and code

countless programmers do so every day when they choose JavaScript (Flanagan 2006).

In this section, we’ll implement a subset of Yegge’s UDP and discuss how it might be

used as the basis for abstraction-oriented programming and polymorphism using Clo-

jure’s multimethods and ad hoc hierarchies.

9.2.1

The parts

In addition to the aforementioned prototype reference, the UDP requires a set of sup-

porting functions to operate: beget, get, put, has?, and forget. The entire UDP is

built on these five functions, but we’ll need the first three for this section.

BEGET

The beget function performs a simple task. It takes a map and associates its prototype

reference to another map, returning a new map:

(ns joy.udp

(:refer-clojure :exclude [get]))

(defn beget [o p] (assoc o ::prototype p))

(beget {:sub 0} {:super 1})

;=> {:joy.udp/prototype {:super 1}, :sub 0}

To participate in the UDP, maps must have a :joy.udp/prototype entry.

PUT

The function put takes a key and an associated value and puts them into the supplied

map, overwriting any existing key of the same name:

(def put assoc)

The put function is asymmetric to the functionality of get. The get method retrieves

values anywhere along the prototype chain, whereas put only ever inserts at the level

of the supplied map.

GET

Because of the presence of the prototype link, get requires more than a simple one-

level lookup. Instead, whenever a value isn’t found in a given map, the prototype

chain is followed until the end:

(defn get [m k]

(when m

(if-let [[_ v] (find m k)]

v

(recur (::prototype m) k))))

(get (beget {:sub 0} {:super 1})

:super)

;=> 1

We don’t explicitly handle the case of “removed ”properties, but instead treat them

like any other associated value. This is fine because the “not found” value of nil is

falsey. Most of the time, it’s sufficient to rely on the fact that looking up a nonexistent

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185

key will return nil. But in cases where you want to allow users of your functions to

store any value at all, including nil, you’ll have to be careful to distinguish nil from

“not found,” and the find function is the best way to do this.

9.2.2

Usage

Using only beget, put, and get, you can leverage the UDP in some simple, yet power-

ful ways. Assume that at birth cats like dogs and only learn to despise them when

goaded. Morris the cat has spent most of his life liking 9-Lives cat food and dogs, until

the day comes when a surly Shih Tzu leaves him battered and bruised. We can model

this unfortunate story as shown:

(def cat {:likes-dogs true, :ocd-bathing true})

(def morris (beget {:likes-9lives true} cat))

(def post-traumatic-morris (beget {:likes-dogs nil} morris))

(get cat :likes-dogs)

;=> true

(get morris :likes-dogs)

;=> true

(get post-traumatic-morris :likes-dogs)

;=> nil

The map post-traumatic-morris is like the old morris in every way except for the

fact that he has learned to hate dogs. Modeling cat and dog societal woes is interesting

but far from the only use case for the UDP, as you’ll see next.

NO NOTION OF SELF

Our implementation of the UDP contains no notion of self-awareness via an implicit

this or self reference. Though adding such a feature would probably be possible,

we’ve intentionally excluded it in order to draw a clear separation between the proto-

types and the functions that work on them (Keene 1989). A better solution, and one

that follows in line with a deeper Clojure philosophy, would be to access, use, and

manipulate these prototypes using Clojure’s multimethods.

9.2.3

Multimethods to the rescue

Adding behaviors to the UDP can be accomplished easily using Clojure’s multimethod

facilities. Multimethods provide a way to perform function polymorphism based on the

result of an arbitrary dispatch function. Coupled with our earlier UDP implementa-

tion, we can implement a prototypal object system with differential inheritance similar

to (although not as elegant as) that in the Io language (Dekorte Io). First, we’ll need

to define a multimethod compiler that dispatches on a key :os:

(defmulti compiler :os)

(defmethod compiler ::unix [m] (get m :c-compiler))

(defmethod compiler ::osx [m] (get m :c-compiler))

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CHAPTER 9 Combining data and code

The multimethod compiler describes a simple scenario: if the function compiler is

called with a prototype map, then the map is queried for an element :os, which has

methods defined on the results for either ::unix or ::osx. We’ll create some proto-

type maps to exercise compiler:

(def clone (partial beget {}))

(def unix {:os ::unix, :c-compiler "cc", :home "/home", :dev "/dev"})

(def osx (-> (clone unix)

(put :os ::osx)

(put :c-compiler "gcc")

(put :home "/Users")))

(compiler unix)

;=> "cc"

(compiler osx)

;=> "gcc"

That’s all there is (Foote 2003) to creating behaviors that work against the specific

“type” of a prototype map. But a problem of inherited behaviors still persists. Because

our implementation of the UDP separates state from behavior, there’s seemingly no

way to associate inherited behaviors. But as we’ll now show, Clojure does provide a way

to define ad hoc hierarchies that we can use to simulate inheritance within our model.

9.2.4

Ad hoc hierarchies for inherited behaviors

Based on the layout of the unix and osx prototype maps, the property :home is over-

ridden in osx. We could again duplicate the use of get within each method defined

(as in compiler), but instead we prefer to say that the lookup of :home should be a

derived function:

(defmulti home :os)

(defmethod home ::unix [m] (get m :home))

(home unix)

;=> "/home"

(home osx)

; java.lang.IllegalArgumentException:

; No method in multimethod 'home' for dispatch value: :user/osx

Clojure allows you to define a relationship stating “::osx is a ::unix” and have the

derived function take over the lookup behavior using Clojure’s derive function:

(derive ::osx ::unix)

Now the home function works:

(home osx)

;=> "/Users"

You can query the derivation hierarchy using the functions parents, ancestors,

descendants, and isa? as shown:

(parents ::osx)

;=> #{:user/unix}

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187

(ancestors ::osx)

;=> #{:user/unix}

(descendants ::unix)

;=> #{:user/osx}

(isa? ::osx ::unix)

;=> true

(isa? ::unix ::osx)

;=> false

The result of the isa? function defines how multimethods dispatch. In the absence of

a derivation hierarchy, isa? can be likened to pure equality, but with it traverses a der-

ivation graph.

9.2.5

Resolving conflict in hierarchies

What if we interject another ancestor into the hierarchy for ::osx and want to again

call the home method? Observe the following:

(derive ::osx ::bsd)

(defmethod home ::bsd [m] "/home")

(home osx)

; java.lang.IllegalArgumentException: Multiple methods in multimethod

; 'home' match dispatch value: :user/osx -> :user/unix and

; :user/bsd, and neither is preferred

As shown in figure 9.6, ::osx derives from both ::bsd and ::unix, so there’s no way

to decide which method to dispatch, because they’re both at the same level in the der-

ivation hierarchy. Fortunately, Clojure provides a way to assign favor to one method

over another using the function prefer-method:

(prefer-method home ::unix ::bsd)

(home osx)

;=> "/Users"

In this case, we used prefer-method to explicitly state that for the multimethod home,

we prefer the method associated with the dispatch value ::unix over the one for

::bsd, as illustrated in figure 9.5. As you’ll recall, the home method for ::unix explic-

itly used get to traverse the prototype chain, which is the preferred behavior.

As you might expect, removing the home method for the ::bsd dispatch value

using remove-method will remove the preferential lookup for ::osx:

(remove-method home ::bsd)

(derive ::osx ::unix)

(home osx)

::unix

;=> "/Users"

::osx

All of these functions manipulate and

Figure 9.6 Hierarchy conflict:

most languages allowing type

operate off of the global hierarchy map

(derive ::osx ::bsd)

derivations use a built-in

directly. If you prefer to reduce these

::unix

::bsd

conflict-resolution strategy. In

potentially confusing side-effects, then

the case of CLOS, it’s fully

::osx

you can define a derivation hierarchy

customizable. Clojure requires

(home osx)

conflicts to be resolved with

using make-hierarchy and derive:

???

prefer-method.

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CHAPTER 9 Combining data and code

(derive (make-hierarchy) ::osx ::unix)

;=> {:parents {:user/osx #{:user/unix}},

:ancestors {:user/osx #{:user/unix}},

:descendants {:user/unix #{:user/osx}}}

Once you have a separate hierarchy in hand, you can provide it to defmulti to specify

the derivation context, thus preserving the global hierarchy map.

9.2.6

Arbitrary dispatch for true maximum power

Until now, we’ve only exercised multimethods using a single privileged :os property,

but this doesn’t accentuate their true power. Instead, multimethods are fully open and

can dispatch on the result of an arbitrary function, even one that can pull apart and/

or combine its inputs into any form:

(defmulti compile-cmd (juxt :os compiler))

(defmethod compile-cmd [::osx "gcc"] [m]

(str "/usr/bin/" (get m :c-compiler)))

(defmethod compile-cmd :default [m]

(str "Unsure where to locate " (get m :c-compiler)))

The dispatch values for the new compile-cmd methods are vectors composed of the

results of looking up the :os key and calling the compiler function defined earlier.

You can now observe what happens when compile-cmd is called:

(compile-cmd osx)

;=> "/usr/bin/gcc"

(compile-cmd unix)

;=> "Unsure where to locate cc"

Using multimethods and the UDP is an interesting way to build abstractions. Multi-

methods and ad hoc hierarchies are open systems, allowing for polymorphic dispatch

based on arbitrary functions. Clojure also provides a simpler model for creating

abstractions and gaining the benefits of polymorphism—types, protocols, and

records—which we’ll cover next.

The handy-dandy juxt function

The juxt function is highly useful in defining multimethod dispatch functions. In a

nutshell, juxt takes a bunch of functions and composes them into a function return-

ing a vector of its argument(s) applied to each given function, as shown:

(def each-math (juxt + * - /))

(each-math 2 3)

;=> [5 6 -1 2/3]

((juxt take drop) 3 (range 9))

[(0 1 2) (3 4 5 6 7 8)]

Having a convenient and succinct way to build vectors of applied functions is powerful

for defining understandable multimethods—although that’s not the limit of juxt’s

usefulness.

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189

9.3

Types, protocols, and records

We showed in the previous section that Clojure multimethods provide a way to

achieve runtime polymorphism based on arbitrary dispatch functions. Though

extremely powerful, multimethods are sometimes less than ideal. Interposing a dis-

patch function into the polymorphism machinery isn’t always conducive to raw speed.

Likewise, dispatching on an arbitrary function is often overkill. Therefore, Clojure

provides facilities for creating logically grouped polymorphic functions that are both

simple and performant—types, records, and protocols. We’ll delve into these topics in

this section and introduce the concept of abstraction-oriented programming, predi-

cated on the creation of logical groupings. But first, we’ll discuss the simplest of the

three topics, records, which you might recognize.

9.3.1

Records

Using maps as data objects is perfectly acceptable and has several lovely features.

Chief among these is that maps require no declaration of any sort: you just use literal

syntax to build them right on the spot. We showed this in section 7.2 when we built an

object like this:

{:val 5, :l nil, :r nil}

This is handy but is missing things that are often desirable, the most significant of

which is a type of its own. The object constructed here is some kind of map, but it

isn’t, as far as Clojure is concerned, a TreeNode. That means that when used in its sim-

ple form as we did here, there’s no clean way2 to determine whether any particular

map is a TreeNode or not.

In such circumstances, records become a compelling3 solution. You define a

record type with a defrecord form. For example, a defrecord for TreeNode looks like

this:

(defrecord TreeNode [val l r])

This creates a new Java class with a constructor that takes a value for each of the fields

listed. It also imports that class into your current namespace so you can easily use it to

create new instances.

Here’s how to create an instance of the TreeNode record:

(TreeNode. 5 nil nil)

;=> #:user.TreeNode{:val 5, :l nil, :r nil}

2

You could test a map for the existence of the keys :val, :l, and :r, a sort of duck-typing but on fields instead

of methods. But because there exists a real possibility than some other kind of object may happen to have

these keys but use them in a different way, undesirable complexity and/or unexpected behavior is likely. For-

tunately, you can mitigate this risk by using namespace-qualified keywords. Despite the general agreement of

experts that ducks are Kosher, we’d definitely classify this particular duck as unclean.

3

There was a pre-Clojure 1.2 convention of attaching :type metadata to an object, which can be looked up

with the type function, but this approach is rarely if ever needed moving forward.

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CHAPTER 9 Combining data and code

Explicit importing of defrecord and deftype classes

It’s important to note that when you define a defrecord and deftype, corresponding

classes are generated. These classes are automatically imported into the same

namespace where the defrecord and deftype declarations occur, but not in any

other namespace. Instead, you must explicitly import defrecord and deftype

classes using the import function or :import namespace declaration:

(ns my-cool-ns

(:import joy.udp.TreeNode))

Loading a namespace via :require or :use won’t be enough to import defrecord

and deftype classes.

The use of defrecord buys you several important benefits. First of all, it provides a

simple and specific idiom for documenting the expected fields of the object. But it

also delivers several important performance improvements. A record will be created

more quickly, consume less memory, and look up keys in itself more quickly than the

equivalent array map or hash map. Data types can also store primitive values (byte, int,

long, and so on), which take up considerably less memory than the equivalent boxed

objects (Byte, Integer, Long, and so on) supported by other collection types.

That’s a lot of benefit, so what does it cost you? The first cost we already men-

tioned—you must define the record type before using it. Another is that currently,

records aren’t printed in a way that the Clojure reader can read, unlike hash maps.

This can be a problem if you’re using Clojure’s print functions to save off or transmit

data. Here’s what it looks like if we try, successfully, with a literal map and then again,

unsuccessfully, with a record:

The downfall of defstructs

Clojure provides a defstruct mechanism, which can be viewed as a way to define a

map that acts as an ad hoc class mechanism. These structs defined a set of keys

that were required to exist in the map and could therefore not be removed via dissoc.

With the advent of defrecord, the need for structs has been nearly eliminated, and

therefore structs aren’t covered in this book. But if you have a code base reliant on

structs, then a record can replace them with minimal code changes, as highlighted

here:

(defn change-age [p] (assoc p :age 286))

(defstruct person :fname :lname)

(change-age (struct person "Immanuel" "Kant"))

;=> {:fname "Immanuel", :lname "Kant", :age 286}

(defrecord Person [fname lname])

(change-age (Person. "Immanuel" "Kant"))

;=> #:user.Person{:fname "Immanuel", :lname "Kant", :age 286}

Note that the change-age function works with either structs or records—no change

is required. Only the definition and the mechanism of instantiation need to be

updated.

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Types, protocols, and records

191

(read-string (pr-str {:val 5, :l nil, :r nil}))

;=> {:val 5, :l nil, :r nil}

(read-string (pr-str (TreeNode. 5 nil nil)))

; java.lang.RuntimeException: java.lang.Exception: No dispatch macro for:

This may change eventually, but there are some tricky problems yet to be worked out

before records can be printed and read back in.

Other noteworthy differences between maps and records include

 Records, unlike maps, can’t serve as functions.

 Records are never equal to maps with the same key/value mappings.

You still look up values in records by doing (:keyword obj); it’s just that if obj is a

record, this code will run dramatically faster. By the way, that means destructuring will

still work as well. Records support metadata using with-meta and meta just like other

Clojure collections, and you can even redefine a record if desired to have different

fields giving you the compiled performance of Java dynamically. All of these together

mean you can build a lot of code on top of simple hash-map objects and then make

minimal changes to switch to using records instead, gaining all the performance ben-

efits we already covered.

You should understand records well enough to be able to reimplement the persis-

tent binary tree from chapter 5 using defrecord instead of maps. This is shown in the

following listing. Note that we had to add the defrecord and change the expressions

in xconj where objects are created, but the xseq function is defined identically to how

it was before.

Listing 9.2 Persistent binary tree built of records

(defrecord TreeNode [val l r])

Define record type

(defn xconj [t v]

Add to tree

(cond

(nil? t) (TreeNode. v nil nil)

(< v (:val t)) (TreeNode. (:val t) (xconj (:l t) v) (:r t))

:else (TreeNode. (:val t) (:l t) (xconj (:r t) v))))

(defn xseq [t]

Convert trees

(when t

to seqs

(concat (xseq (:l t)) [(:val t)] (xseq (:r t)))))

(def sample-tree (reduce xconj nil [3 5 2 4 6]))

Try it all out

(xseq sample-tree)

;=> (2 3 4 5 6)

You can assoc and dissoc any key you want—adding keys that weren’t defined in the

defrecord works, though they have the performance of a regular map. Perhaps more

surprisingly, dissocing a key given in the record works but returns a regular map

rather than a record. In this example, note that the return value is printed as a plain

map, not with the #:user.TreeNode prefix of a record:

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CHAPTER 9 Combining data and code

(dissoc (TreeNodePlus 5 nil nil) :l)

;=> {:val 5, :r nil}

A final benefit of records is how well they integrate with Clojure protocols. But to fully

understand how they relate, we must first explore what protocols are.

9.3.2

Protocols

The establishment of protocols ... creates an obvious way for two people who are

not directly communicating to structure independently developed code so that it

works in a manner that remains coherent when such code is later combined.

—Kent M. Pitman (Pitman 2001)

A protocol in Clojure is simply a set of function signatures, each with at least one param-

eter, that are given a collective name. They fulfill a role somewhat like Java interfaces

or C++ pure virtual classes—a class that claims to implement a particular protocol

should provide specialized implementations of each of the functions in that protocol.

Then, when any of those functions is called, the appropriate implementation is poly-

morphic on the type of the first parameter, just like Java. In fact, the first parameter to

a protocol function corresponds to the target object (the thing to the left of the dot

for a method call used in Java source) of a method in object-oriented parlance.

For example, consider what collections such as stacks (First In, Last Out: FILO)

and queues (First In, First Out: FIFO) have in common. Each has a simple function for

inserting a thing (call it push), a simple function for removing a thing (pop), and usu-

ally a function to see what would be removed if you removed a thing (peek). What we

just gave you was an informal description of a protocol; all that’s missing is the name.

We can replace the changing third item of the acronym with an X and call objects that

provide these functions FIXO. Note that besides stacks and queues, FIXO could

include priority queues, pipes, and other critters.

So now let’s look at that informal description rewritten as a formal Clojure

definition:

(defprotocol FIXO

(fixo-push [fixo value])

(fixo-pop [fixo])

(fixo-peek [fixo]))

...and that’s it. The only reason we prefixed the function names with fixo- is so that

they don’t conflict with Clojure’s built-in functions.4 Besides that, it’s hard to imagine

how there could be much less ceremony, isn’t it?

But in order for a protocol to do any good, something must implement it. Proto-

cols are implemented using one of the extend forms: extend, extend-type,5 or

extend-protocol. Each of these does essentially the same thing, but extend-type and

4

It would be better to fix this problem by defining FIXO in a new namespace and excluding from it the simi-

larly named clojure.core functions, except this would be a distraction from the point of this section. We’ll dis-

cuss interesting interactions between namespaces and protocols later in this chapter.

5

Records are a specialized kind of data type, so extend-type is used for both. We’ll look at data types later

in this section.

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193

extend-protocol are convenience macros for when you want to provide multiple

functions for a given type. For example, the binary TreeNode from listing 9.2 is a

record, so if we want to extend it, extend-type would be most convenient. Because

TreeNode already has a function xconj that works just like push should, we’ll start by

implementing that:

(extend-type TreeNode

FIXO

(fixo-push [node value]

(xconj node value)))

(xseq (fixo-push sample-tree 5/2))

;=> (2 5/2 3 4 5 6)

The first argument to extend-type is the class or interface that the entire rest of the

form will be extending. Following the type name are one or more blocks, each starting

with the name of the protocol to be extended and followed by one or more functions

from that protocol to implement. So in the preceding example, we’re implementing a

single function fixo-push for TreeNode objects, and we call the existing xconj func-

tion. Got it? The reason this is better than simply defining a regular function named

fixo-push is that protocols allow for polymorphism. That same function can have a

different implementation for a different kind of object. Clojure vectors can act like

stacks by extending FIXO to vectors:

(extend-type clojure.lang.IPersistentVector

FIXO

(fixo-push [vector value]

(conj vector value)))

(fixo-push [2 3 4 5 6] 5/2)

;=> [2 3 4 5 6 5/2]

Here we’re extending FIXO to an interface instead of a concrete class. This means that

fixo-push is now defined for all classes that inherit from IPersistentVector. Note

that we can now call fixo-push with either a vector or a TreeNode, and the appropri-

ate function implementation is invoked.

Clojure-style mixins

As you proceed through this section, you’ll notice that we extend the FIXO protocol’s

fixo-push function in isolation. This works fine for our purposes, but you might want

to take note of the implications of this approach. Consider the following:

(use 'clojure.string)

(defprotocol StringOps (rev [s]) (upp [s]))

(extend-type String

StringOps

(rev [s] (clojure.string/reverse s)))

(rev "Works")

;=> "skroW"

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CHAPTER 9 Combining data and code

(continued)

Defining the StringOps protocol and extending its rev function to String seems to

work fine. But observe what happens when the protocol is again extended to cover

the remaining upp function:

(extend-type String

StringOps

(upp [s] (clojure.string/upper-case s)))

(upp "Works")

;=> "WORKS"

(rev "Works?")

; IllegalArgumentException No implementation of method: :rev

; of protocol: #'user/StringOps found for

; class: java.lang.String

The reason for this exception is that for a protocol to be fully populated (all of its func-

tions callable), it must be extended fully, per individual type. Protocol extension is at

the granularity of the entire protocol and not at a per-function basis. This behavior

seems antithetical to the common notion of a mixin—granules of discrete functional-

ity that can be “mixed into” existing classes, modules, and so on. Clojure too has

mixins, but it takes a slightly different approach:

(def rev-mixin {:rev clojure.string/reverse})

(def upp-mixin {:upp (fn [this] (.toUpperCase this))})

(def fully-mixed (merge upp-mixin rev-mixin))

(extend String StringOps fully-mixed)

(-> "Works" upp rev)

;=> SKROW

Mixins in Clojure refer to the creation of discrete maps containing protocol function

implementations that are combined in such a way as to create a complete implemen-

tation of a protocol. Once mixed together (as in the Var fully-mixed), only then are

types extended to protocols. As with many of Clojure’s features, mixins and protocol

extension are fully open.

What we’ve just done is impossible with Java interfaces or C++ classes, at least in the

order we did it. With either of those languages, the concrete type (such as TreeNode

or vector) must name at the time it’s defined all the interfaces or classes it’s going to

implement. Here we went the other way around—both TreeNode and vectors were

defined before the FIXO protocol even existed, and we easily extended FIXO to each of

them. This matters in the real world because the concrete types and even the protocol

could be provided by third-party libraries—possibly even different third-party librar-

ies—and we could still match them up, provide implementations for the appropriate

functions, and get back to work. All this without any adapters, wrappers, monkey-

patching, or other incidental complexity getting in the way. In fact, Clojure polymor-

phism lives in the protocol functions, not in the classes, as shown in figure 9.7.

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195

Monkey Patching

Wrapping

Pr

P o

r to

t co

c ls

TreeNode

TreeNodeWrapper

FIXO

fixo-pop

(fixo-pop TreeNode)

fixo-pop

fixo-push

(fixo-pop Vector)

fixo-push

fixo-peek

(fixo-push TreeNode)

(fixo-push Vector)

TreeNode

fixo-peek

(fixo-peek TreeNode)

(fixo-peek Vector)

Figure 9.7 As opposed to the notion of monkey-patching and wrapping,

the polymorphism in Clojure resides in the functions themselves and not

in the classes worked with.

You can even extend a protocol to nil itself. You’d be forgiven for not immediately

seeing why you’d want to do this; but consider how TreeNode implements fixo-push,

and yet the sample-tree we’re using was built using xconj instead. Trying to build up

a tree the same way with fixo-push runs into a problem:

(reduce fixo-push nil [3 5 2 4 6 0])

; java.lang.IllegalArgumentException:

; No implementation of method: :fixo-push

; of protocol: #'user/FIXO found for class: nil

The xconj implementation specifically handled the initial nil case, but because pro-

tocol methods dispatch on the first argument, we need special support from extend to

get fixo-push to behave similarly. This is done by extending a protocol to the value

nil, like this:

(extend-type nil

FIXO

(fixo-push [t v]

(TreeNode. v nil nil)))

(xseq (reduce fixo-push nil [3 5 2 4 6 0]))

;=> (0 2 3 4 5 6)

All the options and arrangements of code allowed by extend can be disorienting, but

one thing you can keep firmly in mind is that extend is always about a protocol. Each

method listed in an extend form is implementing an intersection between a protocol

and something else. That something else can be a concrete class, an interface, a

record type, or even nil, but it’s always being connected to a protocol.

See the following listing for complete implementations of FIXO for TreeNode and

vectors. As mentioned in the sidebar, in order for the FIXO protocol to be fully realiz-

able, each of its functions should be mixed in. But you might not always require that a

protocol be fully realizable.

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CHAPTER 9 Combining data and code

Listing 9.3 Complete implementations of FIXO for TreeNode and vector

(extend-type TreeNode

FIXO

Delegate 

(fixo-push [node value]

to xconj

(xconj node value))

(fixo-peek [node]

Walk down left nodes

(if (:l node)

to find smallest

(recur (:l node))

(:val node)))

Build new path down

(fixo-pop [node]

left to removed item

(if (:l node)

(TreeNode. (:val node) (fixo-pop (:l node)) (:r node))

(:r node))))

(extend-type clojure.lang.IPersistentVector

FIXO

fixo-push is

(fixo-push [vector value]

vector's conj

(conj vector value))

(fixo-peek [vector]

peek is peek

(peek vector))

(fixo-pop [vector]

pop is pop

(pop vector)))

If you’ve done six impossible things this morning, why not round it off with breakfast

at Milliways, the Restaurant at the End of the Universe?

—Douglas Adams

Each of the function bodies in the previous example have either had no code in com-

mon with each other, or called out to another function such as xconj for implementa-

tion details that they have in common. These techniques work well when there’s a low

level of commonality between the methods being implemented, but sometimes you

have many methods of a protocol or even whole protocol implementations that you

want to extend to multiple classes. In these cases, some languages would encourage

you to create a base class that implements some or all of the methods and then inherit

from that. Clojure has a different approach.

SHARING METHOD IMPLEMENTATIONS

Clojure doesn’t encourage implementation inheritance, so although it’s possible to

inherit from concrete classes as needed for Java interoperability,6 there’s no way to use

extend to provide a concrete implementation and then build another class on top of

that. There are important reasons why Clojure intentionally avoids this, but regardless

of the reasons, we’re left with the question of how best to avoid repeating code when

similar objects implement the same protocol method.

The simplest solution is to write a regular function that builds on the protocol’s

methods. For example, Clojure’s own into takes a collection and uses the conj imple-

mentation provided by the collection. We can write a similar function for FIXO objects

like this:

6 Mechanisms that support something like Java-style implementation inheritance include gen-class, proxy,

and extending protocol methods to Java abstract classes and interfaces.

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197

(defn fixo-into [c1 c2]

(reduce fixo-push c1 c2))

(xseq (fixo-into (TreeNode. 5 nil nil) [2 4 6 7]))

;=> (2 4 5 6 7)

(seq (fixo-into [5] [2 4 6 7]))

;=> (5 2 4 6 7)

But this is only an option when your function can be defined entirely in terms of the

protocol’s methods. If this isn’t the case, you may need the more nuanced solution

provided by the extend function. We mentioned it earlier but so far have only given

examples of a macro built on top of it, extend-type. Though this and extend-

protocol are frequently the most convenient way to implement protocol methods,

they don’t provide a natural way to mix in method implementations. The extend func-

tion takes a map for each protocol you want to implement, and you can build that

map however you’d like, including by merging in implementations that are already

defined. In the following listing, you should note how a FIXO implementation could

be defined early using a map and extended to a protocol/record type later (while still

maintaining every benefit of using the original map).

Listing 9.4 Using a map to extend FIXO to TreeNode

(def tree-node-fixo

Define map of names

{:fixo-push (fn [node value]

to functions

(xconj node value))

:fixo-peek (fn [node]

(if (:l node)

(recur (:l node))

(:val node)))

:fixo-pop (fn [node]

(if (:l node)

(TreeNode. (:val node) (fixo-pop (:l node)) (:r node))

(:r node)))})

Extend protocol

(extend TreeNode FIXO tree-node-fixo)

using map

(xseq (fixo-into (TreeNode. 5 nil nil) [2 4 6 7]))

;=> (2 4 5 6 7)

These record objects and the way protocols can be extended to them result in rather

differently shaped code than the objects built out of closures that we showed in sec-

tion 7.2. Often this ability to define the data and implementation separately is desir-

able, but you’re likely to find yourself occasionally in a circumstance where closures

may feel like a better fit than records, and yet you want to extend a protocol or inter-

face, not just provide ad hoc method names as in section 7.2.

REIFY

The reify macro brings together all the power of function closures and all the perfor-

mance and protocol participation of extend into a single form. For example, say you

want a stack-like FIXO that’s constrained to a certain fixed size. Any attempt to push

items onto one of these fixed-fixos when it’s already full will fail, and an unchanged

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CHAPTER 9 Combining data and code

object will be returned. The wrinkle in the requirements that makes reify a reason-

able option is that you’ll want this size limit to be configurable. Thus you’ll need a

constructor or factory function, shown next, that takes the size limit and returns an

object that will obey that limit.

Listing 9.5 Size-limited stack FIXO using reify

(defn fixed-fixo

([limit] (fixed-fixo limit []))

([limit vector]

(reify FIXO

(fixo-push [this value]

(if (< (count vector) limit)

(fixed-fixo limit (conj vector value))

this))

(fixo-peek [_]

(peek vector))

(fixo-pop [_]

(pop vector)))))

Just like the extend forms, reify has method arglists that include the object itself. It’s

idiomatic to use name the argument this in methods where you need to use it and _

in methods where you ignore its value. But both these conventions should only be fol-

lowed where natural.

NAMESPACED METHODS

A rough analogy can be drawn between protocols and Java interfaces.7 We’ve noted

some of the differences already, but it can be a useful analogy nonetheless. In such a

comparison, where record types are concrete classes, you might see that Java packages

and C++ namespaces are each like Clojure namespaces. It’s normal in all three of

these environments for the interface and the class to each be in a namespace, and not

necessarily the same one. For example, probably few readers were surprised to see

that when we made the class IPersistentVector extend the protocol user/FIXO, they

were each from a different namespace or package.

One way this analogy breaks down is that methods of the protocol itself are

namespaced in a way that Java and C++ interfaces aren’t. In those languages, all meth-

ods of a class share the same effective namespace, regardless of interfaces they’re

implementing. In Clojure, the methods always use the same namespace as the proto-

col itself, which means a record or type can extend (via extend, extend-type, and so

on) identically named methods of two different protocols without any ambiguity. This

is a subtle feature, but it allows you to avoid a whole category of issues that can come

up when trying to combine third-party libraries into a single codebase.

Note that because the methods share the namespace of their protocol, you can’t

have identically named methods in two different protocols if those protocols are in

the same namespace. Because both are under the control of the same person, it’s easy

7 Those of you familiar with Haskell might recognize analogies to its typeclasses in our discussion.

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Types, protocols, and records

199

to resolve this by moving one of the protocols to a different namespace or using more

specific method names.

METHOD IMPLEMENTATIONS IN DEFRECORD

We’ve already shown how both protocols and interfaces can be extended to record

types using the various extend forms, but there’s another way to achieve similar

results. Protocol and interface method implementations can be written directly inside

a defrecord form, which ends up looking like the following.

Listing 9.6 Method implementations in defrecord

(defrecord TreeNode [val l r]

FIXO

Implement FIXO

(fixo-push [t v]

methods inline

(if (< v val)

(TreeNode. val (fixo-push l v) r)

(TreeNode. val l (fixo-push r v))))

(fixo-peek [t]

(if l

(fixo-peek l)

Call method instead

val))

of using recur

(fixo-pop [t]

(if l

(TreeNode. val (fixo-pop l) r)

r)))

(def sample-tree2 (reduce fixo-push (TreeNode. 3 nil nil) [5 2 4 6]))

(xseq sample-tree2)

;=> (2 3 4 5 6)

This isn’t only more convenient in many cases, but it can also produce dramatically

faster code. Calling a protocol method like fixo-peek on a record type that imple-

ments it inline can be several times faster than calling the same method on an object

that implements it via an extend form. Also note that the fields of the object are now

available as locals—we use val instead of (:val t).

Polymorphism and recur

Throughout this section, we’ve implemented the fixo-peek function using different

methodologies, but a more subtle difference is worth noting. The first implementation

uses recur for its recursive call as shown:

(fixo-peek [node]

(if (:l node)

(recur (:l node))

(:val node)))

Because of the nature of recur, the first implementation of fixo-peek isn’t polymor-

phic on the recursive call. But the second version of fixo-peek uses a different

approach:

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CHAPTER 9 Combining data and code

(continued)

(fixo-peek [t]

(if l

(fixo-peek l)

val))

You’ll notice that the recursive call in the second implementation is direct (mundane)

and as a result is polymorphic. In the course of writing your own programs, this dif-

ference will probably not cause issues, but it’s worth storing in the back of your mind.

Putting method definitions inside the defrecord form also allows you to implement

Java interfaces and extend java.lang.Object, which isn’t possible using any extend

form. Because interface methods can accept and return primitive values as well as

boxed objects, implementations of these in defrecord can also support primitives.

This is important for interoperability and can provide ultimate performance parity

with Java code.

We do need to note one detail of these inline method definitions in relation to

recur. Specifically, uses of recur in these definitions can’t provide a new target object:

the initial argument will get the same value as the initial (non-recur) call to the

method. For example, fixo-push takes args t and v, so if it used recur, only a single

parameter would be given: the new value for the v arg.

9.3.3

Building from a more primitive base with deftype

You may have noticed we’ve been using our own function xseq throughout the exam-

ples in this section, instead of Clojure’s seq. This shouldn’t be necessary, as Clojure

provides an ISeqable interface that its seq function can use—all we need to do is to

have our own type implement ISeqable. But an attempt to do this with defrecord is

doomed:

(defrecord InfiniteConstant [i]

clojure.lang.ISeq

(seq [this]

(lazy-seq (cons i (seq this)))))

; java.lang.ClassFormatError: Duplicate method

; name&signature in class file user/InfiniteConstant

This is because record types are maps and implement everything maps should—seq

along with assoc, dissoc, get, and so forth. Because these are provided for us, we

can’t implement them again ourselves, and thus the preceding exception. For the rare

case where you’re building your own data structure instead of just creating applica-

tion-level record types, Clojure provides a lower-level deftype construct that’s similar

to defrecord but doesn’t implement anything at all, so implementing seq won’t con-

flict with anything:

(deftype InfiniteConstant [i]

clojure.lang.ISeq

(seq [this]

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Types, protocols, and records

201

(lazy-seq (cons i (seq this)))))

(take 3 (InfiniteConstant. 5))

;=> (5 5 5)

But that also means that keyword lookups, assoc, dissoc, and so on will remain unim-

plemented unless we implement them ourselves:

(:i (InfiniteConstant. 5))

;=> nil

The fields we declared are still public and accessible (although you should try to avoid

naming them the same at the methods in java.lang.Object); they just require nor-

mal Java interop forms to get at them:

(.i (InfiniteConstant. 5))

;=> 5

With all that in mind, the following listing is a final implementation of TreeNode using

deftype, which lets us implement not only ISeq so that we can use seq instead of

xseq, but also IPersistentStack so we can use peek, pop, and conj as well as the

fixo- versions.

Listing 9.7 Implementing map interfaces with deftype

(deftype TreeNode [val l r]

FIXO

Implement FIXO

(fixo-push [_ v]

methods inline

(if (< v val)

(TreeNode. val (fixo-push l v) r)

(TreeNode. val l (fixo-push r v))))

(fixo-peek [_]

(if l

(fixo-peek l)

Call method instead

val))

of using recur

(fixo-pop [_]

(if l

(TreeNode. val (fixo-pop l) r)

r))

clojure.lang.IPersistentStack

Implement interfaces

(cons [this v] (fixo-push this v))

(peek [this] (fixo-peek this))

(pop [this] (fixo-pop this))

clojure.lang.Seqable

(seq [t]

(concat (seq l) [val] (seq r))))

(extend-type nil

FIXO

(fixo-push [t v]

Redefine to use

(TreeNode. v nil nil)))

new TreeNode

(def sample-tree2 (into (TreeNode. 3 nil nil) [5 2 4 6]))

(seq sample-tree2)

;=> (2 3 4 5 6)

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CHAPTER 9 Combining data and code

One final note about deftype—it’s the one mechanism by which Clojure lets you cre-

ate classes with volatile and mutable fields. We won’t go into it here because using

such classes is almost never the right solution. Only when you’ve learned how Clojure

approaches identity and state, how to use reference types, what it means for a field to

be volatile, and all the pitfalls related to that, should you even consider creating

classes with mutable fields. By then, you’ll have no problem understanding the official

docs for deftype, and you won’t need any help from us.

None of the examples we’ve shown in this section come close to the flexibility of

multimethods. All protocol methods dispatch on just the type of the first argument.

This is because that’s what Java is good at doing quickly, and in many cases it’s all the

polymorphism that’s needed. Clojure once again takes the practical route and makes

the highest-performance mechanisms available via protocols, while providing more

dynamic behavior than Java does and leaving multimethods on the table for when ulti-

mate flexibility is required.

9.4

Putting it all together: a fluent builder for chess moves

People have been known to say that Java is a verbose programming language. This

may be true when compared to the Lisp family of languages, but considerable mind-

share has been devoted to devising ways to mitigate its verbosity. One popular tech-

nique is known as the fluent builder (Fowler 2005) and can be summed up as the

chaining of Java methods to form a more readable and agile instance construction

technique. In this section, we’ll show a simple example of a fluent builder supporting

the construction of chess move descriptions. We’ll then explain how such a technique

is unnecessary within Clojure and instead present an alternative approach that’s sim-

pler, concise, and more extensible. We’ll leverage Clojure’s records in the final solu-

tion, illustrating that Java’s class-based paradigm is counter to Clojure’s basic

principles and often overkill for Java programs.

9.4.1

Java implementation

We’ll start by identifying all of the component parts of a Move class including from and

to squares, a flag indicating whether the move is a castling move, and also the desired

promotion piece if applicable. In order to constrain the discussion, we’ll limit our

idea of a Move to those elements listed. The next step would be to create a simple class

with its properties and a set of constructors, each taking some combination of the

expected properties. We’d then generate a set of accessors for the properties, but not

their corresponding mutators, because it’s probably best for the move instances to be

immutable.

Having created this simple class and rolled it out to the customers of the chess

move API, we begin to notice that our users are sending into the constructor the to

string before the from string, which is sometimes placed after the promotion, and so

on. After some months of intense design and weeks of development and testing, we

release the following elided chess move class:

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Putting it all together: a fluent builder for chess moves

203

public class FluentMove {

String from, to, promotion = "";

boolean castlep;

public static MoveBuilder desc() { return new MoveBuilder(); }

public String toString() {

return "Move " + from +

" to " + to +

(castlep ? " castle" : "") +

(promotion.length() != 0 ? " promote to " + promotion : "");

}

public static final class MoveBuilder {

FluentMove move = new FluentMove();

public MoveBuilder from(String from) {

move.from = from; return this;

}

public MoveBuilder to(String to) {

move.to = to; return this;

}

public MoveBuilder castle() {

move.castlep = true; return this;

}

public MoveBuilder promoteTo(String promotion) {

move.promotion = promotion; return this;

}

public FluentMove build() { return move; }

}

}

For brevity’s sake, our code has a lot of holes, such as missing checks for fence post

errors, null, empty strings, assertions, and invariants; it does allow us to illustrate that

the code provides a fluent builder given the following main method:

public static void main(String[] args) {

FluentMove move = FluentMove.desc()

.from("e2")

.to("e4").build();

System.out.println(move);

move = FluentMove.desc()

.from("a1")

.to("c1")

.castle().build();

System.out.println(move);

move = FluentMove.desc()

.from("a7")

.to("a8")

.promoteTo("Q").build();

System.out.println(move);

}

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CHAPTER 9 Combining data and code

// Move e2 to e4

// Move a1 to c1 castle

// Move a7 to a8 promote to Q

The original constructor ambiguities have disappeared, with the only trade-off being a

slight increase in complexity of the implementation and the breaking of the common

Java getter/setter idioms—both of which we’re willing to live with. But if we’d started

the chess move API as a Clojure project, the code would likely be a very different expe-

rience for the end user.

9.4.2

Clojure implementation

In lieu of Java’s class-based approach, Clojure provides a core set of collection types,

and as you might guess, its map type is a nice candidate for move representation:

{:from "e7", :to "e8", :castle? false, :promotion \Q}

Simple, no?

In a language like Java, it’s common to represent everything as a class—to do oth-

erwise is either inefficient, non-idiomatic, or outright taboo. Clojure prefers simplifi-

cation, providing a set of composite types perfect for representing most categories of

problems typically handled by class hierarchies. Using Clojure’s composite types

makes sense for one simple reason: existing functions, built on a sequence abstrac-

tion, just work:

(defn build-move [& pieces]

(apply hash-map pieces))

(build-move :from "e7" :to "e8" :promotion \Q)

;=> {:from "e7", :to "e8", :promotion \Q}

In two lines, we’ve effectively replaced the Java implementation with an analogous, yet

more flexible representation. The term domain-specific language (DSL) is often thrown

around to describe code such as build-move, but to Clojure (and Lisps in general) the

line between DSL and API is blurred. In the original FluentMove class, we required a

cornucopia of code in order to ensure the API was agnostic of the ordering of move ele-

ments; using a map, we get that for free. Additionally, FluentMove, though relatively

concise, was still bound by fundamental Java syntactical and semantic constraints.

There’s one major problem with our implementation—it doesn’t totally replace

the Java solution. If you recall, the Java solution utilized the toString method to print

its representative form. The existence of a polymorphic print facility in Java is nice,

and it allows a class creator to define a default print representation for an object when

sent to any Java print stream. This means that the same representation is used on the

console, in log files, and so on. Using raw maps can’t give us this same behavior, so

how can we solve this problem?

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Putting it all together: a fluent builder for chess moves

205

USING RECORDS

If we instead use a record, then the solution is as simple as that shown next.

Listing 9.8 A chess move record

(defrecord Move [from to castle? promotion]

Object

(toString [this]

(str "Move " (:from this)

" to " (:to this)

(if (:castle? this) " castle"

(if-let [p (:promotion this)]

(str " promote to " p)

"")))))

As we mentioned in the previous section, within the body of a record we can take up

to two actions: participate in a protocol, or override any of the methods in the

java.lang.Object class. For the Move record, we override toString in order to allow

it to participate in Java’s overarching polymorphic print facility, as shown:

(str (Move. "e2" "e4" nil nil))

;=> "Move e2 to e4"

(.println System/out (Move. "e7" "e8" nil \Q))

; Move e7 to e8 promote to Q

We’ve once again gone back to positional construction using records, but as we’ll

show, Clojure even has an answer for this.

SEPARATION OF CONCERNS

Both FluentMove and build-move make enormous assumptions about the form of the

data supplied to them and do no validation of the input. For FluentMove, object-

oriented principles dictate that the validation of a well-formed move (not a legal

move, mind you) should be determined by the class itself. There are a number of

problems with this approach, the most obvious being that to determine whether a

move is well-formed, the class needs information about the rules of chess. We can

rewrite FluentMove to throw an exception to prevent illegal moves from being con-

structed, but the root problem still remains—FluentMove instances are too smart. Per-

haps you don’t see this as a problem, but if we were to extend our API to include other

aspects of the game of chess, then we’ll find that bits of overlapping chess knowledge

would be scattered throughout the class hierarchy. By viewing the move structure as a

value, Clojure code provides some freedom in the implementation of a total solution,

as shown:

(defn build-move [& {:keys [from to castle? promotion]}]

{:pre [from to]}

(Move. from to castle? promotion))

(str (build-move :from "e2" :to "e4"))

;=> "Move e2 to e4"

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CHAPTER 9 Combining data and code

By wrapping the Move constructor in a build-move function, we put the smarts of con-

structing moves there instead of in the type itself. In addition, using a precondition,

we specified the required fields, and by using Clojure’s named parameters and argu-

ment destructuring we’ve again ensured argument order independence. As a final

added advantage, Clojure’s records are maps and as a result can operate in almost

every circumstance where a map would. As author Rich Hickey proclaimed, any new

class in general is itself an island, unusable by any existing code written by anyone,

anywhere. So our point is this: consider throwing the baby out with the bath water.

9.5

Summary

Clojure disavows the typical object-oriented model of development. But that’s not to

say that it completely dismisses all that OOP stands for. Instead, Clojure wholeheartedly

touts the virtues of interface-oriented programming (or abstraction-oriented program-

ming, as we’ve called it), in addition to runtime polymorphism. But in both cases, the

way that Clojure presents these familiar topics is quite different from what you might

be accustomed to. In almost every circumstance, Clojure’s abstraction-oriented facili-

ties will sufficiently represent your problem domain, but there may be times when they

simply can’t. We’ll preach the virtues of abstractions more throughout the rest of the

book, but for now we’re compelled to take a side path into an explorations of Java

interoperability.

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Java.next

This chapter covers

 Generating objects on the fly with proxy

 Clojure gen-class and GUI programming

 Clojure’s relationship to Java arrays

 All Clojure functions implement...

 Using Clojure data structures in Java APIs

 definterface

 Be wary of exceptions

Regardless of your views on the Java language itself, it’s difficult to deny that the JVM

is a stellar piece of software. The confluence of the just-in-time (JIT) compiler, gar-

bage collection, HotSpot, and the flexible bytecode have created an environment

that many programmers have chosen to grow their alternative programming lan-

guages. Additionally, the deluge of library options hosted on the JVM further make

the JVM the language target of choice. From Clojure to Groovy to Scala to Fantom to

Frink to Ioke to Jess to JRuby to Jython, there seems to be no lack of options for the

enthusiastic polyglot programmer. We may soon see job listings for “JVM program-

mers.” But where does that leave Java the programming language?

Java the language isn’t dead.

207

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CHAPTER 10 Java.next

The JVM is optimized for running Java bytecode, and only recently1 have Java.next

languages been a consideration. You may ask yourself whether JVM bytecode is equiva-

lent to Java source code, and the answer is no. Instead, languages such as Clojure and

Scala compile directly to bytecode and can access Java compiled libraries as needed.

Because of their reliance on the JVM as the runtime environment, Clojure and the

other Java.next languages will be fundamentally constrained by the limitations of the

JVM itself. The limitations of the JVM as defined by the limitations of the Java language

specification set the beat by which the Java.next languages dance. Java isn’t dead; it’s

alive and well, and it runs the show.

THE JAVA.NEXT MANTRA The apprentice avoids all use of Java classes. The jour-

neyman embraces Java classes. The master knows which classes to embrace

and which to avoid.

An expert understanding of the Java Virtual Machine isn’t required for writing power-

ful applications in Clojure, but it’ll help when issues stemming from host limitations

arise. Thankfully, Clojure does a good job of mitigating many of the limitations inher-

ent in its host, but some are too deeply embedded in the fibers of the JVM to avoid. Clo-

jure provides a specific set of interoperability tools: gen-class, proxy, definterface,

its exceptions facility, and a host of array functions. We’ll touch on each of these in

turn, but we’ll begin with the creation of anonymous objects using proxy.

10.1

Generating objects on the fly with proxy

There’s a saying within the Clojure community stating (Halloway 2009) that Clojure

does Java better than Java. This is a bold statement, but not one without merit, as we’ll

show throughout this chapter. Java programmers are accustomed to drawing a severe

distinction between development time and runtime. Using Clojure’s proxy feature

allows you to blur this distinction.

CLOJURE APHORISM Many software projects require a lot of planning because

their implementation languages don’t foster change. Clojure makes it a lot

easier to plan for change.

Clojure’s proxy mechanism is meant strictly for interoperability purposes. In section

9.3, we discussed how reify was intended to realize a single instance of a type, proto-

col, or interface—in other words, abstractions. But when dealing with Java libraries,

you’re at times required to extend concrete classes, and it’s in this circumstance where

proxy shines. Be aware that by using proxy, you bring a lot of Java’s semantics into

your Clojure programs. Though extending concrete classes is seen often in Java,

doing so in Clojure is considered poor design, leading to fragility, and should there-

fore be restricted to those instances where interoperability demands it.

1 More details can be found in JSR-000292, “Supporting Dynamically Typed Languages on the Java Platform.”

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Generating objects on the fly with proxy

209

10.1.1 A simple dynamic web service

Using Clojure breaks the ponderous code/compile/run development cycle by adding

an element of dynamism into the fold. Take for example a scenario where we want to

develop a web service using an existing Java 1.5 API.

Listing 10.1 A simple dynamic web service

(ns joy.web

(:import (com.sun.net.httpserver HttpHandler HttpExchange HttpServer)

(java.net InetSocketAddress HttpURLConnection)

(java.io IOException FilterOutputStream)

(java.util Arrays)))

(defn new-server [port path handler]

Create service

(doto (HttpServer/create (InetSocketAddress. port) 0)

(.createContext path handler)

(.setExecutor nil)

(.start)))

(defn default-handler [txt]

Print hello message

(proxy [HttpHandler] []

(handle [exchange]

(.sendResponseHeaders exchange HttpURLConnection/HTTP_OK 0)

(doto (.getResponseBody exchange)

(.write (.getBytes txt))

Close over txt

(.close)))))

(def server (new-server 8123

"/joy/hello"

(default-handler "Hello Cleveland")))

After entering the code in listing 10.1, you should see the message “Hello Cleveland”

in your web browser at address http://localhost:8123/joy/hello. This is only margin-ally interesting, especially because the source is organized in a way that doesn’t take

advantage of Clojure’s flexibility.

If we instead organize the code to bind the return of default-handler, we can

manipulate the handler independently and update its behavior at runtime, as shown:

(.stop server 0)

(def p (default-handler

"There's no problem that can't be solved

with another level of indirection"))

(def server (new-server 8123 "/joy/hello" p))

At this point, visiting the aforementioned URL will show the new message, making this

simple server more compelling. But we can take it one step further by making changes

without taking the server instance down in such a clumsy fashion. Ideally, we’d like to

be able to call a function to change the message at any time:

(change-message p "Our new message")

The implementation of change-message is given in the following listing.

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CHAPTER 10 Java.next

Listing 10.2 Convenience functions for changing the web service message

(defn make-handler-fn [fltr txt]

(fn [this exchange]

Name explicit this

(let [b (.getBytes txt)]

(-> exchange

.getResponseHeaders

(.set "Content-Type" "text/html"))

(.sendResponseHeaders exchange

HttpURLConnection/HTTP_OK

0)

(doto (fltr (.getResponseBody exchange))

Pass through filter

(.write b)

(.close)))))

(defn change-message

"Convenience method to change a proxy's output message"

([p txt] (change-message p identity txt))

Use identity filter

([p fltr txt]

(update-proxy p

{"handle" (make-handler-fn fltr txt)})))

We’ve added a few extras to the implementation that will be useful later, but for now

concentrate on the fact that change-message calls the function update-proxy with

the proxy object p and a map containing an anonymous function keyed on a string

referencing a method name to override. The anonymous function looks similar to the

handle method defined in the returned proxy from the original default-handler

function, with some extras added for flexibility’s sake. You can test this by entering the

following function call:

(change-message p "Hello Dynamic!")

Refreshing your browser will reflect the change made by displaying the string "Hello

Dynamic!". If so inclined, you can also inspect the current proxy mappings using the

function proxy-mappings. The question remains—how does update-proxy change

the behavior of a previously generated proxy class?

IT’S CALLED PROXY FOR A REASON

As we mentioned, the proxy function generates the bytecode for an actual class on

demand, but it does so in such a way to provide a more dynamic implementation.

Instead of inserting the bytecode for the given function bodies directly into the proxy

class, Clojure instead generates a proper proxy in which each method looks up the

function implementing a method in a map. This trades highly useful dynamic behav-

ior for some runtime cost, but in many cases this is a fair trade.

Based on the method name, the corresponding function is retrieved from a map

and invoked with the this reference and the argument(s).

PROXIES FOR TRUE POWER DYNAMISM

Working from the abstract model in figure 10.1, observe how Clojure updates the

mapped functions within a proxy at runtime. This web service is a humble example,

but there’s a point to take away from this exercise: to perform this same task in Java

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Generating objects on the fly with proxy

211

isn’t impossible but would require an enormous

p

amount of scaffolding to implement properly,

this = HttpHandler.

whereas in Clojure it’s built into the language.

handle(exchange)

PROXIES AS PROPER CITIZENS

In the original change-message function, we

provided a hook named fltr that took the result

(fn [this exchange])

of the call to the .getResponseBody method.

Because the result of this method call is a

java.io. OutputStream, we can use that infor-

mation to our advantage when creating a filtering

function. The use of the identity function as the

Figure 10.1 Proxy lookup: the

default filter ensures that the usage doesn’t break

instance returned by proxy is a proper

proxy that does method dispatch to

in the default case; but if we’re to utilize our own

functions in a lookup table. These

filtering function, we must ensure that we prop-

functions can therefore be swapped out

erly wrap the original, which again is a perfect use

with replacements as needed.

case for proxy. A simple implementation of a

screaming-filter would be implemented as such:

(defn screaming-filter [o]

(proxy [FilterOutputStream] [o]

(write [b]

(proxy-super write (.getBytes (str "<strong>"

(.toUpperCase (String. b))

"</strong>"))))))

The proxy returned by screaming-filter extends the Java class java.io.Filter-

OutputStream to the superclass constructor (via the [o] vector). It passes the argument

o, which corresponds to the OutputStream obtained from the .getResponseBody

method.

ANAPHORIC PROXY In section 8.5, we mentioned that it’s non-idiomatic to

write anaphoric macros, yet you might’ve noticed that proxy is a contradic-

tion of that statement. The use of the anaphora this is subject to the same

nesting limitations as previously mentioned and is a good candidate for

change in later versions of Clojure. You might notice that the reify macro,

though similar to proxy, doesn’t use an anaphoric this but instead requires

that it be named explicitly—the preferred approach for your own, and likely

the way forward for all future Clojure core macros.

The call to the proxy-super function is similar to Java’s super.method() semantics. If

we now execute the call to change-message passing in screaming-filter, we’ll see

the expected filtered message in all caps and bold on a browser refresh:

(change-message p screaming-filter "whisper")

Note that in a break from almost every other construct in Clojure, proxy-super is not

thread-safe. If some other thread were to call this proxy instance’s write method

while proxy-super was still running, the base class’s method would be called directly,

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CHAPTER 10 Java.next

incorrectly skipping the proxy implementation. So be careful using proxy-super and

multiple threads in close proximity to each other.

FINAL POINTS ABOUT PROXY

Clojure’s proxy capabilities are truly dynamic, allowing you to create fully stubbed

proxies using either construct-proxy, get-proxy-class, or init-proxy. In both

cases, a partially to fully realized proxy will be constructed, allowing programmatic

customization using update-proxy and arbitrary mixin maps.

There’s a universe of difference between the code outlined in this subsection and

systems employing true code hot-loading, but it’s a reasonable facsimile. Using proxy

is powerful, but doing so creates unnamed instances unavailable for later extension. If

you instead wish to create named classes then you’ll need to use Clojure’s gen-class

mechanism, which we’ll discuss next.

10.2

Clojure gen-class and GUI programming

In section 9.1, we mentioned that Clojure namespaces can be used as the basis for

generating a named class. In this section, we’ll address this topic and others related to

Clojure’s gen-class function and :gen-class namespace directive in the context of

writing a simple graphical user interface (GUI) library.

10.2.1 Namespaces as class specifications

Similarly to the ns example in section 9.1, the explanation of gen-class begs a declar-

ative approach for a namespace defining a class named joy.gui.DynaFrame. We’d like

this class to extend javax.swing.JFrame and declare the Vars providing its overriding

method implementations to be prefixed2 by the symbol df-. In addition, we’d like the

class to implement the clojure.lang.IMeta interface. We’d also like a place to store

information about instances of this class in state and would like the initialization func-

tion called on construction to be named df-init. We’d like to define a single construc-

tor, taking a string and passing it onto the superclass constructor also taking a string.

We then want to declare two public methods: the first named display taking a

java.awt.Container and returning void, and the second static method version tak-

ing no arguments and returning a string. Finally, we’ll declare the required imports

needed.

The worded DynaFrame class declaration is complex but has the advantage of hav-

ing a direct code translation, as shown next.

Listing 10.3 The DynaFrame class namespace declaration

(ns joy.gui.DynaFrame

(:gen-class

:name joy.gui.DynaFrame

:extends javax.swing.JFrame

Superclass

:implements [clojure.lang.IMeta]

Interface

:prefix df-

2 If you don’t specify a :prefix, then the default - will be used.

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Clojure gen-class and GUI programming

213

:state state

Instance state

:init init

Init function

Constructor

:constructors {[String] [String]}

mapping

:methods [[display [java.awt.Container] void]

Public

^{:static true} [version [] String]])

method

(:import (javax.swing JFrame JPanel)

Static method

(java.awt BorderLayout Container)))

You can compile this namespace by saving it in a directory joy/gui, located on the

classpath, in a file named DynaFrame.clj and executing the function (compile

'joy.gui.DynaFrame) in a fresh REPL. This allows a compiled class to be immediately

available. But trying to create an instance in the same REPL will prove fruitless:

(joy.gui.DynaFrame. "1st")

; java.lang.UnsupportedOperationException:

; joy.gui.DynaFrame/df-init not defined

Clearly we haven’t defined the df-init function, so we’ll do that now by switching to

the joy.gui.DynaFrame namespace, defining it outright:

(in-ns 'joy.gui.DynaFrame)

(defn df-init [title]

[[title] (atom {::title title})])

Now run the following in your REPL:

(joy.gui.DynaFrame. "2nd")

; java.lang.UnsupportedOperationException:

; meta (joy.gui.DynaFrame/df-meta not defined?)

Because we told the Clojure compiler that the class should implement the IMeta inter-

face, we should’ve provided a concrete implementation, which you can do at the REPL:

(defn df-meta [this] @(.state this))

(defn version [] "1.0")

As an added bonus, we implemented the static method version. To see the effects of

these functions, execute the following:

(meta (joy.gui.DynaFrame. "3rd"))

;=> {:joy.gui.DynaFrame/title "3rd"}

(joy.gui.DynaFrame/version)

;=> "1.0"

We’ve filled in most of the implementation of the DynaFrame class except for the

display function, which you can implement as follows:

(defn df-display [this pane]

(doto this

(-> .getContentPane .removeAll)

(.setContentPane (doto (JPanel.)

(.add pane BorderLayout/CENTER)))

(.pack)

(.setVisible true)))

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CHAPTER 10 Java.next

Figure 10.2 A simple use of DynaFrame: now

Figure 10.3 A simple dynamic update of

that you’ve compiled the DynaFrame class,

DynaFrame: we can update the

you can start using it to display simple GUIs.

DynaFrame on the fly without restarting.

You can see df-display in action within the REPL by running the following:

(def gui (joy.gui.DynaFrame. "4th"))

(.display gui (doto (javax.swing.JPanel.)

(.add (javax.swing.JLabel. "Charlemagne and Pippin"))))

This will now display the GUI frame seen in figure 10.2.

And because it’s a DynaFrame we should be able to change it on the fly, right? Right:

(.display gui (doto (javax.swing.JPanel.)

(.add (javax.swing.JLabel. "Mater semper certa est." ))))

This will change the view to that in figure 10.3.

But now that you have this interesting little frame, what can you do with it? Next,

we’ll experiment with DynaFrame as the foundation for agile GUI prototyping.

THE GUTS OF NAMESPACE COMPILATION

So what exactly does the :gen-class directive provide in terms of generated class

files? With or without :gen-class, Clojure will generate a set of classes correspond-

ing to each function in a namespace. For the function joy.gui.DynaFrame/df-dis-

play, a class file will be generated on the classpath of joy.gui.DynaFrame

$df_display containing (at least) a method invoke, at the location CLASSPATH/

joy/gui/DynaFrame$df_display.class, as shown:

package joy.gui;

public class DynaFrame$df_display extends AFunction {

. . .

public Object invoke(Object that, Object container) {

. . . display actions . . .

}

}

Of course, this describes implementation details and shouldn’t be considered fact in

future version of Clojure. In fact, as shown before, you were able to add implementa-

tions for the parts of the DynaFrame class at the REPL because Clojure generates a stub

that looks up concrete implementations through Vars. But these details are useful for

describing the logical product of :gen-class and compile. The :gen-class directive

with the argument :name joy.gui.DynaFrame creates a class vaguely resembling the

following Java source:

package joy.gui;

public class DynaFrame extends javax.swing.JFrame {

public final Object state;

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Clojure gen-class and GUI programming

215

public DynaFrame(String title) {

Object r = clojure.lang.RT.var("joy.gui.DynaFrame", "df-init")

.invoke(title);

Object cargs = clojure.lang.RT.nth(r, 0);

state = clojure.lang.RT.nth(r, 1);

super((String) clojure.lang.RT.nth(cargs, 0));

}

public static String version() { return "1.0"; }

// Delegate to the display function var

public void display(Object the_this, java.awt.Container c) {

return clojure.lang.RT.var("joy.gui.DynaFrame", "df-display")

.invoke(the_this, c);

}

. . .

}

The :gen-class directive creates a class that’s a delegate for the Vars (prefixed as spec-

ified with df-) located in the corresponding namespace, contains the state, and also

holds any static methods. This is a lot of detail to contend with, but understanding it’s

important when arranging your Clojure projects to take advantage of code compilation.

One final important point when using gen-class is the semantics surrounding the

:impl-ns directive. Our example relies on the fact that the gen-class namespace is

the same as the implementation namespace (the :impl-ns ), meaning that the compi-

lation will transitively compile all of the implementation functions. On the other

hand, when your implementation and gen-class namespaces are distinct, you no lon-

ger suffer transitive compilation. This provides the benefit of allowing a mixture of

compiled (class files) and uncompiled (.clj files) Clojure products.

10.2.2 Exploring user interface design and development with Clojure

Before we begin, we’ll devise a simple model (_why 20073) for exploring user inter-

face design. We don’t have to complicate matters, because the goal is only to get a gen-

eral idea of how Clojure makes a typically painful task like Java GUI development a joy.

To achieve this modest goal, we’ll need some simple containers illustrated in figure

10.4: shelves, stacks, and splitters.

Because DynaFrame requires a java.awt.

shelf

stack

splitter

Container as its displayed element, we’ll make

each container a derivative thereof. This allows

the containers to nest, helping to build richer

GUIs. Finally, their forms should mirror their

graphical layout, within reason. These three

containers are implemented in the following

Figure 10.4 Basic GUI containers: using

only a handful of rudimentary containers,

listing.

we can build neato GUI prototypes.

3 Our GUI model in this section is based loosely on the Ruby framework Shoes created by _why. Thank you sir,

wherever you are.

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CHAPTER 10 Java.next

Listing 10.4 Simple GUI containers

(ns joy.gui.socks

(:import

(joy.gui DynaFrame)

(javax.swing Box BoxLayout JTextField JPanel

JSplitPane JLabel JButton

JOptionPane)

(java.awt BorderLayout Component GridLayout FlowLayout)

(java.awt.event ActionListener)))

(defn shelf [& components]

(let [shelf (JPanel.)]

(.setLayout shelf (FlowLayout.))

(doseq [c components] (.add shelf c))

shelf))

(defn stack [& components]

(let [stack (Box. BoxLayout/PAGE_AXIS)]

(doseq [c components]

(.setAlignmentX c Component/CENTER_ALIGNMENT)

(.add stack c))

stack))

(defn splitter [top bottom]

(doto (JSplitPane.)

(.setOrientation JSplitPane/VERTICAL_SPLIT)

(.setLeftComponent top)

(.setRightComponent bottom)))

These simple GUI elements are built on top of the Java Swing library, where each sub-

widget in the components argument is added to the properly configured Container-

derived parent. These are good as a starting point, but still there’s nothing to display

unless we dive into the Swing API directly. We can do one better than that by providing

a simple base set of widgets: buttons, labels, and text boxes.

Listing 10.5 A set of simple widgets

(defn button [text f]

(doto (JButton. text)

(.addActionListener

(proxy [ActionListener] []

(actionPerformed [_] (f))))))

(defn txt [cols t]

(doto (JTextField.)

(.setColumns cols)

(.setText t)))

(defn label [txt] (JLabel. txt))

The button element takes a function executed on a mouse-click, so we’ll now provide

a JavaScript-like alert function as a simple action:

(defn alert

([msg] (alert nil msg))

([frame msg]

(javax.swing.JOptionPane/showMessageDialog frame msg)))

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Clojure gen-class and GUI programming

217

Figure 10.5 DynaFrame alerts: we

Figure 10.6 A much more elaborate DynaFrame GUI:

can create slightly more complex

there’s no limit to the complexity of this simple GUI

GUIs and attach actions on the fly.

model. Go ahead and experiment to your heart’s content.

Having built all of these GUI elements, we’ll describe the first simple GUI as shown in

figure 10.5.

It seems simple, if not pointless. But you might be pleasantly surprised with the con-

cise code used to describe it:

(.display gui

(splitter

(button "Procrastinate" #(alert "Eat Cheetos"))genclass

(button "Move It" #(alert "Couch to 5k"))))

These widgets are adequate enough to create richer user interfaces, and to illustrate

we’ll add one more widget builder for grid-like elements:

(defn grid [x y f]

(let [g (doto (JPanel.)

(.setLayout (GridLayout. x y)))]

(dotimes [i x]

(dotimes [j y]

(.add g (f))))

g))

With a small amount of code, we can build the richer user interface in figure 10.6.

Listing 10.6 A more complex GUI example

(.display gui

(let [g1 (txt 10 "Charlemagne")

g2 (txt 10 "Pippin")

r (txt 3 "10")

d (txt 3 "5")]

(splitter

(stack

(shelf (label "Player 1") g1)

(shelf (label "Player 2") g2)

(shelf (label "Rounds ") r

(label "Delay ") d))

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CHAPTER 10 Java.next

(stack

(grid 21 11 #(label "-"))

(button "Go!" #(alert (str (.getText g1) " vs. "

(.getText g2) " for "

(.getText r) " rounds, every "

(.getText d) " seconds.")))))))

Though not perfect, it gives you a good idea how to extend these functions to provide

a finer level of control over layout and positioning, as well as ways to provide more

functionality to create richer interfaces. How would you go about creating an agile

environment for incremental GUI development using plain Java? Clojure allows you to

start with a powerful set of primitives and incrementally refine them until they suit

your exact needs.

Though this section started as a description of creating a simple dynamic frame

using the gen-class facility, we felt it was worthwhile to expand into the realm of

dynamic, incremental development. There are times when AOT compilation is abso-

lutely necessary (such as client requirements), but our advice is to avoid it if at all pos-

sible. Instead, leverage the dynamic nature of Clojure to its fullest, designing your

system to fit into that model.

10.3 Clojure’s relationship to Java arrays

In general, the need to delve into arrays should be limited, but such casual dismissal

isn’t always apropos. In this section, we’ll cover some of the uses for Java arrays in Clo-

jure, including but not limited to arrays as multimethod dispatch, primitive versus ref-

erence arrays, calling variadic functions and constructors, and multi-dimensional

arrays.

10.3.1 Types of arrays: primitive and reference

As mentioned in section 4.1, Clojure numbers are of the boxed variety, but in many

cases the Clojure compiler can resolve the correct call for primitive interoperability

calls. But it can never resolve the need to pass a primitive array when a reference array

is provided instead.

CREATING PRIMITIVE ARRAYS

The Java class java.lang.StringBuilder provides4 a method .append(char[]) that

appends the primitive chars in the passed array to its end. But our first instinct for

making this happen in Clojure won’t bear fruit:

(doto (StringBuilder. "abc")

(.append (into-array [\x \y \z])))

;=> #<StringBuilder abc[Ljava.lang.Character;@65efb4be>

The problem lies in that Clojure’s into-array function doesn’t return a primitive

array of char[], but instead a reference array of Character[], forcing the Clojure

4 When dealing with and manipulating strings, your best options can almost always be found in the core

clojure.string namespace or the clojure.contrib.string namespace in the Clojure contrib library.

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compiler to resolve the call as to the StringBuilder.append(Object) method

instead. That the Array class is a subclass of Object is a constant cause for headache in

Java and clearly can be a problem5 for Clojure as well. What we really want to do is

ensure that a primitive array is used as the argument to .append, which we do here:

(doto (StringBuilder. "abc")

(.append (char-array [\x \y \z])))

;=> #<StringBuilder abcxyz>

Clojure provides a number of primitive array-building functions that work similarly to

char-array, as summarized in the following list.

 boolean-array

 double-array

 long-array

 byte-array

 float-array

 object-array

 char-array

 int-array

 short-array

You could also use the make-array and into-array functions to create primitive

arrays:

(let [ary (make-array Integer/TYPE 3 3)]

(dotimes [i 3]

(dotimes [j 3]

(aset ary i j (+ i j))))

(map seq ary))

;=> ((0 1 2) (1 2 3) (2 3 4))

(into-array Integer/TYPE [1 2 3])

;=> #<int[] [I@391be9d4>

Populating arrays can often be an iterative affair, as seen in the previous snippet, but

there are often more concise ways to do so when creating reference arrays.

CREATING REFERENCE ARRAYS

To intentionally create an array of a particular reference type, or of compatible types,

use the into-array function, passing in a sequence of objects:

(into-array ["a" "b" "c"])

;=> #<String[] [Ljava.lang.String;@3c3ac93e>

(into-array [(java.util.Date.) (java.sql.Time. 0)])

;=> #<Date[] [Ljava.util.Date;@178aab40>

(into-array ["a" "b" 1M])

; java.lang.IllegalArgumentException: array element type mismatch

(into-array Number [1 2.0 3M 4/5])

;=> #<Number[] [Ljava.lang.Number;@140b6e46>

The function into-array determines the type of the resulting array based on the first

element of the sequence, and each subsequent element type must be compatible (a

5

In this example, it’s preferred that a “java.lang.IllegalArgumentException: No matching method found”

exception be thrown, because StringBuilder doesn’t have a method matching .append(Character[]) or

even .append(Object[]).

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CHAPTER 10 Java.next

subclass). To create a heterogeneous array of java.lang.Object, use the to-array or

to-array-2d function:

(to-array-2d [[1 2 3]

[4 5 6]])

;=> #<Object[][] [[Ljava.lang.Object;@bdccedd>

(to-array ["a" 1M #(%) (proxy [Object] [])])

;=> #<Object[] [Ljava.lang.Object;@18987a33>

(to-array [1 (int 2)])

;=> #<Object[] [Ljava.lang.Object;@6ad3c65d>

Be wary: primitives will be autoboxed when using either to-array or to-array-2d.

10.3.2 Array mutability

Because JVM arrays are mutable, you need to be aware that their contents can change

at any point. For example:

(def ary (into-array [1 2 3]))

(def sary (seq ary))

sary

;=> (1 2 3)

What happens to sary if we change the contents of ary?

(aset ary 0 42)

sary

;=> (42 2 3)

The seq view of an array is that of the live array and therefore subject to concurrent

modification. Be cautious when sharing arrays from one function to the next, and

especially across threads. Note that this can be especially disastrous should an array

change in the middle of a sequential operation, such as the use of the higher-order

array functions amap and areduce, as might be used to define a sum-of-squares func-

tion6 for arrays:

(defn asum-sq [xs]

(let [dbl (amap xs i ret

(* (aget xs i)

(aget xs i)))]

(areduce dbl i ret 0

(+ ret (aget dbl i)))))

(asum-sq (float-array [1 2 3 4 5]))

;=> 55.0

At any point during the processing of asum-sq, the underlying array could change,

causing inaccurate results or worse. You should take great care when using Java’s

mutable arrays, though sharing only the seq of an array is perfectly safe because

there’s no way to get at the array when you only have a reference to the seq.

6 This function is fairly clear but slower than it should be. We’ll make it faster in sections 12.1 and 12.5.

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10.3.3 That unfortunate naming convention

You might’ve noticed (how could you miss?) the ugly names printed by the Clojure

REPL whenever an array is evaluated. There’s logic to this madness, as part of the jum-

ble is the legal name of the class corresponding to the array—the part formed as

[Ljava.lang.String;. For example, the previous name corresponded to a 1D array

of strings. The representation for a 2D array of strings is then [[Ljava.lang.String;,

and it therefore follows that [[[Ljava.lang.String; is a 3D array of strings. Are you

sensing a pattern here? Table 10.1 lays it out.

Using what you know about arrays, the class representation names can be used to

do things such as multimethod dispatch:

(what-is (into-array ["a" "b"]))

;=> "1d String"

(what-is (to-array-2d [[1 2][3 4]]))

;=> "2d Object"

(what-is (make-array Integer/TYPE 2 2 2 2))

;=> "Primitive 4d int"

You can create methods for identifying arrays and returning a descriptive string using

the <indexterm><primary>java.lang.Class/forName</primary></indexterm>Class/

forName method as shown:

(defmulti what-is class)

(defmethod what-is (Class/forName "[Ljava.lang.String;") [a] "1d String")

(defmethod what-is (Class/forName "[[Ljava.lang.Object;") [a] "2d Object")

(defmethod what-is (Class/forName "[[[[I") [a] "Primitive 4d int")

Though not the most beautiful task to perform in Clojure, it’s easy to understand

once you’ve grasped how the array class names are constructed.

Representation

Array type

[Ljava.lang.Object;

Reference array

[B

Primitive byte array

[I

Primitive int array

[C

Primitive char array

[S

Primitive short array

[F

Primitive float array

[D

Primitive double array

[J

Primitive long array

[Z

Primitive boolean array

Representation

Dimension

[

1D

[[

2D

Table 10.1 Array type class

...

and so on...

names and dimensions

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10.3.4 Multidimensional arrays

Observe what happens when the following call is tried:

(what-is (into-array [[1.0] [2.0]]))

; java.lang.IllegalArgumentException: No method in multimethod

; 'what-is' for dispatch value: class [Lclojure.lang.PersistentVector;

The problem is that the into-array function builds a 1D array of persistent vectors,

but we wanted a 2D array of doubles. In order to do this, the array would have to be

built differently:

(defmethod what-is (Class/forName "[[D") [a] "Primitive 2d double")

(defmethod what-is (Class/forName "[Lclojure.lang.PersistentVector;") [a]

"1d Persistent Vector")

(what-is (into-array (map double-array [[1.0] [2.0]])))

;=> "Primitive 2d double"

(what-is (into-array [[1.0] [2.0]]))

;=> "1d Persistent Vector"

We had to use the map function with double-array on the inner arrays in order to

build the properly typed outer array. When working with multidimensional arrays, be

sure that you know what your inner elements should be on creation and create them

accordingly.

10.3.5 Variadic method/constructor calls

There’s no such thing as a variadic constructor or method at the bytecode level,

although Java provides syntactic sugar at the language level. Instead, variadic methods

expect an array as their final argument, and this is how they should be accessed in Clo-

jure interop scenarios. Take, for example, the call to the String/format function:

(String/format "An int %d and a String %s" (to-array [99, "luftballons"]))

;=> "An int 99 and a String luftballons"

That covers most of the high points regarding arrays in Clojure interoperability. We’ll

touch on them briefly when we talk about performance considerations in chapter 12,

but for now we’ll move on to a more interesting topic: the interoperability underpin-

nings relating to Clojure’s implementation.

10.4

All Clojure functions implement...

Clojure functions are highly amenable to interoperability. Their underlying classes

implement a number of useful interfaces that you can investigate by running

(ancestors (class #())). Most of the resulting classes are only applicable to the

internals of Clojure itself, but a few interfaces are useful in interop scenarios:

java.util.concurrent.Callable, java.util.Comparator, and java.lang.Runnable.

In this section, we’ll talk briefly about each and also provide simple examples.

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10.4.1 java.util.Comparator

Simply put, the java.util.Comparator interface defines the signature for a single

method .compare that takes two objects l and r and returns -1 if l < r, 0 if l == r, and

> 0 if l > r. The static Java method Collections/sort provides an implementation

that takes a derivative of java.util.List and a Comparator and destructively sorts the

list provided. Using this knowledge, we can provide some basic infrastructure for the

remainder of this subsection:

(import '[java.util Comparator Collections ArrayList])

(defn gimme [] (ArrayList. [1 3 4 8 2]))

(doto (gimme)

(Collections/sort (Collections/reverseOrder)))

;=> #<ArrayList [8, 4, 3, 2, 1]>

In order to write a simple comparator that provides a reverse-sort Comparator, we

might naively do so:

(doto (gimme)

(Collections/sort

(reify Comparator

(compare [this l r]

(cond

(> l r) -1

(= l r) 0

:else 1)))))

;=> #<ArrayList [8, 4, 3, 2, 1]>

Though this works, Clojure provides a better way by allowing the use of a function as

the Comparator directly. You can couple this knowledge with the fact that Clojure

already provides numerous functions useful for comparison, as shown next.

Listing 10.7 Useful comparison functions

(doto (gimme) (Collections/sort #(compare %2 %1)))

compare function

;=> #<ArrayList [8, 4, 3, 2, 1]>

(doto (gimme) (Collections/sort >))

Less-than function

;=> #<ArrayList [8, 4, 3, 2, 1]>

(doto (gimme) (Collections/sort <))

Greater-than function

;=> #<ArrayList [1, 2, 3, 4, 8]>

(doto (gimme) (Collections/sort (complement <)))

complement function

;=> #<ArrayList [8, 4, 3, 2, 1]>

When presented with numerous possible implementation strategies, often the best

one in Clojure is the simplest.

10.4.2 java.lang.Runnable

Java threads expect an object implementing the java.lang.Runnable interface meant

for computations returning no value. We won’t get into the specifics of threaded

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CHAPTER 10 Java.next

computation until the next chapter, but the next two examples are simple enough to

require little a priori knowledge on the matter. If you wish to pass a function to

another Java thread, it’s as simple as providing it as an argument to the Thread

constructor:

(doto (Thread. #(do (Thread/sleep 5000)

(println "haikeeba!")))

.start)

; => #<Thread Thread[Thread-3,5,main]>

; ... 5 seconds later

; haikeeba!

This scenario is unlikely to occur often, because Clojure’s core concurrency features

are often sufficient for most needs. But that’s not always the case, and therefore it’s nice

to know that raw Clojure functions can be used seamlessly in the JVM’s concurrency API.

10.4.3 java.util.concurrent.Callable

The Java interface java.util.concurrent.Callable is specifically meant to be used

in a threaded context for computations returning a value. You can use a Clojure func-

tion using Java’s java.util.concurrentFutureTask class representing a “computa-

tion to occur later”:

(import '[java.util.concurrent FutureTask])

(let [f (FutureTask. #(do (Thread/sleep 5000) 42))]

(.start (Thread. #(.run f)))

(.get f))

; ... 5 seconds later

;=> 42

The call to FutureTask.get as the last expression will stop execution (a behavior

known as blocking) until the function passed to the constructor completes. Because the

function in question sleeps for 5 seconds, the call to .get must wait.

Clojure’s interoperability mechanisms are a two-way street. Not only do they allow

Java APIs to work seamlessly within Clojure, but they also provide ways for Clojure

functions to work in Java APIs. In the next section, we’ll continue on this theme of

bidirectional interop with a discussion on the ways that Clojure’s collection types can

also be used in traditional Java APIs.

10.5

Using Clojure data structures in Java APIs

Clojure functions are ready to use in many Java APIs, and as it turns out, so are its col-

lection types. Just as the Clojure collections are separated along three distinct equality

partitions7 (maps, sequences, and sets), so too are its levels of Java collection interop-

erability support. The Java Collections Framework has a nice high-level design philos-

ophy centered around working against interfaces. These interfaces are additionally

cognizant of immutability, in that the mutable parts are optional and the immutable

7 A refresher on equality partitions can be found in section 5.1.2 and throughout the remainder of chapter 5.

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225

parts are clearly demarcated. In this section, we’ll give a brief rundown of possible

ways that Clojure collections can be used within traditional Java APIs adhering to the

immutable collection protocols.

10.5.1 java.util.List

Clojure sequential collections conform to the immutable parts of the java.util.List

interface, which in turn extends the java.util.Collection and java.lang.Iterable

interfaces. You can see this conformance in action in the following listing.

Listing 10.8 java.util.List conformance for sequences and seqs

(.get '[a b c] 1)

Vectors

;=> b

(.get (repeat :a) 138)

Lazy seqs

;=> :a

(.containsAll '[a b c] '[b c])

Vectors also collections

;=> true

(.add '[a b c] 'd)

Sequences not mutable

; java.lang.UnsupportedOperationException

That Clojure sequences and seqs don’t provide the mutable API of typical Java collec-

tions is obvious. But the implications are that you can’t use them in all Java APIs, such

as you might attempt when requiring that a vector be sorted destructively with a Java

API call:

(java.util.Collections/sort [3 4 2 1])

; java.lang.UnsupportedOperationException

A better approach is to either use the method used in the previous section using a Clo-

jure function, or even better to use the Clojure’s sort function instead.

10.5.2 java.lang.Comparable

The interface java.lang.Comparable is the cousin of the Comparator interface.

Comparator refers to objects that can compare two other objects, whereas Comparable

refers to an object that can compare itself to another object:

(.compareTo [:a] [:a])

;=> 0

(.compareTo [:a :b] [:a])

;=> 1

(.compareTo [:a :b] [:a :b :c])

;=> -1

(sort [[:a :b :c] [:a] [:a :b]])

;=> ([:a] [:a :b] [:a :b :c])

One thing to note is that Clojure’s vector implementation is currently the only collec-

tion type that implements the java.lang.Comparable interface providing the

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CHAPTER 10 Java.next

.compareTo method. As a result, attempting to compare a different collection type to

a vector leads to a confusing error message:

(.compareTo [1 2 3] '(1 2 3))

; java.lang.ClassCastException: clojure.lang.PersistentList

; cannot be cast to clojure.lang.IPersistentVector

Pay no attention to that class-cast exception behind the curtain.

10.5.3 java.util.RandomAccess

In general, the java.util.RandomAccess interface is used to indicate that the data

type provides constant time indexed access to its elements. This allows for algorithms

to follow optimized paths accordingly. This optimization is generally performed by

using the .get method for access rather than an iterator:

(.get '[a b c] 2)

;=> c

Vectors are currently the only Clojure collection type that can make such guarantees.

10.5.4 java.util.Collection

The java.util.Collection interface lies at the heart of the Java Collections Frame-

work, and classes implementing it can play in many of Java’s core collections APIs. A

useful idiom taking advantage of this fact is the use of a Clojure sequence as a model

to build a mutable sequence for use in the Java Collections API, as shown:

(defn shuffle [coll]

(seq (doto (java.util.ArrayList. coll)

java.util.Collections/shuffle)))

(shuffle (range 10))

;=> (3 9 2 5 4 7 8 6 1 0)

It’s difficult to write a proper sequence-shuffling function, so the shuffle function

takes full advantage of an existing Java API that has been tested and used extensively

for years. As an added bonus, shuffle is mostly8 functional, idiomatic, and fast. Clo-

jure favors immutability but doesn’t trap you into it when there are practical solutions

to be leveraged.

JAVA.UTIL.MAP

Like most of the Clojure collections, its maps are analogous to Java maps in that they

can be used in nonmutating contexts. But immutable maps have the added advantage

of never requiring defensive copies and will act exactly the same as unmodifiable Java

maps:

(java.util.Collections/unmodifiableMap

(doto (java.util.HashMap.) (.put :a 1)))

;=> #<UnmodifiableMap {:a=1}>

8 shuffle isn’t referentially transparent. Can you see why?

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227

(into {} (doto (java.util.HashMap.) (.put :a 1)))

;=> {:a 1}

In both cases, any attempt to modify the map entry classes of the maps will throw an

exception.

10.5.5 java.util.Set

In the case of Java and Clojure sets, the use of mutable objects9 as elements is highly

frowned upon:

(def x (java.awt.Point. 0 0))

(def y (java.awt.Point. 0 42))

(def points #{x y})

points

;=> #{#<Point java.awt.Point[x=0,y=0]> #<Point java.awt.Point[x=0,y=42]>}

Everything looks peachy at this point, but introducing mutability into the equation

has devastating costs:

(.setLocation y 0 0)

points

;=> #{#<Point java.awt.Point[x=0,y=0]> #<Point java.awt.Point[x=0,y=0]>}

Oh boy. Not only have we confused the set points by modifying its entries out from

underneath it, but we’ve also circumvented Clojure’s value-based semantics and the

nature of set-ness. Dealing with mutable objects is extremely difficult to reason about,

especially when dealing with collections of them. The gates of a mutable class are wide

open, and at any point during the execution of your programs this fact can be

exploited, willingly or not. But you can’t always avoid dealing with mutable nasties in

Clojure code because of a strict adherence to fostering interoperability.

We’ve covered the two-way interop for functions and now collection types, but we

have one final path to traverse: the use and benefits of Clojure’s definterface macro.

10.6

definterface

As we mentioned in section 9.3, Clojure was built on abstractions in the host platform

Java. Types and protocols help to provide a foundation for defining your own abstrac-

tions in Clojure itself, for use within a Clojure context. But when interoperating with

Java code, protocols and types won’t always suffice. Therefore, you need to be able to

generate interfaces in some interop scenarios, and also for performance in cases

involving primitive argument and return types. In this section, we’ll talk briefly about

generating Java interfaces as the syntax, use cases, and purposes are likely familiar.

10.6.1 Generating interfaces on the fly

When you AOT-compile a protocol, you generate a public interface by the same name,

with the methods defined. The code in listing 10.9 uses definterface to define an

9 Clojure’s mutable reference types used to represent a logical identity are perfectly safe to use in sets. We’ll

explore the reference types in exquisite detail in the next chapter.

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CHAPTER 10 Java.next

interface ISliceable. This interface is used to define an abstract thing that has the

ability to be sliced using a method slice, which takes start and end indices of type int.

Likewise, the interface defines a method sliceCount that returns an int representing

the number of possible slices.

Listing 10.9 An interface defining a sliceable object

(definterface ISliceable

(slice [^int s ^int e])

(^int sliceCount []))

;=> user.ISliceable

You’ll notice the inclusion of the type decoration ^int on the arguments to slice and

the return type of sliceCount. For now you can assume that they operate the same as

a type declaration in most languages providing them. They look similar to type hints

discussed in section 12.1, except that only in definterface are primitive hints sup-

ported. Now we can create an instance implementing the user.ISliceable interface,

as shown next.

Listing 10.10 A dummy reified ISliceable

(def dumb

(reify user.ISliceable

(slice [_ s e] [:empty])

(sliceCount [_] 42)))

(.slice dumb 1 2)

;=> [:empty]

(.sliceCount dumb)

;=> 42

There’s nothing terribly surprising about dumb, but you can instead implement it via

deftype, proxy, gen-class, or even a Java class. Note that definterface works even

without AOT compilation.

We can now take definterface to the next logical step and extend the ISliceable

interface to other types using a well-placed protocol.

Listing 10.11 Using a protocol to extend ISliceable

(defprotocol Sliceable

(slice [this s e])

(sliceCount [this]))

(extend user.ISliceable

Sliceable

{:slice (fn [this s e] (.slice this s e))

:sliceCount (fn [this] (.sliceCount this))})

(sliceCount dumb)

;=> 42

(slice dumb 0 0)

;=> [:empty]

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229

By extending the ISliceable interface along Sliceable, ISliceable is able to partic-

ipate in the protocol, meaning that you have the possibility for extending other types,

even final types such as String, as shown next.

Listing 10.12 Extending strings along the Sliceable protocol

(defn calc-slice-count [thing]

"Calculates the number of possible slices using the formula:

(n + r - 1)!

------------

r!(n - 1)!

where n is (count thing) and r is 2"

(let [! #(reduce * (take % (iterate inc 1)))

n (count thing)]

(/ (! (- (+ n 2) 1))

(* (! 2) (! (- n 1))))))

(extend-type String

Sliceable

(slice [this s e] (.substring this s (inc e)))

(sliceCount [this] (calc-slice-count this)))

(slice "abc" 0 1)

;=> "ab"

(sliceCount "abc")

;=> 6

The advantages of using definterface over defprotocol are restricted entirely to the

fact that the former allows primitive types for arguments and returns. At some point in

the future, the same advantages will likely be extended to the interfaces generated, so

use definterface sparingly and prefer protocols unless absolutely necessary.

10.7

Be wary of exceptions

There’s been much debate on the virtues of checked exceptions in Java, so we won’t

cover that here. Instead, we’ll stick to the facts regarding the nuances the JVM imposes

on Clojure’s error-handling facilities. Before we begin, consider the following view on

the use of exceptions in Clojure source:

When writing Clojure code, use errors to mean can’t continue and exceptions to mean

can or might continue .

We’ll attempt to constrain ourselves to the generalities of exception handling in this

section. If you desire information on deciphering exception messages, we talked about

that in section 3.4. If you’re curious about the effects of exceptions on continuation-

passing style, then refer back to section 7.3.4. We discussed the behavior of Clojure to

attempt to supplant numerical inaccuracies by throwing exceptions in section 4.1.3. If

you instead want to learn about the interplay between exceptions and Clojure’s refer-

ence types, then such matters can be found throughout chapter 11. Finally, if you have

no idea what an exception is, then we discuss the basics in section 1.5.8.

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10.7.1 A bit of background regarding exceptions

The behavior of Clojure’s exception features directly spawns from the JVM enforcing

the promulgation of checked exceptions. Virtuous or not in the context of Java devel-

opment, checked exceptions are antithetical to closures and higher-order functions.

Checked exceptions require that not only should the thrower and the party responsi-

ble for handling them declare interest, but every intermediary is also forced to partic-

ipate. These intermediaries don’t have to actively throw or handle exceptions

occurring within, but they must declare that they’ll be “passing through.” Therefore,

by including the call to a Java method throwing a checked exception within a closure,

Clojure has two possible alternatives:

 Provide a cumbersome exception declaration mechanism on every single func-

tion, including closures.

 By default, declare that all functions throw the root Exception or Runtime-

Exception.

And as you can probably guess, Clojure takes the second approach, which leads to a

condition of multilevel wrapping of exceptions as they pass back up the call stack. This

is why you see, in almost any (.printStackTrace *e) invocation, the point of origin of

an error offset by some number of layers of java.lang.RuntimeException. Because

Java interfaces and classes get to decide what types of problems potential derivative

classes and even callers can have, Clojure needs to handle the base

java.lang.Exception at every level, because it has to preserve dynamism in the face

of a closed system. Unless you’re directly calling something that throws typed excep-

tions, your best bet is to catch Exception and then see what you have in context.

10.7.2 Runtime versus compile-time exceptions

There are two contexts in Clojure where exceptions can be thrown: runtime and com-

pile time. In this section we’ll touch on both, explaining how and when to use them.

RUNTIME EXCEPTIONS

The case of runtime exceptions might be the most familiar, because it’s likely to have

been encountered and utilized in your own code. There are two types of runtime

exceptions: errors and exceptions. We can illustrate the difference between the two by

showing you the following:

(defn explode [] (explode))

(try (explode) (catch Exception e "Stack is blown"))

; java.lang.StackOverflowError

So why were we unable to catch the java.lang.StackOverflowError? The reason lies

in Java’s exception class hierarchy and the fact that StackOverflowError isn’t a deriv-

ative of the Exception class, but instead of the Error class:

(try (explode) (catch StackOverflowError e "Stack is blown"))

;=> "Stack is blown"

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Be wary of exceptions

231

(try (explode) (catch Error e "Stack is blown"))

;=> "Stack is blown"

(try (explode) (catch Throwable e "Stack is blown"))

;=> "Stack is blown"

(try (throw (RuntimeException.))

(catch Throwable e "Catching Throwable is Bad"))

;=> "Catching Throwable is Bad"

We started with a catch of the most specific exception type StackOverflowError and

gradually decreased specificity until catching Throwable, which as you’ll notice also

catches a RuntimeException. In Java, catching exceptions at the level of Throwable is

considered bad form, and it should generally be viewed the same in Clojure. There-

fore, we suggest that you follow the advice stated in the opening to this section and

reserve those deriving from Errors for conditions that can’t be continued from and

those from Exception indicating possible continuation.

COMPILE-TIME EXCEPTIONS

There are a few ways that you might come across compile-time exceptions, the most

obvious occurring within the body of a macro:

(defmacro do-something [x] `(~x))

(do-something 1)

; java.lang.ClassCastException:

; java.lang.Integer cannot be cast to clojure.lang.IFn

Though the type of the exception is a java.lang.ClassCastException, it was indeed

thrown by the compiler, which you’d see if you were to trace the stack using some-

thing like (for [e (.getStackTrace *e)] (.getClassName e)).10 It’s perfectly accept-

able (and even encouraged) to throw exceptions within your own macros, but it’s

important to make a distinction between a compile-time and runtime exception.

COMPILE-TIME EXCEPTIONS Why delay until runtime the reporting of an error

that at compile time you know exists?

The way to throw a compile-time exception is to make sure your throw doesn’t occur

within a syntax-quoted form, as we show in the following listing.

Listing 10.13 Throwing a compile-time exception

(defmacro pairs [& args]

(if (even? (count args))

`(partition 2 '~args)

(throw (Exception. (str "pairs requires an even number of args")))))

(pairs 1 2 3)

; java.lang.Exception: pairs requires an even number of args

(pairs 1 2 3 4)

;=> ((1 2) (3 4))

10 This is a limited analogy to Groovy’s .? operator. Clojure also provides convenience functions for displaying

and handling stack traces in the clojure.stacktrace namespace.

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CHAPTER 10 Java.next

Nothing is preventing the exception from being thrown at runtime, but because we

know that pairs requires an even number of arguments, we instead prefer to fail as

early as possible—at compilation time. This difference is clearly demonstrated by

repeating the preceding test in a function definition:

(fn [] (pairs 1 2 3))

; java.lang.Exception: pairs requires an even number of args

A runtime exception wouldn’t have been thrown until this function was called, but

because the pairs macro threw an exception at compile time, users are notified of

their error immediately. Though powerful, you should always try to balance the bene-

fits of compile-time error checking with macros and the advantages that implement-

ing as a function provides (the use in higher-order functions, apply, and so on).

10.7.3 Handling exceptions

There are two ways to handle exceptions and errors, each defined by the way in which

the error-handling mechanisms “flow” through the source. Imagine that you want a

macro that provides a limited11 null-safe (Koenig 2007) arrow that catches any occur-

rence of a NullPointerException in a pipeline:

(defmacro -?> [& forms]

exceptions

`(try (-> ~@forms)

(catch NullPointerException _# nil)))

(try

(+

(-?> 25 Math/sqrt (+ 100))

(Math/sqrt 25)

;=> 105.0

100)

(catch NPE e nil))

(-?> 25 Math/sqrt (and nil) (+ 100))

;=> nil

binding

The flow of any occurrence of NullPointer-

Exception happens from the inner functions of the

(binding [handle prn]

stitched forms. Conceptually, this flow can be viewed

(try

(+

as in figure 10.7, which describes the way that errors

(Math/sqrt 25)

can be caught depending on the direction in which

100)

(catch NPE e

data is moving along the stack.

(handle e))))

The typical (try ... (catch ...)) form would

therefore be used for the case where the handler

Figure 10.7 Outside-in and inside-

out error handling. There are two

catches errors bubbling outward from inner func-

ways to handle errors in Clojure. The

tions and forms, as seen in the -?> macro. But if you

typical way is to let exceptions flow

want to catch errors at their point of origin, you’ll

from the inner forms to the outer.

need a way to pass handlers up the stack. Fortunately,

The other way, discussed in section

13.4, uses dynamic bindings to

Clojure provides a way to do this via its dynamic Var

“reach into” the inner forms to

feature, which will be discussed in section 13.5.

handle them immediately.

11 There are much more comprehensive -?> and .?. macros found in the clojure.contrib.core

namespace, and those are recommended above the one in this section.

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Summary

233

10.7.4 Custom exceptions

If you’re inclined to write your own exception and error types, then you’ll need to do

so using the gen-class feature described in section 10.2. JVM exceptions again are a

closed system, and it might be better to explore other possibilities (Houser EK) for

reporting and handling errors in your Clojure code. But, should you wish to ignore

this advice, then bear in mind that it’s rare for Clojure core functions to throw excep-

tions, and even more rarely are they checked exceptions. The idiom is for Clojure to

throw derivatives of RuntimeException or Error, and thus your code should also strive

for this when appropriate.

10.8

Summary

Clojure provides an extensive set of data abstractions via its types and protocols. It

also provides an extensive interoperability facility through proxy, gen-class,

definterface, exception handling, and the implementation of core Java collection

interfaces. Though we stress that types and protocols will give you the performant

abstractions needed for solving most problems, we realize that not all interop scenar-

ios are solved this way. In these circumstances, you should use the features listed in

this chapter to push you the remainder of the way toward your solution. Clojure

embraces Java interoperability, but it does so in specific ways, and with a specific set

of tools.

In the next chapter, we move on to a rather complex topic, and one that Clojure

helps to simplify—shared state concurrency and mutation.

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Mutation

This chapter covers

 Software transactional memory with multiversion

concurrency control and snapshot isolation

 When to use Refs

 When to use Agents

 When to use Atoms

 When to use locks

 When to use futures

 When to use promises

 Parallelism

 Vars and dynamic binding

Clojure’s main tenet isn’t the facilitation of concurrency. Instead, Clojure at its core

is concerned with the sane management of state, and facilitating concurrent pro-

gramming naturally falls out of that. The JVM operates on a shared-state concur-

rency model built around juggling fine-grained locks that protect access to shared

data. Even if you can keep all of your locks in order, rarely does such a strategy scale

well, and even less frequently does it foster reusability. But Clojure’s state manage-

ment is simpler to reason about and promotes reusability.

234

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Software transactional memory

235

CLOJURE APHORISM A tangled web of mutation means that any change to

your code potentially occurs in the large.

In this chapter, we’ll take the grand tour of

the mutation primitives and see how Clojure

Concurrency 

makes concurrent programming not only

vs. parallelism

possible, but fun. Our journey will take us

Concurrency refers to the execu-

through Clojure’s four major mutable refer-

tion of disparate tasks at roughly

ences: Refs, Agents, Atoms, and Vars. When

the same time, each sharing a

common resource. The results of

possible and appropriate, we’ll also point out

concurrent tasks often affect the

the Java facilities for concurrent program-

behavior of other concurrent

ming (including locking) and provide infor-

tasks, and therefore contain an

mation on the trade-offs involved in

element of nondeterminism. Paral-

choosing them. We’ll also explore parallel-

lelism refers to partitioning a task

ism support in Clojure using futures, prom-

into multiple parts, each run at the

same time. Typically, parallel

ises, and a trio of functions pmap, pvalues,

tasks work toward an aggregate

and pcalls.

goal and the result of one doesn’t

Before we dive into the details of Clo-

affect the behavior of any other

jure’s reference types, let’s start with a high-

parallel task, thus maintaining

level overview of Clojure’s software transac-

determinacy.

tional memory ( STM ).

11.1 Software transactional memory with multiversion

concurrency control and snapshot isolation

Software transactional memory

A faster program that doesn’t work right is useless.

—Simon Peyton-Jones

in “Beautiful Concurrency”

In chapter 1, we defined three important terms:

 Time —The relative moments when events occur

 State —A snapshot of an entity’s properties at a moment in time

 Identity —The logical entity identified by a common stream of states occurring

over time

These terms form the foundation for Clojure’s model of state management and muta-

tion. In Clojure’s model, a program must accommodate the fact that when dealing

with identities, it’s receiving a snapshot of its properties at a moment in time, not nec-

essarily the most recent. Therefore, all decisions must be made in a continuum. This

model is a natural one, as humans and animals alike make all decisions based on their

current knowledge of an ever-shifting world. Clojure provides the tools for dealing

with identity semantics via its Ref reference type, the change semantics of which are

governed by Clojure’s software transactional memory; this ensures state consistency

throughout the application timeline, delineated by a transaction.

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CHAPTER 11 Mutation

11.1.1 Transactions

Within the first few moments of using Clojure’s STM, you’ll notice something different

than you may be accustomed to: no locks. Consequently, because there’s no need for

ad-hoc locking schemes when using STM, there’s no chance of deadlock. Likewise, Clo-

jure’s STM doesn’t require the use of monitors and as a result is free from lost wakeup

conditions. Behind the scenes, Clojure’s STM uses multiversion concurrency control

( MVCC ) to ensure snapshot isolation. In simpler terms, snapshot isolation means that

each transaction gets its own view of the data that it’s interested in. This snapshot is

made up of in-transaction reference values, forming the foundation of MVCC (Ullman

1988). As illustrated in figure 11.1, each transaction merrily chugs along making

changes to in-transaction values only, oblivious to and ambivalent about other transac-

tions. At the conclusion of the transaction, the local values are examined against the

modification target for conflicts. An example of a simple possible conflict is if another

transaction B committed a change to a target reference during the time that transac-

tion A was working, thus causing A to retry. If no conflicts are found, then the in-trans-

action values are committed and the target references are modified with their updated

values. Another advantage that STM provides is that in the case of an exception during

a transaction, its in-transaction values are thrown away and the exception propagated

outward. In the case of lock-based schemes, exceptions can complicate matters ever

more, because in most cases locks need to be released (and in some cases, in the cor-

rect order) before an exception can be safely propagated up the call stack.

Because each transaction has its own isolated snapshot, there’s no danger in retry-

ing—the data is never modified until a successful commit occurs. STM transactions

can easily nest without taking additional measures to facilitate composition. In lan-

guages providing explicit locking for concurrency, matters of composability are often

difficult, if not impossible. The reasons for this are far-reaching and the mitigating

forces (Goetz 2006) complex, but the primary reasons tend to be that lock-based

concurrency schemes often hinge on a secret incantation not explicitly understand-

able through the source itself: for example, the order in which to take and release a

set of locks.

11.1.2 Embedded transactions

In systems providing embedded transactions, it’s often common for transactions to be

nested, thus limiting the scope of restarts (Gray 1992). Embedding transactions within

Clojure operates differently, as summarized in figure 11.2.

In some database systems, transactions can be used to limit the scope of a restart as

shown when transaction embedded.b restarts only as far back as its own scope. Clojure

has but one transaction per thread, thus causing all subtransactions to be subsumed

into the larger transaction. Therefore, when a restart occurs in the (conceptual) sub-

transaction clojure.b, it causes a restart of the larger transaction. Though not shown,

some transaction systems provide committal in each subtransaction; in Clojure, com-

mit only occurs at the outermost larger transaction.

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Software transactional memory

237

Figure 11.1 Illustrating an STM retry: Clojure’s STM works much like a database.

embedded

clojure

A

A

B

B

b

b

Figure 11.2 Clojure’s embedded transactions: a restart in

restart

restart

any of Clojure’s embedded transactions A, B, b, and C causes

C

C

a restart in the whole subsuming transaction. This is unlike a

fully embedded transaction system where the subtransactions

commit!

commit!

can be used to restrain the scope of restarts.

11.1.3 The things that STM makes easy

The phrase TANSTAAFL, meaning “There ain’t no such thing as a free lunch,” was

popularized in the excellent sci-fi novel The Moon Is a Harsh Mistress (Heinlein 1966)

and is an apt response to the view that STM is a panacea for concurrency complexities.

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238

CHAPTER 11 Mutation

As you proceed through this chapter, we urge you to keep this in the back of your

mind, because it’s important to realize that though Clojure facilitates concurrent pro-

gramming, it doesn’t solve it for you. But there are a few things that Clojure’s STM

implementation simplifies in solving difficult concurrent problems.

CONSISTENT INFORMATION

The STM allows you to perform arbitrary sets of read/write operations on arbitrary

sets of data in a consistent (Papadimitriou 1986) way. By providing these assurances,

the STM allows your programs to make decisions given overlapping subsets of informa-

tion. Likewise, Clojure’s STM helps to solve the reporting problem—the problem of

getting a consistent view of the world in the face of massive concurrent modification

and reading, without stopping (locking).

NO NEED FOR LOCKS

In any sized application, the inclusion of locks for managing concurrent access to

shared data adds complexity. There are many factors adding to this complexity, but

chief among them are the following:

 You can’t use locks without supplying extensive error handling. This is critical

in avoiding orphaned locks (locks held by a thread that has died).

 Every application requires that you reinvent a whole new locking scheme.

 Locking schemes often require that you impose a total ordering that’s difficult

to enforce in client code, frequently leading to a priority inversion scenario.

Locking schemes are difficult to design correctly and become increasingly so as the

number of locks grows. Clojure’s STM eliminates the need for locking and as a result

eliminates dreaded deadlock scenarios. Clojure’s STM provides a story for managing

state consistently. Adhering to this story will go a long way toward helping you solve

software problems effectively. This is true even when concurrent programming isn’t a

factor in your design.

ACI

In the verbiage of database transactions is a well-known acronym ACID, which refers to

the properties ensuring transactional reliability. Clojure’s STM provides the first three

properties: atomicity, consistency, and isolation. The other, durability, is missing due to the fact that Clojure’s STM resides in-memory and is therefore subject to data loss in the

face of catastrophic system failure. Clojure relegates the problem of maintaining dura-

bility to the application developer instead of supplying common strategies by default:

database persistence, external application logs, serialization, and so on.

11.1.4 Potential downsides

There are two potential problems inherent in STMs in general, which we’ll only touch

on briefly here.

WRITE SKEW

For the most part, you can write correct programs simply by putting all access and

changes to references in appropriately scoped transactions. The one exception to this

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Software transactional memory

239

is write skew, which occurs in MVCC systems such as Clojure’s. Write skew can occur

when one transaction uses the value of a reference to regulate its behavior but doesn’t

write to that reference. At the same time, another transaction updates the value for

that same reference. One way to avoid this would be to do a “dummy write” in the first

transaction, but Clojure provides a less costly solution: the ensure function. This sce-

nario is rare in Clojure applications, but possible.

LIVE-LOCK

Live-lock refers to a set of transaction(s) that repeatedly restart one another. Clojure

combats live-lock in a couple of ways. First, there are transaction restart limits that will

raise an error when breached. Generally this occurs when the units of work within

some number of transactions is too large. The second way that Clojure combats live-

lock is called barging. Barging refers to some careful logic in the STM implementation

allowing an older transaction to continue running while younger transactions retry.

11.1.5 The things that make STM unhappy

Certain things can rarely (if ever) be safely performed within a transaction, and in this

section we’ll talk briefly about each.

I/O

Any I/O operation in the body of a transaction is highly discouraged. Due to restarts,

the embedded I/O could at best be rendered useless, and cause great harm at worst.

It’s advised that you employ the io! macro whenever performing I/O operations:

(io! (.println System/out "Haikeeba!"))

; Haikeeba!

When this same statement is used in a transaction, an exception is thrown:

(dosync (io! (.println System/out "Haikeeba!")))

; java.lang.IllegalStateException: I/O in transaction

Though it may not be feasible to use io! in every circumstance, it’s a good idea to do

so whenever possible.

CLASS INSTANCE MUTATION

Unrestrained instance mutation is often not idempotent, meaning that running a set of

mutating operations multiple times often displays different results.

LARGE TRANSACTIONS

Though the size of transactions is highly subjective, the general rule of thumb when

partitioning units of work should always be get in and get out as quickly as possible.

Though it’s important to understand that transactions will help to simplify the

management of state, you should strive to minimize their footprint in your code. The

use of I/O and instance mutation is often an essential part of many applications; it’s

important to work to separate your programs into logical partitions, keeping I/O and

its ilk on one side, and transaction processing and mutation on the other. Fortunately

for us, Clojure provides a powerful toolset for making the management of mutability

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CHAPTER 11 Mutation

sane, but none of the tools provide a shortcut to thinking. Multithreaded program-

ming is a difficult problem, independent of specifics, and Clojure’s state-management

tools won’t solve this problem magically. We’ll help to guide you through the proper

use of these tools starting with Clojure’s Ref type.

11.2

When to use Refs

Clojure currently provides four different reference types to aide in concurrent pro-

gramming: Refs, Agents, Atoms, and Vars. All but Vars are considered shared refer-

ences and allow for changes to be seen across threads of execution. The most

important point to remember about choosing between reference types is that

although their features sometimes overlap, each has an ideal use. All the reference

types and their primary characteristics are shown in figure 11.3.

Ref Agent Atom Var

Figure 11.3 Clojure’s four reference types are listed across the

Coordinated

top, with their features listed down the left. Atoms are for lone

Asynchronous

synchronous objects. Agents are for asynchronous actions. Vars

Retriable

are for thread-local storage. Refs are for synchronously

Thread-local

coordinating multiple objects.

The unique feature of Refs is that they’re coordinated. This means that reads and writes

to multiple refs can be made in a way that guarantees no race conditions. Asynchronous

means that the request to update is queued to happen in another thread some time

later, while the thread that made the request continues immediately. Retriable indicates

that the work done to update a reference’s value is speculative and may have to be

repeated. Finally, thread-local means that thread safety is achieved by isolating changes

to state to a single thread.

Value access via the @ reader feature or the deref function provide a uniform client

interface, regardless of the reference type used. On the other hand, the write mecha-

nism associated with each reference type is unique by name and specific behavior, but

dothreads

To illustrate some major points, we’ll use a function dothreads! that launches a

given number of threads each running a function a number of times:

(import '(java.util.concurrent Executors))

(def *pool* (Executors/newFixedThreadPool

(+ 2 (.availableProcessors (Runtime/getRuntime)))))

(defn dothreads! [f & {thread-count :threads

exec-count :times

:or {thread-count 1 exec-count 1}}]

(dotimes [t thread-count]

(.submit *pool* #(dotimes [_ exec-count] (f)))))

The dothreads! function is of limited utility—throwing a bunch of threads at a func-

tion to see if it breaks.

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When to use Refs

241

similar in structure. Each referenced value is changed through the application1 of a

pure function. The result of this function will become the new referenced value.

Finally, all reference types allow the association of a validator function via set-

validator that will be used as the final gatekeeper on any value change.

11.2.1 Coordinated, synchronous change using alter

A Ref is a reference type allowing synchronous, coordinated change to its contained

value. What does this mean? By enforcing that any change to a Ref’s value occurs in a

transaction, Clojure can guarantee that change happens in a way that maintains a con-

sistent view of the referenced value in all threads. But there’s a question as to what

constitutes coordination. We’ll construct a simple vector of Refs to represent a 3 x 3

chess board:

(def initial-board

[[:- :k :-]

[:- :- :-]

[:- :K :-]])

(defn board-map [f bd]

(vec (map #(vec (for [s %] (f s))) bd)))

Just as in section 2.4, the lowercase keyword represents a dark king piece and the

uppercase a light king piece. We’ve chosen to represent the board as a 2D vector of

Refs (which are created by the board-map function). There are other ways to repre-

sent our board, but we’ve chosen this because it’s nicely illustrative—the act of moving

a piece would require a coordinated change in two reference squares, or else a change

to one square in one thread could lead to another thread observing that square as

occupied. Likewise, this problem requires synchronous change, because it would be

no good for pieces of the same color to move consecutively. Refs are the only game in

town to ensure that the necessary coordinated change occurs synchronously. Before

you see Refs in action, we need to define auxiliary functions:

(defn reset!

"Resets the board state. Generally these types of functions are a

bad idea, but matters of page count force our hand."

[]

(def board (board-map ref initial-board))

(def to-move (ref [[:K [2 1]] [:k [0 1]]]))

(def num-moves (ref 0)))

(def king-moves (partial neighbors

[[-1 -1] [-1 0] [-1 1] [0 -1] [0 1] [1 -1] [1 0] [1 1]] 3))

(defn good-move? [to enemy-sq]

(when (not= to enemy-sq) to))

(defn choose-move [[[mover mpos][_ enemy-pos]]]

[mover (some #(good-move? % enemy-pos)

(shuffle (king-moves mpos)))])

1 Except for ref-set on Refs, reset! on Atoms, and set! on Vars.

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CHAPTER 11 Mutation

The to-move structure describes the order of moves, so in the base case, it states that

the light king :K at y=2,x=1 moves before the dark king :k at y=0,x=1. We reuse the

neighbors function from section 7.4 to build a legal-move generator for chess king

pieces. We do this by using partial supplied with the kingly position deltas and the

board size. The good-move? function states that a move to a square is legal only if the

enemy isn’t already located there. The function choose-move destructures the to-

move vector and chooses a good move from a shuffled sequence of legal moves. The

choose-move function can be tested in isolation:

(reset!)

(take 5 (repeatedly #(choose-move @to-move)))

;=> ([:K [1 0]] [:K [1 1]] [:K [1 1]] [:K [1 0]] [:K [2 0]])

And now we’ll create a function to make a random move for the piece at the front of

to-move, shown next.

Listing 11.1 Using alter to update a Ref

(defn place [from to] to)

(defn move-piece [[piece dest] [[_ src] _]]

(alter (get-in board dest) place piece)

Place moving piece

(alter (get-in board src ) place :-)

Empty from square

(alter num-moves inc))

(defn update-to-move [move]

(alter to-move #(vector (second %) move)))

Swap

(defn make-move []

(dosync

(let [move (choose-move @to-move)]

(move-piece move @to-move)

(update-to-move move))))

The alter function appears four times within the dosync, so that the from and to posi-

tions, as well as the to-move Refs, are updated in a coordinated fashion. We’re using

the place function as the alter function, which states “given a to piece and a from

piece, always return the to piece.” Observe what occurs when make-move is run once:

(make-move)

;=> [[:k [0 1]] [:K [2 0]]]

(board-map deref board)

;=> [[:- :k :-] [:- :- :-] [:K :- :-]]

@num-moves

;=> 1

We’ve successfully made a change to two board squares, the to-move structure, and

num-moves using the uniform state change model. By itself, this model of state change

is compelling. The semantics are simple to understand: give a reference a function

that determines how the value changes. This is the model of sane state change that

Clojure preaches. But we can now throw a bunch of threads at this solution and still

maintain consistency:

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When to use Refs

243

(alter num-moves inc)

-- in-transaction --

(apply inc @num-moves)

9

(apply inc 9)

. . .

Figure 11.4 Alter path: the in-transaction value 9 for

the Ref num-moves is retrieved in the body of the

-- commit time --

transaction and manipulated with the alter function

(commit num-moves)

10

inc. This resulting value 10 is eventually used for the

commit-time value, unless a retry is required.

(defn go [move-fn threads times]

(dothreads! move-fn :threads threads :times times))

(go make-move 100 100)

(board-map #(dosync (deref %)) board)

;=> [[:k :- :-] [:- :- :-] [:K :- :-]]

@to-move

;=> [[:k [0 0]] [:K [2 0]]]

@num-moves

;=> 10001

Figure 11.4 shows that at the time of the transaction, the in-transaction value of the to

square is set to (apply place @SQUARE-REF PIECE). At the end of the transaction, the

STM uses this in-transaction value as the commit value. If any other transaction had

updated any other coordinated Ref before commit time, then the whole transaction

would be retried.

Clojure’s retry mechanism guarantees that the Refs in a transaction are always

coordinated upon commit because all other transactions line up waiting their turn to

commit their coordinated values. Look at what happens should the Ref updates hap-

pen in separate transactions:

(defn bad-make-move []

(let [move (choose-move @to-move)]

(dosync (move-piece move @to-move))

(dosync (update-to-move move))))

(go bad-make-move 100 100)

(board-map #(dosync (deref %)) board)

;=> [[:- :K :-] [:- :- :-] [:- :K :-]]

Clearly something has gone awry, and as we mentioned, the reason lies in splitting the

updates of the to and from Refs into different transactions. Being separated into two

transactions means that they’re (potentially) running on different timelines. Because

board and to-move are dependent, their states must be coordinated, but we’ve broken

that necessity with bad-make-move. Therefore, somewhere along the line board was

updated from two subsequent timelines where it was :K’s turn to move!

As shown in figure 11.5, either transaction can commit or be restarted; but because

the two Refs are no longer in the same transaction, the occurrences of these condi-

tions become staggered over time, leading to inconsistent values.

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CHAPTER 11 Mutation

Transaction for A

Transaction for B

-- in-transaction --

-- in-transaction --

(apply f @A)

a

(apply f @B)

b

(apply f a)

(apply f b)

. . .

. . .

-- commit time --

-- commit time --

(commit A)

a'

RESTART

Figure 11.5 Splitting coordinated Refs: if Refs A and B should be coordinated,

then splitting their updates across different transactions is dangerous. Value a?

is eventually committed to A, but the update for B never commits due to retry and

coordination is lost. Another error occurs if B’s change depends on A’s value and

A and B are split across transactions. There are no guarantees that the dependent

values refer to the same timeline.

11.2.2 Commutative change with commute

Figure 11.4 showed that using alter can cause a transaction to retry if a Ref it

depends on is modified and committed while it’s running. But there may be circum-

stances where the value of a Ref within a given transaction isn’t important to its com-

pletion semantics. For example, the num-moves Ref is a simple counter, and surely its

value at any given time is irrelevant for determining how it should be incremented. To

handle these loose dependency circumstances, Clojure offers an operation named

commute. What if we were to change the make-move function to use the commute func-

tion instead of alter?

(defn move-piece [[piece dest] [[_ src] _]]

(commute (get-in board dest) place piece)

(commute (get-in board src ) place :-)

(commute num-moves inc))

(reset!)

(go make-move 100 100)

(board-map deref board)

;=> [[:K :- :-] [:- :- :-] [:- :- :k]]

@to-move

;=> [[:K [0 0]] [:k [2 2]]]

Everything looks great! But you can’t assume that the same will work for update-to-

move:

(defn update-to-move [move]

(commute to-move #(vector (second %) move)))

(go make-move 100 100)

(board-map #(dosync (deref %)) board)

;=> [[:- :- :-] [:- :K :-] [:- :- :K]]

@to-move

[[:K [2 2]] [:K [1 1]]]

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245

(commute num-moves inc)

-- in-transaction --

(apply inc @num-moves)

9

(apply inc 9)

. . .

-- commit time --

Figure 11.6 Commute path: the in-transaction value 9 in the

(apply inc @num-moves)

13 num-moves Ref is retrieved in the body of the transaction and

manipulated with the commute function. But the commute

(apply inc 13)

function inc is again run at commit time with the current value

(commit num-moves)

14 13 contained in the Ref. The result of this action serves as the

committed value 14.

Thanks to our rash decision, we’ve once again introduced inconsistency into the system.

But why? The reason lies in the fact that the new update-to-move isn’t amenable to the

semantics of the commute function. commute allows for more concurrency in the STM by

devaluing in-transaction value disparity resulting from another transaction’s commit. In

other words, figure 11.6 shows that the in-transaction value of a Ref is initially set as

when using alter, but the commit time value is reset just before commute commits.

By retrieving the most current value for a Ref at the time of commit, the values

committed might not be those corresponding to the in-transaction state. This leads to

a condition of update reordering that your application must accommodate. Of course,

this new function isn’t commutative because vector doesn’t give the same answer if its

argument order is switched.

Using commute is useful as long as the following conditions aren’t problematic:

 The value you see in-transaction may not be the value that gets committed at

commit time.

 The function you give to commute will be run at least twice—once to compute

the in-transaction value, and again to compute the commit value. It might be

run any number of times.

11.2.3 Vulgar change with ref-set

The function ref-set is different from alter and commute in that instead of changing

a Ref based on a function of its value, it does so given a raw value:

(dosync (ref-set to-move '[[:K [2 1]] [:k [0 1]]]))

;=> [[:K [2 1]] [:k [0 1]]]

In general, this sort of vulgar change should be avoided. But because the Refs have

become out of sync, perhaps you could be forgiven in using ref-set to fix it—just this

once.

11.2.4 Fixing write-skew with ensure

Snapshot isolation means that within a transaction, all enclosed Ref states represent the

same moment in time. Any Ref value that you see inside a transaction will never change

unless you change it within that transaction. Your algorithms should be devised so that

all you care about is that the values of the references haven’t changed before commit

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CHAPTER 11 Mutation

(unless your change function is commutative, as mentioned previously). If those val-

ues have changed, then the transaction retries, and you try again. Earlier, we talked

about write skew, a condition occurring when you make decisions based on the in-

transaction value of a Ref that’s never written to, which is also changed at the same

time. Avoiding write skew is accomplished using Clojure’s ensure function, which

guarantees a read-only Ref isn’t modified by another thread. The make-move function

isn’t subject to write skew because it has no invariants on read data and in fact never

reads a Ref that it doesn’t eventually write. This design is ideal because it allows other

threads to calculate moves without having to stop them, while any given transaction

does the same. But in your own applications, you may be confronted with a true read

invariant scenario, and it’s in such a scenario that ensure will help.

11.2.5 Refs under stress

After you’ve created your Refs and written your transactions, and simple isolated tests

are passing, you may yet run into difficulties in larger integration tests because of how

Refs behave under stress from multiple transactions. As a rule of thumb, it’s best to

avoid having both short- and long-running transactions interacting with the same Ref.

Clojure’s STM implementation will usually compensate eventually regardless, but

you’ll soon see some less-than-ideal consequences of ignoring this rule.

To demonstrate this problem, listing 11.2 shows a function designed specifically to

over-stress a Ref. It does this by starting a long-running or slow transaction in another

thread, where work is simulated by a 200ms sleep, but all it’s really doing is reading

the Ref in a transaction. This requires the STM to know of a stable value for the Ref for

the full 200ms. Meanwhile, the main thread runs quick transactions 500 times in a

row, each one incrementing the value in the Ref and thereby frustrating the slow

transaction’s attempts to see a stable value. The STM works to overcome this frustra-

tion by growing the history of values kept for the Ref. But by default this history is lim-

ited to 10 entries, and our perverse function can easily saturate that:

(stress-ref (ref 0))

;=> :done

; r is: 500, history: 10, after: 26 tries

You may see a slightly different number of tries, but the important detail is that the

slow transaction is unable to successfully commit and print the value of r until the

main thread has finished its frantic looping and returned :done. The Ref’s history

started at a default of 0 and grew to 10, but this was still insufficient.

Listing 11.2 How to make a Ref squirm

(defn stress-ref [r]

(let [slow-tries (atom 0)]

(future

Long-running

(dosync

transaction

(swap! slow-tries inc)

(Thread/sleep 200)

@r)

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When to use Agents

247

(println (format "r is: %s, history: %d, after: %d tries"

@r (ref-history-count r) @slow-tries)))

(dotimes [i 500]

500 very quick

(Thread/sleep 10)

transactions

(dosync (alter r inc)))

:done))

Remember that our real problem here is mixing short- and long-running transactions

on the same Ref. But if this is truly unavoidable, Clojure allows us to create a Ref with

a more generous cap on the history size:

(stress-ref (ref 0 :max-history 30))

; r is: 410, history: 20, after: 21 tries

;=> :done

Again, your numbers may be different, but this time the Ref’s history grew sufficiently

(reaching 20 in this run) to allow the slow transaction to finish first and report about

r before all 500 quick transactions completed. In this run, only 410 had finished when

the slow transaction committed.

But the slow transaction still had to be retried 20 times, with the history growing

one step large each time, before it was able to complete. If our slow transaction were

doing real work instead of just sleeping, this could represent a lot of wasted comput-

ing effort. If your tests or production environment reveal this type of situation and the

underlying transaction size difference can’t be resolved, one final Ref option can

help. Because you can see that the history will likely need to be 20 anyway, you may as

well start it off closer to its goal:

(stress-ref (ref 0 :min-history 15 :max-history 30))

; r is: 97, history: 19, after: 5 tries

;=> :done

This time the slow transaction finished before even 100 of the quick transactions had

finished; and even though the history grew to roughly the same size, starting it off at

15 meant the slow transaction only retried 4 times before succeeding.

The use of Refs to guarantee coordinated change is generally simple for managing

state in a synchronous fashion, and tuning with :min-history and :max-history is

rarely required. But not all changes in your applications will require coordination,

nor will they need to be synchronous. For these circumstances, Clojure also provides

another reference type, the Agent, that provides independent asynchronous changes,

which we’ll discuss next.

11.3

When to use Agents

Like all Clojure reference types, an Agent represents an identity, a specific thing whose

value can change over time. Each Agent has a queue to hold actions that need to be

performed on its value, and each action will produce a new value for the Agent to

hold and pass to the subsequent action. Thus the state of the Agent advances through

time, action after action, and by their nature only one action at a time can be operat-

ing on a given Agent. Of course, other actions can be operating on other Agents at

the same time, each in its own thread.

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CHAPTER 11 Mutation

You can queue an action on any Agent by using send or send-off, the minor differ-

ence between which we’ll discuss later. Agents are integrated with STM transactions,

and within a transaction any actions sent are held until the transaction commits or are

thrown away if the transaction retries. Thus send and send-off are not considered side-

effects in the context of a dosync, because they handle retries correctly and gracefully.

11.3.1 In-process versus distributed concurrency models

Both Clojure and Erlang are designed (Armstrong 2007) specifically with concurrent

programming in mind, and Erlang’s process2 model is similar in some ways to Clojure

Agents, so it’s fair to briefly compare how they each approach the problem.

Erlang takes a distributed, share-nothing (Armstrong 2007b) approach; Clojure

instead promotes shared, immutable data. The key to Clojure’s success is the fact that

its composite data structures are immutable, because immutable structures can be

freely shared among disparate threads. Erlang’s composite data structures are also

immutable, but because the communication model is distributed, the underlying

theme is always one of dislocation. The implications of this are that all knowledge of

the world under consideration is provided via messages. But with Clojure’s in-process

model, data structures are always accessible directly, as illustrated in figure 11.7,

whereas Erlang makes copies of the data sent back and forth between processes. This

works well for Erlang and allows it to provide its fault recovery guarantees, but many

application domains can benefit from the shared-memory model provided by Clojure.

The second difference is that Erlang messages block on reception, opening up the

possibility for deadlock. On the other hand, when interacting with Clojure Agents,

both sends and derefs proceed immediately and never block or wait on the Agent.

Clojure Agent

Erlang Actor

1

1

Agent

Actor

inc

inc

inc

inc

2

2

Agent

Actor

3

3

Agent

Actor

de

Figure 11.7 Clojure agents versus Erlang

deref

ref

get

get

processes: each Agent and process starts with 

3

3

3

the value 1. Both receive an inc request

Actor

simultaneously but can only process one at a time,

resp

3

so more are queued. Requests to the process are

3

queued until a response can be delivered, whereas

Actor

resp

any number of simultaneous derefs can be done 

on an Agent. Despite what this illustration may

3

suggest, an Agent is not just an actor with a hat on.

2

It’s interesting that popular opinion has tagged Erlang processes with the “actor” tag although the language

implementers rarely, if ever, use that term. Therefore, because the Erlang elite choose not to use that term,

we’ll avoid doing so also... almost.

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When to use Agents

249

Clojure does have an await function that can be used to block a thread until a partic-

ular Agent has processed a message, but this function is specifically disallowed in

Agent threads (and also STM transactions) in order to prevent accidentally creating

this sort of deadlock.

The final difference lies in the fact that Agents allow for arbitrary update func-

tions whereas Erlang processes are bound to static pattern-matched message han-

dling routines. In other words, pattern matching couples the data and update logic,

whereas the former decouples them. Erlang is an excellent language for solving the

extremely difficult problem of distributed computation, but Clojure’s concurrency

mechanisms service the in-process programming model more flexibly than Erlang

allows (Clementson 2008).

11.3.2 Controlling I/O with an Agent

One handy use for Agents is to serialize access to a resource, such as an file or other

I/O stream. For example, imagine we want to provide a way for multiple threads to report

their progress on various tasks, giving each report a unique incrementing number.

Because the state we want to hold is known, we can go ahead and create the Agent:

(def log-agent (agent 0))

Now we’ll supply an action function to send to log-agent. All action functions take as

their first argument the current state of the Agent and can take any number of other

arguments that are sent:

(defn do-log [msg-id message]

(println msg-id ":" message)

(inc msg-id))

Here msg-id is the state—the first time do-log is sent to the Agent, msg-id will be 0.

The return value of the action function will be the new Agent state, incrementing it to

1 after that first action.

Now we need to do some work worth logging about, but for this example we’ll just

pretend:

(defn do-step [channel message]

(Thread/sleep 1)

(send-off log-agent do-log (str channel message)))

(defn three-step [channel]

(do-step channel " ready to begin (step 0)")

(do-step channel " warming up (step 1)")

(do-step channel " really getting going now (step 2)")

(do-step channel " done! (step 3)"))

To see how log-agent will correctly queue and serialize the messages, we need to start

a few threads, each yammering away at the Agent, shown next:

(defn all-together-now []

(dothreads! #(three-step "alpha"))

(dothreads! #(three-step "beta"))

(dothreads! #(three-step "omega")))

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CHAPTER 11 Mutation

(all-together-now)

; 0 : alpha ready to being (step 0)

; 1 : omega ready to being (step 0)

; 2 : beta ready to being (step 0)

; 3 : alpha warming up (step 1)

; 4 : alpha really getting going now (step 2)

; 5 : omega warming up (step 1)

; 6 : alpha done! (step 3)

; 7 : omega really getting going now (step 2)

; 8 : omega done! (step 3)

; 9 : beta warming up (step 1)

; 10 : beta really getting going now (step 2)

; 11 : beta done! (step 3)

Your output is likely to look different, but one thing that should be exactly the same is

the stable, incrementing IDs assigned by the Agent, even while the alpha, beta, and

omega threads fight for control.

There are several other possible approaches to solving this problem, and it can be

constructive to contrast them. The simplest alternative would be to hold a lock while

printing and incrementing. Besides the general risk of deadlocks when a complex

program has multiple locks, there are some specific drawbacks even if this would be

the only lock in play. For one, each client thread would block anytime there was con-

tention for the lock, and unless some fairness mechanism were used, there’d be at

least a slight possibility of one or more threads being “starved” and never having an

opportunity to print or proceed with their work. Because Agent actions are queued

and don’t block waiting for their action to be processed, neither of these is a concern.

Another option would be to use a blocking queue to hold pending log messages.

Client threads would be able to add messages to the queue without blocking and with

adequate fairness. But you’d generally need to dedicate a thread to popping messages

from the queue and printing them, or write code to handle starting and stopping the

printing thread as needed. Why write such code when Agents do this for you already?

When no actions are queued, the Agent in our example has no thread assigned to it.3

Agents have other features that may or may not be useful in any given situation.

One is that the current state of an Agent can be observed cheaply. In the previous

example, this would allow us to discover the ID of the next message to be written out,

as follows:

@log-agent

;=> 11

Here the Agent is idle—no actions are queued or running, but the same expression

would work equally well if the Agent were running.

Other features include the await and await-for functions, which allow a sending

thread to block until all the actions it’s sent to a given set of Agents have completed.

3

Using Agents for logging might not be appropriate in all cases. For example, in probing scenarios, the num-

ber of log events could be extremely high. Coupling this volume with serialization could make the Agent

unable to catch its ever-growing queue.

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When to use Agents

251

This could be useful in this logging example if we wanted to be sure a particular mes-

sage had been written out before proceeding:

(do-step "important: " "this must go out")

(await log-agent)

The await-for function is similar but allows you to specify a number of milliseconds

after which to time out, even if the queued actions still haven’t completed.

A final feature Agents provide is that the set of actions you can send to an Agent is

open. You can tell an Agent to do something that wasn’t even conceived of at the time

the Agent was designed. For example, we could tell the Agent to skip ahead several

IDs, and this action would be queued up along with all the log-message actions and

executed by the Agent when its turn came:

(send log-agent (fn [_] 1000))

(do-step "epsilon " "near miss")

; 1000 : epsilon near miss

This is another area in which Clojure allows you to extend your design on the fly

instead of requiring recompiling or even restarting your app. If you’re paying atten-

tion, you might wonder why we used send in that last example rather than send-off.

11.3.3 The difference between send and send-off

You can use either send or send-off with any Agent. When you use send-off as we

did in most of the examples so far, only a single action queue is involved: the one man-

aged by the individual Agent. Anytime the Agent has a send-off action queued, it has

a thread assigned to it, working through the queue. With send, there’s a second

queue—actions still go into the Agent’s queue, but then the Agent itself queues up

waiting for a thread from a fixed-sized pool of threads. The size of this fixed pool is

based on the number of processors the JVM is running on, so it’s a bad idea to use

send with any actions that might block, tying up one of these limited number of

threads. These differences are illustrated in figure 11.8.

Idle agent. Empty queue,

no thread assigned.

Action queue

Unbounded

thread

send-off

pool

New agent state becomes

action applied to old state.

Figure 11.8 Agents using send versus send-off. When

Action queue

send

an Agent is idle, no CPU resources are being consumed.

Agent

queue

Each action is sent to an Agent using either send or

send-off, which determines which thread pool will be

The limited number of

used to dequeue and apply the action. Because actions

threads distribute CPU time

fairly among the queued

queued with send are applied by a limited thread pool, the

Bounded

agents.

thread

Agents queue up for access to these threads, a constraint

pool

that doesn’t apply to actions queued with send-off.

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CHAPTER 11 Mutation

We can make this scenario play out if we make a gaggle of Agents and send them

actions that sleep for a moment. Here’s a little function that does this, using which-

ever send function we specify, and then waits for all the actions to complete:

(defn exercise-agents [send-fn]

(let [agents (map #(agent %) (range 10))]

(doseq [a agents]

(send-fn a (fn [_] (Thread/sleep 1000))))

(doseq [a agents]

(await a))))

If we use send-off, all the agents will begin their one-second wait more or less simul-

taneously, each in its own thread. So the entire sequence of them will complete in

slightly over one second:

(time (exercise-agents send-off))

; "Elapsed time: 1008.771296 msecs"

Now we can demonstrate why it’s a bad idea to mix send with actions that block:

(time (exercise-agents send))

; "Elapsed time: 3001.555086 msecs"

The exact elapsed time you’ll see will depend on the number of processors you have,

but if you have fewer than eight you’ll see this example takes at least two seconds to

complete. The threads in the fixed-size pool are all clogged up waiting for sleep to

finish, so the other Agents queue up waiting for a free thread. Because clearly the

computer could complete all 10 actions in about one second using send-off, using

send is a bad idea.

So that’s it: send is for actions that stay busy using the processor and not blocking

on I/O or other threads, whereas send-off is for actions that might block, sleep, or

otherwise tie up the thread. This is why we used send-off for the threads that printed

log lines and send for the one that did no I/O at all.

11.3.4 Error handling

We’ve been fortunate so far—none of these Agent actions have thrown an exception.

But real life is rarely so kind. Most of the other reference types are synchronous and

so exceptions thrown while updating their state bubble up the call stack in a normal

way, to be caught with a regular try/catch in your application (or not). Because

Agent actions run in other threads after the sending thread has moved on, we need a

different mechanism for handling exceptions that are thrown by Agent actions. As of

Clojure 1.2, you can choose between two different error-handling modes for each

Agent: :continue and :fail.

:FAIL

By default, new Agents start out using the :fail mode, where an exception thrown by

an Agent’s action will be captured by the Agent and held so that you can see it later.

Meanwhile, the Agent will be considered failed or stopped and will stop processing its

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When to use Agents

253

action queue—all the queued actions will have to wait patiently until someone clears

up the Agent’s error.

One common mistake when dealing with Agents is to forget that your action func-

tion must take at least one argument for the Agent’s current state. For example, we

might try to reset the log-agent’s current message ID like this:

(send log-agent (fn [] 2000)) ; incorrect

@log-agent

;=> 1001

At first glance it looks like the action we sent had no effect, or perhaps hasn’t been

applied yet. But we’d wait in vain for that Agent to do anything ever again without

intervention, because it’s stopped. One way to determine this is with the agent-error

function:

(agent-error log-agent)

;=> #<IllegalArgumentException java.lang.IllegalArgumentException:

; Wrong number of args passed to: user$eval--509$fn>

This returns the error of a stopped Agent, or nil if it’s still running fine. Another way

to see whether an Agent is stopped is to try to send another action to it:

(send log-agent (fn [_] 3000))

; java.lang.RuntimeException: Agent is failed, needs restart

Even though this action would’ve worked fine, the Agent has failed and so no further

sends are allowed. The state of log-agent remains unchanged:

@log-agent

;=> 1001

In order to get the Agent back into working order, we need to restart it:

(restart-agent log-agent 2500 :clear-actions true)

;=> 2500

This resets the value of log-agent to 2500 and deletes all those actions patiently wait-

ing in their queue. If we hadn’t included the :clear-actionstrue option, those

actions would’ve survived and the Agent would continue processing them. Either way,

the Agent is now in good working order again, and so we can again send and send-

off to it:

(send-off log-agent do-log "The agent, it lives!")

; 2500 : The agent, it lives!

;=> #<Agent@72898540: 2500>

Note that restart-agent only makes sense and thus is only allowed when the Agent

has failed. If it hasn’t failed, any attempt to restart it throws an exception in the thread

making the attempt, and the Agent is left undisturbed:

(restart-agent log-agent 2500 :clear-actions true)

;=> java.lang.RuntimeException: Agent does not need a restart

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CHAPTER 11 Mutation

This mode is perhaps most appropriate for manual intervention. Agents that normally

don’t have errors but in a running system end up failing can use the :fail mode to

keep from doing anything too bad until a human can take things in hand, check to

see what happened, choose an appropriate new state for the Agent, and restart it just

as we did here.

:CONTINUE

The other error mode that Agents currently support is :continue, where any action

that throws an exception is skipped and the Agent proceeds to the next queued action

if any. This is most useful when combined with an error handler—if you specify an

:error-handler when you create an Agent, that Agent’s error mode defaults to

:continue. The Agent calls the error handler when an action throws an exception

and doesn’t proceed to the next action until the handler returns. This gives the han-

dler a chance to report the error in some appropriate way. For example, we could

have log-agent handle faulty actions by logging the attempt:

(defn handle-log-error [the-agent the-err]

(println "An action sent to the log-agent threw " the-err))

(set-error-handler! log-agent handle-log-error)

(set-error-mode! log-agent :continue)

With the error mode and handler set up, sending faulty actions does cause reports to

be printed as we wanted:

(send log-agent (fn [x] (/ x 0))) ; incorrect

; An action sent to the log-

agent threw java.lang.ArithmeticException: Divide by zero

;=> #<Agent@66200db9: 2501>

(send log-agent (fn [] 0)) ; also incorrect

; An action sent to the log-agent threw java.lang.IllegalArgumentException:

; Wrong number of args passed to: user$eval--820$fn

;=> #<Agent@66200db9: 2501>

And the Agent stays in good shape, always ready for new actions to be sent:

(send-off log-agent do-log "Stayin' alive, stayin' alive...")

; 2501 : Stayin' alive, stayin' alive...

Note that error handlers can’t change the state of the Agent (ours keeps its current

message id of 2501 throughout the preceding tests). Error handlers are also sup-

ported in the :fail error mode, but handlers can’t call restart-agent so they’re less

often useful for :fail than they are for the :continue error mode.

11.3.5 When not to use Agents

It can be tempting to repurpose Agents for any situation requiring the spawning of

new threads. Their succinct syntax and “Clojurey” feel often make this temptation

strong. But though Agents perform beautifully when each one is representing a real

identity in your application, they start to show weaknesses when used a sort of “green

thread” abstraction. In cases where you just need a bunch of worker threads banging

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When to use Atoms

255

away on some work, or you have a specific long-running thread polling or blocking on

events, or any other kind of situation where it doesn’t seem useful that the Agent

maintain a value, you’ll usually be able to find a better mechanism than Agents. In

these cases, there’s every reason to consider using a Java Thread directly, or a Java

executor (as we did with dothreads!) to manage a pool of threads, or in some cases

perhaps a Clojure future.

Another common temptation is to use Agents when you need state held but you

don’t actually want the sending thread to proceed until the Agent action you sent is

complete. This can be done by using await, but it’s another form of abuse that should

be avoided. For one, because you’re not allowed to use await in an Agent’s action, as

you try to use this technique in more and more contexts you’re likely to run into a sit-

uation where it won’t work. But in general, there’s probably a reference type that will

do a better job of behaving the way you want. Because this is essentially an attempt to

use Agents as if they were synchronous, you may have more success with one of the

other shared synchronous types. In particular, Atoms are shared and uncoordinated

just like Agents, but they’re synchronous and so may fit better.

11.4

When to use Atoms

Atoms are like Refs in that they’re synchronous but are like Agents in that they’re inde-

pendent (uncoordinated). An Atom may seem at first glance similar to a variable, but

as we proceed you’ll see that any similarities are at best superficial. The use cases for

Atoms are similar to those of compare-and-swap ( CAS ) spinning operations. Anywhere

you might want to atomically compute a value given an existing value and swap in the

new value, an Atom will suffice. Atom updates occur locally to the calling thread, and

execution continues after the Atom value has been changed. If another thread B

changes the value in an Atom before thread A is successful, then A retries. But these

retries are spin-loop and don’t occur within the STM, and thus Atom changes can’t be

coordinated with changes to other reference types. You should take care when embed-

ding changes to Atoms within Clojure’s transactions because as you know, transactions

can potentially be retried numerous times. Once an Atom’s value is set, it’s set, and it

doesn’t roll back when a transaction is retried, so in effect this should be viewed as a

side effect. Therefore, use Atoms in transactions only when you’re certain that an

attempt to update its value, performed numerous times, is idempotent.

Aside from the normal use of @ and deref to query an Atom’s value, you can also

use the mutating functions swap!, compare-and-set!, and reset!.

11.4.1 Sharing across threads

As we mentioned, Atoms are thread safe and can be used when you require a light-

weight mutable reference to be shared across threads. A simple case is one of a glob-

ally accessible incrementing timer created using the atom function:

(def *time* (atom 0))

(defn tick [] (swap! *time* inc))

(dothreads! tick :threads 1000 :times 100)

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CHAPTER 11 Mutation

@*time*

;=> 100000

Though this will work, Java already provides a concurrent class for just such a purpose,

java.util.concurrent.atomic.AtomicInteger, which can be used similarly:

(def *time* (java.util.concurrent.atomic.AtomicInteger. 0))

(defn tick [] (.getAndIncrement *time*))

(dothreads! tick :threads 1000 :times 100)

*time*

;=> 100000

Though the use of AtomicInteger is more appropriate in this case, the use of an

Atom works to show that it’s safe to use across threads.

11.4.2 Using Atoms in transactions

Just because we said that Atoms should be used carefully within transactions, that’s not

to say that they can never be used in that way. In fact, the use of an Atom as the refer-

ence holding a function’s memoization cache is idempotent on update.

MEMOIZATION Memoization is a way for a function to store calculated values in

a cache so that multiple calls to the function can retrieve previously calcu-

lated results from the cache, instead of performing potentially expensive cal-

culations every time. Clojure provides a core function memoize that can be

used on any referentially transparent function.

Individual requirements from memoization are highly personal, and a generic

approach isn’t always the appropriate solution for every problem. We’ll discuss per-

sonalized memoization strategies in section 12.4, but for now we’ll use an illustrative

example appropriate for Atom usage.

ATOMIC MEMOIZATION

The core memoize function is great for creating simple function caches, but it has

some limitations. First, it doesn’t allow for custom caching and expiration strategies.

Additionally, memoize doesn’t allow you to manipulate the cache for the purposes of

clearing it in part or wholesale. Therefore, we’ll create a function manipulable-

memoize that allows us to get at the cache and perform operations on it directly.

Throughout the book, we’ve mentioned Clojure’s metadata facility, and for this exam-

ple it will come in handy. We can take in the function to be memoized and attach

some metadata4 with an Atom containing the cache itself for later manipulation.

Listing 11.3 A resettable memoize function

(defn manipulable-memoize [function]

(let [cache (atom {})]

Store cache

(with-meta

in Atom

(fn [& args]

4 The ability to attach metadata to functions is a recent addition to Clojure version 1.2.

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257

(or (second (find @cache args))

Check cache first

(let [ret (apply function args)]

Else calculate, 

(swap! cache assoc args ret)

store, and return

ret)))

{:cache cache})))

Attach metadata

As shown in listing 11.3, we’ve slightly modified the core memoize function to attach

the Atom to the function being memoized. You can now observe manipulable-

memoize in action:

(def slowly (fn [x] (Thread/sleep 3000) x))

(time [(slowly 9) (slowly 9)])

; "Elapsed time: 6000.63 msecs"

;=> [9 9]

(def sometimes-slowly (manipulable-memoize slowly))

(time [(sometimes-slowly 108) (sometimes-slowly 108)])

; "Elapsed time: 3000.409 msecs"

;=> [108 108]

The call to slowly is always... well... slow, as you’d expect. But the call to sometimes-

slowly is only slow on the first call given a certain argument. This too is just as you’d

expect. Now we can inspect sometimes-slowly’s cache and perform some operations

on it:

(meta sometimes-slowly)

;=> {:cache #<Atom@e4245: {(108) 108}>}

(let [cache (:cache (meta sometimes-slowly))]

(swap! cache dissoc '(108)))

;=> {}

You may wonder why we used swap! to dissoc the cached argument 108 instead of

using (reset! cache {}). There are certainly valid use cases for the wholesale reset

of an Atom’s value, and this case is arguably one. But it’s good practice to set your ref-

erence values via the application of a function rather than the in-place value setting.

In this way, you can be more selective about the value manipulations being per-

formed. Having said that, here are the consequences our actions had:

(meta sometimes-slowly)

;=> {:cache #<Atom@e4245: {}>}

(time (sometimes-slowly 108))

; "Elapsed time: 3000.3 msecs"

;=> 108

And yes, you can see that we were able to remove the cached argument value 108

using the metadata map attached to the function sometimes-slowly. There are better

ways to allow for pointed cache removal than this, but for now you can take heart in

that using an Atom, we’ve allowed for the local mutation of a reference in a thread-

safe way. Additionally, because of the nature of memoization, you can use these

memoized functions in a transaction without ill effect. Bear in mind that if you do use

this in a transaction, then any attempt to remove values from the cache may not be

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CHAPTER 11 Mutation

met with the results expected. Depending on the interleaving of your removal and any

restarts, the value(s) you remove might be reinserted on the next time through the

restart. But even this condition is agreeable if your only concern is reducing total

cache size.

11.5

When to use locks

Clojure’s reference types and parallel primitives cover a vast array of use cases. Addi-

tionally, Java’s rich set of concurrency classes found in the java.util.concurrent

package are readily available. But even with this arsenal of tools at your disposal, there

still may be circumstances where explicit locking is the only option available, the com-

mon case being the modification of arrays concurrently. We’ll start with a simple pro-

tocol to describe a concurrent, mutable, safe array that holds an internal array

instance, allowing you to access it or mutate it safely. A naive implementation can be

seen in the following listing.

Listing 11.4 A simple SafeArray protocol

(ns joy.locks

(:refer-clojure :exclude [aget aset count seq])

(:require [clojure.core :as clj]))

(defprotocol SafeArray

Small set 

(aset [this i f])

of functions

(aget [this i])

(count [this])

(seq [this]))

(defn make-dumb-array [t sz]

(let [a (make-array t sz)]

(reify

SafeArray

(count [_] (clj/count a))

(seq [_] (clj/seq a))

aget and aset

(aget [_ i] (clj/aget a i))

are unguarded

(aset [this i f]

(clj/aset a i (f (aget this i)))))))

If you’ll notice, we used the :refer-clojure namespace directive to :exclude the

array and sequence functions that the SafeArray protocol overrides. We did this not

only because it’s important to know how to use :refer-clojure, but also because

we’re changing the semantics of aset to take a mutating function as its last argument

instead of a raw value. We then used the :require directive to alias the Clojure

namespace as clj, thus avoiding the need to use the fully qualified function names a

la clojure.core/aget.

The dumb array created by make-dumb-array is stored in a closure created by

reify, and unguarded access is provided without concern for concurrent matters.

Using this implementation across threads is disastrous, as shown:

(defn pummel [a]

(dothreads! #(dotimes [i (count a)] (aset a i inc)) :threads 100))

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When to use locks

259

(def D (make-dumb-array Integer/TYPE 8))

(pummel D)

;; wait for pummel to terminate

(seq D)

;=> (82 84 65 63 83 65 83 87)

This is very wrong—100 threads incrementing concurrently should result in 100 for

each array slot. To add insult to injury, Clojure didn’t throw a Concurrent-

ModificationException as you might’ve expected, but instead just silently went

along doing very bad things. Next, we’ll talk a little about locking and provide an

alternate implementation for SafeArray using locking primitives.

11.5.1 Safe mutation through locking

Currently, the only5 way to safely modify and see consistent values for a mutable object

across threads in Clojure is through locking.

REFERENCES AROUND EVIL MUTABLE THINGS

Wrapping a mutable object in a Clojure reference type provides absolutely no guarantees

for safe concurrent modification. Doing this will at best explode immediately or, worse,

provide inaccurate results.

If at all possible, locking should be avoided; but for those times when it’s unavoid-

able, the locking macro will help. The locking macro takes a single parameter acting

as the locking monitor and a body that executes in the monitor context. Any writes

and reads to the monitor object are thread safe, and as a bonus the monitor is always

released at the end of the block. One of the major complexities in concurrent pro-

gramming using locks is that all errors must be handled fully and appropriately; other-

wise you risk orphaned locks, and they spell deadlock. But the locking macro will

always release the lock, even in the face of exceptions.

Listing 11.5 An implementation of the SafeArray protocol using the locking macro

(defn make-safe-array [t sz]

(let [a (make-array t sz)]

Array creation

(reify

is same

SafeArray

(count [_] (clj/count a))

(seq [_] (clj/seq a))

(aget [_ i]

aget is locked

(locking a

(clj/aget a i)))

(aset [this i f]

aset is locked

(locking a

(clj/aset a i (f (aget this i))))))))

aset uses aget

(def A (make-safe-array Integer/TYPE 8))

(pummel A)

5 Although a potential future addition to Clojure named pods may provide another.

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CHAPTER 11 Mutation

;; wait for pummel to terminate

(seq A)

;=> (100 100 100 100 100 100 100 100)

We used the locking macro on both the aget and aset functions so that they can

both maintain consistency concurrently. Because aset calls aget, the locking macro

is called twice. This isn’t a problem because locking is reentrant, or able to be called

multiple times in the same thread. Typically, you’d have to manage the releasing of

reentrant locking mechanism to match the number of times called, but fortunately

locking manages that for us.

The locking macro is the simplest way to perform primitive locking in Clojure.

But the implementation of make-safe-array is coarse in that the locks used are

guarding the entire array. Any readers or writers wishing to access or update any slot

in the array must wait their turn, a bottleneck known as contention. If you need finer-

grained locking, the locking facilities provided by Java will help to gain more control,

a topic we cover next.

11.5.2 Using Java’s explicit locks

Java provides a set of explicit locks in the java.util.concurrent.locks package that

can also be used as shown in the following listing. One such lock is provided by the

java.util.concurrent.locks.ReentrantLock class.

Listing 11.6 An implementation of the SafeArray protocol using ReentrantLock

(defn lock-i [target-index num-locks]

(mod target-index num-locks))

(import 'java.util.concurrent.locks.ReentrantLock)

(defn make-smart-array [t sz]

(let [a (make-array t sz)

Array

Lsz (quot sz 2)

L (into-array (take Lsz

Locks

(repeatedly #(ReentrantLock.))))]

(reify

SafeArray

(count [_] (clojure.core/count a))

(seq [_] (clojure.core/seq a))

(aget [_ i]

(let [lk (clojure.core/aget L (lock-i (inc i) Lsz))]

(.lock lk)

Explicit

(try

locking

(clojure.core/aget a i)

Explicit

(finally (.unlock lk)))))

unlocking

(aset [this i f]

(let [lk (clojure.core/aget L (lock-i (inc i) Lsz))]

(.lock lk)

(try

Reentrant

(clojure.core/aset a i (f (aget this i)))

locking

(finally (.unlock lk))))))))

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When to use futures

261

(def S (make-smart-array Integer/TYPE 8))

(pummel S)

;; wait for pummel to terminate

(seq S)

;=> (100 100 100 100 100 100 100 100)

The first point of note is that we use a technique (simplified for clarity) called lock

striping (Herlihy 2008) to reduce the contention of guarding the array as a whole

using locking. The target array a’s slots are guarded by half the number of locks, each

chosen using the simple formula (mod target-index num-locks). This scheme

allows readers and writers to (potentially) act independently when accessing different

array slots. It’s crucial that we closed over the lock instance array L because for explicit

locks to work, each access must lock and unlock the same instance. Additionally, we’re

calling the .unlock method in the body of a finally expression, because failing to do

so is a recipe for disaster. Unlike the locking macro, the ReentrantLock class doesn’t

manage lock release automatically. Finally, you can also use the ReentrantLock in a

way equivalent to using the locking macro, but using ReentrantLock gives you the

choice of using proxy to provide more complex semantics than locking can provide.

One flaw of the make-smart-array function is that it uses the same locks for read-

ers and writers. But you can allow for more concurrency if you enable some number

of readers to access array slots without blocking at all by using the java.util.

concurrent.locks.ReentrantReadWriteLock class. The ReentrantReadWriteLock

class holds two lock instances, one for reads and one for writes, and by adding another

lock array you can take advantage of this fact. We won’t get into that exercise here, but

if you choose to do so then you can use the implementation of make-smart-array as a

guide.

Using the various locking mechanisms, you can guarantee consistency across

threads for mutable objects. But as we showed with explicit locks, there’s an expected

incantation to unlocking that must be strictly observed. Though not necessarily com-

plex in the SafeArray implementations, the conceptual baggage incurred in the

semantics of explicit locking scheme doesn’t scale well. The java.util.concurrent

package contains a cacophony of concurrency primitives above and beyond simple

locks, but it’s not our goal to provide a comprehensive survey herein.

Now that we’ve covered the matter of guaranteeing coordinated state across dispa-

rate threads, we turn our attention to a different topic: parallelization.

11.6

When to use futures

Clojure includes two reference types supporting parallelism: futures and promises.

Futures, the subject of this section, are simple yet elegant constructs useful for parti-

tioning a typically sequential operation into discrete parts. These parts can then be

asynchronously processed across numerous threads that will block if the enclosed

expression hasn’t finished. All subsequent dereferencing will return the calculated

value. The simplest example of the use of a future is as shown:

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CHAPTER 11 Mutation

(time (let [x (future (do (Thread/sleep 5000) (+ 41 1)))]

[@x @x]))

; "Elapsed time: 5001.682 msecs"

;=> [42 42]

The processing time of the do block is only paid for on the first dereference of the

future x. Futures represent expressions that have yet to be computed.

11.6.1 Futures as callbacks

One nice use case for futures is in the context of a callback mechanism. Normally you

might call out to a remote-procedure call (RPC), wait for it to complete, and then pro-

ceed with some task depending on the return value. But what happens if you need to

make multiple RPC calls? Should you be forced to wait for them all serially? Thanks to

futures, the answer is no. In this section, we’ll use futures to create an aggregate task

that finds the total number of occurrences of a string within a given set of Twitter6

feeds. This aggregate task will be split into numerous parallel subtasks via futures.

COUNTING WORD OCCURRENCES IN A SET OF TWITTER FEEDS

Upon going to a personal Twitter page such as http://twitter.com/fogus, you can find

a link to the RSS 2.0 feed for that user. We’ll use this feed as the input to our functions.

An RSS 2.0 feed is an XML document used to represent a piece of data that’s con-

stantly changing. The layout of a Twitter RSS entry is straightforward:

<rss version="2.0">

<channel>

<title>Twitter / fogus</title>

<link>http://twitter.com/fogus</link>

<item>

<title>fogus: Thinking about #Clojure futures.</title>

<link>http://twitter.com/fogus/statuses/12180102647/</link>

</item>

</channel>

</rss>

There’s more to the content of a typical RSS feed, but for our purposes we wish to only

retrieve the title element of the item elements (there can be more than one). To do

this, we need to first parse the XML and put it into a convenient format. If you recall

from section 8.4, we created a domain DSL to create a tree built on a simple node

structure of tables with the keys :tag, :attrs, and :content. As mentioned, that struc-

ture is leveraged in many Clojure libraries, and we’ll take advantage of this fact. Clo-

jure provides some core functions in the clojure.xml and clojure.zip namespaces

to help make sense of the feed:

(require '(clojure [xml :as xml]))

(require '(clojure [zip :as zip]))

(defmulti rss-children class)

(defmethod rss-children String [uri-str]

6 Twitter is online at http://twitter.com.

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When to use futures

263

(-> (xml/parse uri-str)

zip/xml-zip

zip/down

zip/children))

Using the function clojure.xml/parse, we can retrieve the XML for a Twitter RSS

feed and convert it into the familiar tree format. That tree is then passed into a func-

tion clojure.zip/xml-zip that converts that structure into another data structure

called a zipper. The form and semantics of the zipper are beyond the scope of this book

(Huet 1997), but using it in this case allows us to easily navigate down from the root

rss XML node to the channel node, where we then retrieve its children. The child

nodes returned from rss-children contain other items besides item nodes (title,

link, and so forth) that need to be filtered out. Once we have those item nodes, we

then want to retrieve the title text and count the number of occurrences of the tar-

get text (case-insensitive). We perform all of these tasks using the function count-

tweet-text-task, defined in the following listing.

Listing 11.7 Creating a future task to count word occurrences in a tweet

(import '(java.util.regex Pattern))

(defn count-tweet-text-task [txt feed]

(let [items (rss-children feed)

Get kids

re (Pattern/compile (Pattern/quote txt))]

(count

(mapcat #(re-seq re (first %))

Get matches

(for [item (filter (comp #{:item} :tag) items)]

Filter non-items

(-> item :content first :content))))))

Get title

We’ll now try to count some text in a Twitter feed to see what happens:

(count-tweet-text-task

"#clojure"

"http://twitter.com/statuses/user_timeline/46130870.rss")

;=> 7

The result you see is highly dependent on when you run this function, because the

RSS feeds are ever-changing. But using the count-tweet-text-task function, we can

build a sequence of tasks to be performed over some number of Twitter feeds. Before

we do that, we’ll create a convenience macro as-futures to take said sequence and

dispatch the enclosed actions across some futures.

Listing 11.8 A macro to dispatch a sequence of futures

(defmacro as-futures [[a args] & body]

(let [parts (partition-by #{'=>} body)

[acts _ [res]] (partition-by #{:as} (first parts))

[_ _ task] parts]

`(let [~res (for [~a ~args] (future ~@acts))]

~@task)))

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CHAPTER 11 Mutation

The as-futures macro implemented in listing 11.8 names a binding corresponding

to the arguments for a given action, which is then dispatched across a number of

futures, after which a task is run against the futures sequence. The body of as-

futures is segmented so that we can clearly specify the needed parts—the action

arguments, the action to be performed for each argument, and the tasks to be run

against the resulting sequence of futures:

(as-futures [<arg-name> <all-args>]

<actions-using-args>

:as <results-name>

=>

<actions-using-results>)

To simplify the macro implementation, we use the :as keyword and => symbol to

clearly delineate its segments. The as-futures body only exits after the task body fin-

ishes—as determined by the execution of the futures. We can use as-futures to per-

form the original task with a new function tweet-occurrences, implemented in the

following listing.

Listing 11.9 Counting string occurrences in Twitter feeds fetched in parallel

(defn tweet-occurrences [tag & feeds]

(as-futures [feed feeds]

(count-tweet-text-task tag feed)

:as results

=>

(reduce (fn [total res] (+ total @res))

0

results)))

The as-futures macro builds a sequence of futures named results, enclosing the

call to count-tweet-text-task across the unique set of Twitter feeds provided. We

then sum the counts returned from the dereferencing of the individual futures, as

shown:

(tweet-occurrences "#Clojure"

"http://twitter.com/statuses/user_timeline/46130870.rss"

"http://twitter.com/statuses/user_timeline/14375110.rss"

"http://twitter.com/statuses/user_timeline/5156041.rss"

"http://twitter.com/statuses/user_timeline/21439272.rss")

;=> 22

And that’s that. Using only a handful of functions and macros, plus using the built-in

core facilities for XML parsing and navigation, we’ve created a simple Twitter occur-

rences counter. Our implementation has some trade-offs made in the name of page

count. First, we blindly dereference the future in tweet-occurrences when calculat-

ing the sum. If the future’s computation freezes, then the dereference would likewise

freeze. Using some combination of future-done?, future-cancel, and future-can-

celled? in your own programs, you can skip, retry, or eliminate ornery feeds from the

calculation. Futures are only one way to perform parallel computation in Clojure, and

in the next section we’ll talk about another—promises.

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When to use promises

265

11.7 When to use promises

Another tool that Clojure provides for parallel computation is the promise and

deliver mechanisms. Promises are similar to futures, in that they represent a unit of

computation to be performed on a separate thread. Likewise, the blocking semantics

when dereferencing an unfinished promise are also the same. Whereas futures encap-

sulate an arbitrary expression that caches its value in the future upon completion,

promises are placeholders for values whose construction is fulfilled by another thread

via the deliver function. A simple example is as follows:

(def x (promise))

(def y (promise))

(def z (promise))

(dothreads! #(deliver z (+ @x @y)))

(dothreads!

#(do (Thread/sleep 2000) (deliver x 52)))

(dothreads!

#(do (Thread/sleep 4000) (deliver y 86)))

(time @z)

; "Elapsed time: 3995.414 msecs"

;=> 138

Promises are write-once; any further attempt to deliver will throw an exception.

11.7.1 Parallel tasks with promises

We can create a macro similar to as-futures for handling promises, but because of

the more advanced value semantics, the implementation is thus more complicated.

We again wish to provide a named set of tasks, but we’d additionally like to name the

corresponding promises so that we can then execute over the eventual results, which

we do next.

Listing 11.10 A macro to dispatch a sequence of promises across a number of threads

(defmacro with-promises [[n tasks _ as] & body]

(when as

`(let [tasks# ~tasks

n# (count tasks#)

promises# (take n# (repeatedly promise))]

(dotimes [i# n#]

(dothreads!

(fn []

(deliver (nth promises# i#)

((nth tasks# i#))))))

(let [~n tasks#

~as promises#]

~@body))))

We could then build a rudimentary parallel testing facility, dispatching tests across dis-

parate threads and summing the results when all of the tests are done:

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CHAPTER 11 Mutation

(defrecord TestRun [run passed failed])

(defn pass [] true)

(defn fail [] false)

(defn run-tests [& all-tests]

(with-promises

[tests all-tests :as results]

(into (TestRun. 0 0 0)

(reduce #(merge-with + %1 %2) {}

(for [r results]

(if @r

{:run 1 :passed 1}

{:run 1 :failed 1}))))))

(run-tests pass fail fail fail pass)

;=> #:user.TestRun{:run 5, :passed 2, :failed 3}

This unit-testing model is simplistic by design in order to illustrate parallelization

using promises and not to provide a comprehensive testing framework.

11.7.2 Callback API to blocking API

Promises, much like futures, are useful for executing RPC on separate threads. This

can be useful if you need to parallelize a group of calls to an RPC service, but there’s a

converse use case also. Often, RPC APIs take arguments to the service calls and also a

callback function to be executed when the call completes. Using the rss-children

function from the previous section, we can construct an archetypal RPC function:

(defn tweet-items [k feed]

(k

(for [item (filter (comp #{:item} :tag) (rss-children feed))]

(-> item :content first :content))))

The tweet-items function is a distillation of the count-tweet-text-task function

from the previous chapter, as shown:

(tweet-items

count

"http://twitter.com/statuses/user_timeline/46130870.rss")

;=> 16

The argument k to tweet-items is the callback, or continuation, that’s called with the

filtered RPC results. This API is fine, but there are times when a blocking call is more

appropriate than callback based call. We can use a promise to achieve this blocking

behavior with the following:

(let [p (promise)]

(tweet-items #(deliver p (count %))

"http://twitter.com/statuses/user_timeline/46130870.rss")

@p)

;=> 16

And as you see, the call blocks until the deliver occurs. This is a fine way to transform

the callback into a blocking call, but we’d like a way to do this generically. Fortunately,

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When to use promises

267

most well-written RPC APIs follow the same form for their callback functions/methods,

so we can create a macro to wrap this up nicely in the following listing.

Listing 11.11 A macro for transforming a callback-based function to a blocking call

(defmacro cps->fn [f k]

`(fn [& args#]

(let [p# (promise)]

(apply ~f (fn [x#] (deliver p# (~k x#))) args#)

@p#)))

(def count-items (cps->fn tweet-items count))

(count-items "http://twitter.com/statuses/user_timeline/46130870.rss")

;=> 16

This is a simple solution to a common problem that you may have already encoun-

tered in your own applications.

11.7.3 Deterministic deadlocks

You can cause a deadlock in your applications by never delivering to a promise. One

possibly surprising advantage of using promises is that if a promise can deadlock, it’ll

deadlock deterministically. Because only a single thread can ever deliver on a promise,

only that thread will ever cause a deadlock. We can create a cycle in the dependencies

between two promises to observe a deadlock using the following code:

(def kant (promise))

(def hume (promise))

(dothreads!

#(do (println "Kant has" @kant) (deliver hume :thinking)))

(dothreads!

#(do (println "Hume is" @hume) (deliver kant :fork)))

The Kant thread is waiting for the delivery of the value for kant from the Hume

thread, which in turn is waiting for the value for hume from the Kant thread. Attempt-

ing either @kant or @hume in the REPL will cause an immediate deadlock. Further-

more, this deadlock will happen every time; it’s deterministic rather than dependent

on odd thread timings or the like. Deadlocks are never nice, but deterministic dead-

locks are better than nondeterministic.7

We’ve only touched the surface for the potential that promises represent. In fact,

the pieces that we’ve assembled in this section represent some of the basic building

blocks of dataflow (Van Roy 2004) concurrency. But any attempt to serve justice to

dataflow concurrency in a single section would be a futile effort. At its essence,

7

There are experts in concurrent programming who will say that naïve locking schemes are also deterministic.

Our simple example is illustrative, but alas it isn’t representative of a scheme that you may devise for your own

code. In complex designs where promises are created in one place and delivered in a remote locale, deter-

mining deadlock will naturally be more complex. Therefore, we’d like to use this space to coin a new phrase:

“determinism is relative.”

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CHAPTER 11 Mutation

dataflow deals with the process of dynamic changes in values causing dynamic changes

in dependent “formulas.” This type of processing finds a nice analogy in the way that

spreadsheet cells operate, some representing values and others dependent formulas

that change as the former also change.

Continuing our survey of Clojure’s parallelization primitives, we’ll next discuss

some of the functions provided in the core library.

11.8

Parallelism

In the previous two sections we built two useful macros as-futures and with-

promises, allowing you to parallelize a set of operations across numerous threads. But

Clojure has functions in its core library providing similar functionality named pmap,

pvalues, and pcalls, which we’ll cover briefly in this section.

11.8.1 pvalues

The pvalues macro is analogous to the as-futures macro, in that it executes an arbi-

trary number of expressions in parallel. Where it differs is that it returns a lazy

sequence of the results of all the enclosed expressions, as shown:

(pvalues 1 2 (+ 1 2))

;=> (1 2 3)

The important point to remember when using pvalues is that the return type is a lazy

sequence, meaning that your access costs might not always present themselves as

expected:

(defn sleeper [s thing] (Thread/sleep (* 1000 s)) thing)

(defn pvs [] (pvalues

(sleeper 2 :1st)

(sleeper 3 :2nd)

(keyword "3rd")))

(-> (pvs) first time)

; "Elapsed time: 2000.309 msecs"

;=> :1st

The total time cost of accessing the first value in the result of pvs is only the cost of its

own calculation. But accessing any subsequent element costs as much as the most

expensive element before it, which you can verify by accessing the last element:

(-> (pvs) last time)

; "Elapsed time: 4001.435 msecs"

;=> :3rd

This may prove a disadvantage if you want to access the result of a relatively cheap

expression that happens to be placed after a more costly expression. More accurately,

all seq values within a sliding window 8 are forced, so processing time is limited by the

most costly element therein.

8 Currently, the window size is N+2, where N is the number of CPU cores. But this is an implementation detail, so it’s enough to know only that the sliding window exists.

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Parallelism

269

11.8.2 pmap

The pmap function is the parallel version of the core map function. Given a function

and a set of sequences, the application of the function to each matching element hap-

pens in parallel:

(->> [1 2 3]

(pmap (comp inc (partial sleeper 2)))

doall

time)

; "Elapsed time: 2000.811 msecs"

;=> (2 3 4)

The total cost of realizing the result of mapping a costly increment function is again

limited by the most costly execution time within the aforementioned sliding window.

Clearly, in this contrived case, using pmap provides a benefit, so why not just replace

every call to map in your programs with a call to pmap? Surely this would lead to faster

execution times if the map functions were all applied in parallel, no? The answer is a

resounding: it depends. A definite cost is associated with keeping the resulting

sequence result coordinated, and to indiscriminately use pmap might actually incur

that cost unnecessarily, leading to a performance penalty. But if you’re certain that the

cost of the function application outweighs the cost of the coordination, then pmap

might help to realize performance gains. Only through experimentation will you be

able to determine whether pmap is the right choice.

11.8.3 pcalls

Finally, Clojure provides a pcalls function that takes an arbitrary number of func-

tions taking no arguments and calls them in parallel, returning a lazy sequence of the

results. The use shouldn’t be a surprise by now:

(-> (pcalls

#(sleeper 2 :1st)

#(sleeper 3 :2nd)

#(keyword "3rd"))

doall

time)

; "Elapsed time: 3001.039 msecs"

;=> (:1st :2nd :3rd)

The same benefits and trade-offs associated with pvalues and pmap also apply to

pcalls and should be considered before use.

Executing costly operations in parallel can be a great boon when used properly,

but should by no means be considered a magic potion guaranteeing speed gains.

There’s currently no magical formula for determining which parts of an application

can be parallelized—the onus is on you to determine your application’s parallel

potential. What Clojure provides is a set of primitives, including futures, promises,

pmap, pvalues, and pcalls as the building blocks for your own personalized paral-

lelization needs.

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CHAPTER 11 Mutation

In the next section, we’ll cover the ubiquitous Var, but from a different perspective

than we have thus far.

11.9

Vars and dynamic binding

The last reference type we’ll explore is perhaps the most commonly used—the Var.

Vars are most often used because of two main features:

 Vars can be named and interned in a namespace.

 Vars can provide thread-local state.

It’s through the second feature that Vars contribute most usefully to the reference

type landscape. The thread-local value of a Var by definition can only be read from or

written to a single thread, and thus provides the thread-safe semantics you’ve come to

expect from a Clojure reference type.

But before you can start experimenting with Vars at the REPL, we to need address

some consequences of the first feature. The other reference objects you’ve looked at

aren’t themselves named and so are generally stored in something with a name. This

means that when the name is evaluated, you get the reference object, not the value.

To get the object’s value, you have to use deref. Named Vars flip this around—evaluat-

ing their name gives the value, so if you want the Var object, you need to pass the

name to the special operator var.

With this knowledge in hand, you can experiment with an existing Var. Clojure

provides a Var named *read-eval*,9 so you can get its current value by evaluating its

name:

*read-eval*

;=> true

No deref needed, because *read-eval* is a named Var. Now for the Var object itself:

(var *read-eval*)

;=> #'clojure.core/*read-eval*

That’s interesting—when a named Var object is printed, it starts with #' and is then

followed by the fully qualified name of the Var. The #' reader feature expands to the

Var operator—it means the same thing:

#'*read-eval*

;=> #'clojure.core/*read-eval*

Now that you’ve seen how to refer to Var objects, you can look at how they behave.

The Var *read-eval* is one of those provided by Clojure that’s specifically meant to

be given thread-local bindings but by default has only a root binding. You should’ve

seen its root binding when you evaluated it earlier—by default, *read-eval* is bound

to true.

9 *read-eval* happens to be a Var that has a default configuration useful for this discussion about Vars —its

actual purpose is unimportant here.

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271

11.9.1 The binding macro

The root binding of a Var can act as the base of a stack, with each thread’s local bind-

ings pushing onto that stack and popping off of it as requested. The most common

mechanism for pushing and popping thread-local bindings is the macro binding. It

takes one or more Var names and a value for each that will initialize the new binding

when it’s pushed. These bindings remain in effect until control passes out of the

binding macro, at which point they’re popped off the stack.

Here’s a simple example of a function that prints the current value of the Var

*read-eval*, either the root or thread-local value, whichever is currently in effect:

(defn print-read-eval []

(println "*read-eval* is currently" *read-eval*))

This function calls print-read-eval three times, the first and last of which will print

the root binding. The middle time, binding is in effect:

(defn binding-play []

Thread A

(print-read-eval)

(binding [*read-eval* false]

(print-read-eval))

Thread B

Var root

(print-read-eval))

binding

push binding

This results in the Var temporarily having a thread-

local value of false:

pop binding

(binding-play)

; *read-eval* is currently true

Thread C

; *read-eval* is currently false

push binding

; *read-eval* is currently true

This is a like thread B in figure 11.9, which also shows a

push binding

simpler scenario than thread A and a more complex

one than thread C.

pop binding

11.9.2 Creating a named Var

pop binding

Vars are most commonly created with the special oper-

ator def or one of the many macros that expands to a

Figure 11.9 Thread-local Var

form that has a def inside:

bindings. This illustration depicts

a single Var being used from three

 defn—For putting a function in a Var

different threads. Each rounded

 defmacro—For putting a macro in a Var

box is a Var binding, either thread-



local or root. Each star is the Var

defonce—For setting the value of an unbound Var

being deref'ed, with the solid

 defmulti—For putting a multimethod in a Var

arrow pointing to the binding used.

The dotted lines point from a

There are a few others in clojure.core10 and many

thread-local binding to the next

more in contrib. What they have in common is that binding on the stack.

10 It's likely that starting with Clojure 1.3 Vars will only have the ability to take on thread-local values when defined using defdynamic or marked with metadata like ^{:dynamic true}. Throughout this book, we will

take the latter approach with high confidence that it will just work in 1.3.

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CHAPTER 11 Mutation

each of these will intern a Var in the current namespace. Clojure will search for the

named Var in the current namespace. If one is found, it’s used; otherwise, a new Var is

created and added to the namespace, and that one is used.11 The Var (specifically the

root binding of the Var) is bound to whatever value, function, or macro (and so on)

was given. The Var itself is returned:

(def favorite-color :green)

#'user/favorite-color

When a Var is printed, its fully qualified name is given, along with the namespace where

the Var is interned (user) and the Var’s name itself (favorite-color). These are pre-

ceded by #' because unlike the other reference types, a named Var is automatically

dereferenced when its name is evaluated—no explicit @ or call to deref is required:

favorite-color

;=> :green

So in order to refer to a Var instead of the value it’s bound to, you need to use #' or

the special form var, which are equivalent:

(var favorite-color)

;=> #'user/favorite-color

A Var can exist (or not exist) in any of four states. The precise state a Var is in can be

determined using the functions resolve, bound?, and thread-bound? as shown in

Table 11.1.

Table 11.1 Var states

Initialization mechanism

(resolve 'x)

(bound? #'x)

(thread-bound? #'x)

(def x)

#'user/x

false

false

(def x 5)

#'user/x

true

false

(binding [x 7] ...)

#'user/x

true

true

(with-local-vars [x 9] ...)

nil

true (bound? x)

true (thread-bound? x)

The first row of the table shows the results of resolve, bound?, and thread-bound?

when a var x is unbound. The remaining rows show how to change x to cause those

functions to return the values shown.

11.9.3 Creating anonymous Vars

Vars don’t always have names, nor do they need to be interned in a namespace. The

with-local-vars macro creates Vars and gives them thread-local bindings all at once,

but it won’t intern them. Instead, they’re bound to locals, which means that the

associated Var isn’t implicitly looked up by symbolic name. You need to use deref or

var-get to get the current value of the Var. Here’s an example of a Var x created and

11 Not all macros starting with def necessarily create or intern Vars. Some that don’t: defmethod, defrecord,

and deftype.

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Vars and dynamic binding

273

interned with def, and then a local x that shadows it and is bound to a new var via

with-local-vars:

(def x 42)

{:outer-var-value x

:with-locals (with-local-vars [x 9]

{:local-var x

:local-var-value (var-get x)})}

;=> {:outer-var-value 42,

:with-locals {:local-var #<Var: --unnamed-->,

:local-var-value 9}}

Within the body of the with-local-vars macro, the bound value can bet set using

(var-set <var> <value>), which will of course only affect the thread-local value. It’s

almost stunning how rarely with-local-vars is useful.

11.9.4 Dynamic scope

Vars have dynamic scope, which contrasts with the lexical scope of let locals. The most

obvious difference is that with a lexical local, you can easily see where it was initialized

by looking at the nested structure of the code. A Var, on the other hand, may have been

initialized by a binding anywhere earlier in the call stack, not necessarily nearby in the

code at all. This difference can create unexpectedly complex interactions and is one of

the few areas where Clojure does little to help you address such complexity.

An example of this complexity is shown by using the binding macro or any macro

built on top of it, such as with-precision and with-out-str. For example, we can

use the with-precision macro to conveniently set up the *math-context* Var:

(with-precision 4

(/ 1M 3))

;=> 0.3333M

We need to use with-precision here because if we don’t tell BigDecimal we’re okay

with it rounding off the result, it’ll refuse to return anything in this case:

(/ 1M 3)

; java.lang.ArithmeticException: Non-terminating decimal expansion;

; no exact representable decimal result.

With that in mind, can you see why with-precision isn’t doing its job in the next

snippet? The only thing that makes it different from the example that worked earlier

is we’re using map to produce a sequence of three numbers instead of just one:

(with-precision 4

(map (fn [x] (/ x 3)) (range 1M 4M)))

; java.lang.ArithmeticException: Non-terminating decimal expansion;

; no exact representable decimal result.

The problem is that map is lazy and therefore doesn’t call the function given to it

immediately. Instead, it waits until the REPL tries to print it, and then does the divi-

sion. Although the map and the function it calls are within the lexical scope of with-

binding, and with-binding itself uses a thread-local binding internally, it doesn’t care

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CHAPTER 11 Mutation

about lexical scope. When the division operation is performed, we’ve already left the

dynamic scope of the with-precision, and it no longer has any effect. The BigDecimal

behavior drops back to its default, and it throws an exception.

One way to solve this is to make sure that all the division is done before leaving the

dynamic scope. Clojure’s doall function is perfect for this:

(with-precision 4

(doall (map (fn [x] (/ x 3)) (range 1M 4M))))

;=> (0.3333M 0.6667M 1M)

One drawback is that it completely defeats map’s laziness. An alternate solution is to

have the function provided to map re-create, when it’s run, the dynamic scope in which

the function was created. Clojure provides a handy macro bound-fn to do exactly that:

(with-precision 4

(map (bound-fn [x] (/ x 3)) (range 1M 4M)))

;=> (0.3333M 0.6667M 1M)

Now the sequence being returned is still lazy, but before each item is computed, the

dynamic scope of *math-context* is re-created and the exception is avoided.

This kind of mismatch between a function definition that appears lexically inside a

form like with-precision or binding and yet has a different dynamic scope when

called doesn’t cause problems with lazy sequences alone. You may also see problems

with functions sent to Agents as actions or with the body of a future, because these are

executed in other threads outside the dynamic scope where they’re set up.

Problems related to dynamic scope aren’t even exclusive to Vars. The scope of a

try/catch is also dynamic and can have similarly unexpected behavior. For example,

with-open uses try/finally to close a file automatically when execution leaves its

dynamic scope. Failing to account for this can lead to an error when trying to write to

a closed file, because the dynamic scope of with-open has been left. Though bound-

fn can help make the dynamic scope of a Var borrow from its lexical scope, the only

way to deal with try/catch is to make sure everything is executed before leaving its

dynamic scope.

11.10 Summary

This has been the most complex chapter of the book. State management is a compli-

cated process that can quickly lose all semblance of sanity in the face of concurrent

modifications. Clojure’s main tenet is not to foster concurrency, but instead to pro-

vide the tools for the sane management of state. As a result of this focus, sane concur-

rency follows. Clojure also provides the building blocks for you to parallelize

computations across disparate threads of execution. From the expression-centric

future, to the function-centric set-once “variable” promise, to the core functions

pcalls, pvalues, and pmap, Clojure gives you the raw materials for your specialized

needs. Finally, we talked in depth about Clojure’s Var, dynamic binding, and the

mechanics of thread-locals.

The next chapter deals with performance considerations and how to make your

Clojure programs much faster.

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Part 5

Tangential

considerations

Some topics are so interesting and important that we must include them,

even if they don’t fit well in another chapter or warrant a chapter to themselves.

In this part, we’ll cover several such topics, including transient collections,

domain-specific languages, and testing.

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Performance

This chapter covers

 Type hints

 Transients

 Chunked sequences

 Memoization

 Understanding coercion

Now that we’ve spent a book’s worth of material learning the why and how of Clo-

jure, it’s high time we turned our attention to the subject of performance. There’s

a meme in programming that can be summarized as follows: make it work first,

then make it fast. Throughout this book, we’ve taught you the ways that Clojure

allows you to “make it work,” and now we’re going to tell how to make it fast.

In many cases, Clojure’s compiler will be able to highly optimize idiomatic Clo-

jure source code. But there are times when the form of your functions, especially in

interoperability scenarios, will prove to be ambiguous or even outright counter to

compiler optimizations. Therefore, we’ll lead you through optimization techniques

such as type hints, transients, chunked sequences, memoization, and coercion.

Using some combination of these techniques will help you approach, and some-

times exceed, the performance of Java itself.

The most obvious place to start, and the one you’re most likely encounter, is

type hinting—so this is where we’ll begin.

277

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CHAPTER 12 Performance

12.1

Type hints

The path of least resistance in Clojure often produces the fastest and most efficient

compiled code, but not always. The beauty of Clojure is that this path of least resis-

tance allows simple techniques for gaining speed via type hints. The first thing to

know about type hints is that they’re used to indicate that an object is an instance of

some class—never a primitive.

THE RULE OF TYPE HINTING Write your code so that it’s first and foremost cor-

rect; then add type-hint adornment to gain speed. Don’t trade the efficiency

of the program for the efficiency of the programmer.

12.1.1 Advantages of type adornment

There are epic debates about the virtues of static versus dynamic type systems; we

won’t engage in those arguments here. But there are a few advantages to a dynamic

type system like Clojure’s that also allows type hinting to occur after the bulk of devel-

opment. One such advantage is that in a static type system, the cost of changing argu-

ment lists is extended to all of the callers, whereas in Clojure the cost is deferred until

adornment time or even outright avoided.1 This scenario isn’t limited to the case of

function arguments in Clojure nor to statically typed languages, but instead to any

typed element. This dynamic type system provides an agile experience in general to

Clojure, which can later be optimized when there’s a need.

12.1.2 Type-hinting arguments and returns

If you recall from section 10.3, we created a function asum-sq that took an array of

floats and performed a sum of squares on its contents. Unfortunately, asum-sq wasn’t

as fast as it could’ve been. We can illuminate the cause of its inefficiency using a REPL

flag named *warn-on-reflection*, which by default is set to false:

(set! *warn-on-reflection* true)

;=> true

What this seemingly innocuous statement does is to signal to the REPL to report when

the compiler encounters a condition where it can’t infer the type of an object and

must use reflection to garner it at runtime. You’ll see a reflection warning by entering

asum-sq into the REPL:

(defn asum-sq [xs]

(let [dbl (amap xs i ret

(* (aget xs i)

(aget xs i)))]

(areduce dbl i ret 0

(+ ret (aget dbl i)))))

; Reflection warning - call to aclone can't be resolved.

; ...

1 Aside from the case where type hints don’t require client changes, the use of keyword arguments as seen in

section 7.1 can help to localize additional function requirements to only the callers needing them.

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Type hints

279

Though not terribly informative in and of itself, the fact that a reflection warning

occurs is portentous. Running the call to asum-sq in a tight loop verifies that some-

thing is amiss:

(time (dotimes [_ 10000] (asum-sq (float-array [1 2 3 4 5]))))

; "Elapsed time: 410.539 msecs"

;=> nil

Though the reflection warning didn’t point to the precise inefficiency, you can infer

where it could be given that Clojure deals with the java.lang.Object class across

function boundaries. Therefore, you can assume that the problem lies in the argu-

ment xs coming into the function as something unexpected. Adding two type hints to

xs and dbl (because it’s built from xs) might do the trick:

(defn asum-sq [ ^floats xs]

(let [^floats dbl (amap xs i ret

...

Rerunning the tight loop verifies that the assumption was correct:

(time (dotimes [_ 10000] (asum-sq (float-array [1 2 3 4 5]))))

; "Elapsed time: 17.087 msecs"

;=> nil

This is a dramatic increase in speed using a simple type hint that casts the incoming

array xs to one containing primitive floats. The whole range of array type hints is

shown next:

 objects

 floats

 shorts

 ints

 doubles

 bytes

 longs

 chars

 booleans

The problems might still not be solved, especially if you want to do something with the

return value of asum-sq, as shown:

(.intValue (asum-sq (float-array [1 2 3 4 5])))

; Reflection warning, reference to field intValue can't be resolved.

;=> 55

This is because the compiler can’t garner the type of the return value and must there-

fore use reflection to do so. By hinting the return type of asum-sq, the problem goes

away:

(defn ^Float asum-sq [ ^floats xs]

...

(.intValue (asum-sq (float-array [1 2 3 4 5])))

;=> 55

With minor decoration on the asum-sq function, we’ve managed to increase its speed

as well as potentially increasing the speed of expressions downstream.

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CHAPTER 12 Performance

12.1.3 Type-hinting objects

In addition to allowing for the hinting of function arguments and return values, you

can also hint arbitrary objects. If you didn’t have control over the source to asum-sq,

then these reflection problem would be insurmountable when executing (.intValue

(asum-sq (float-array [1 2 3 4 5]))). But you can instead hint at the point of usage

and gain the same advantage as if asum-sq had been hinted all along:

(.intValue ^Float (asum-sq (float-array [1 2 3 4 5])))

;=> 55

All isn’t lost when you don’t own a piece of code causing performance problems,

because Clojure is flexible in the placement of type hints.

12.2

Transients

We’ve harped on you for this entire book about the virtues of persistent data struc-

tures and how wonderful they are. In this section, we’ll present an optimization tech-

nique provided by Clojure called transients, which offer a mutable view of a collection.

It seems like blasphemy, but we assure you there’s a good reason for their existence,

which we’ll discuss currently.

12.2.1 Ephemeral garbage

The design of Clojure is such that it presumes that the JVM is extremely efficient at

garbage collection of ephemeral (or short-lived) objects, and in fact it is. But as you

can imagine based on what you’ve learned so far, Clojure does create a lot of young

objects that are never again accessed, shown (in spirit) here:

(reduce merge [{1 3} {1 2} {3 4} {3 5}])

;=> {1 2, 3 5}

A naive implementation2 of reduce would build intermediate maps corresponding to

the different phases of accumulation. The accumulation of these short-lived instances

can in some circumstances cause inefficiencies, which transients are meant to address.

THE RULE OF TRANSIENTS Write your code so that it’s first and foremost cor-

rect using the immutable collections and operations; then, make changes to

use transients for gaining speed. But you might be better served by writing idi-

omatic and correct code and letting the natural progression of speed

enhancements introduced in new versions of Clojure take over. Spot optimi-

zations often quickly become counter-optimizations by preventing the lan-

guage/libraries from doing something better.

We’ll explore how you can use transients in the next section.

2 The actual implementation of reduce follows a reduce protocol that delegates to a smart “internal reduce”

mechanism that’s meant for data structures that know the most efficient way to reduce themselves.

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Transients

281

12.2.2 Transients compare in efficiency to mutable collections

Mutable objects generally don’t make new allocations during intermediate phases of

an operation on a single collection type, and comparing persistent data structures

against that measure assumes a lesser memory efficiency. But you can use transients to

provide not only efficiency of allocation, but often of execution as well. Take a function

zencat, intended to work similarly to Clojure’s concat, but with vectors exclusively:

(defn zencat1 [x y]

(loop [src y, ret x]

(if (seq src)

(recur (next src) (conj ret (first src)))

ret)))

(zencat1 [1 2 3] [4 5 6])

;=> [1 2 3 4 5 6]

(time (dotimes [_ 1000000] (zencat1 [1 2 3] [4 5 6])))

; "Elapsed time: 486.408 msecs"

;=> nil

The implementation is simple enough, but it’s not all that it could be. The effects of

using transients is shown next.

Listing 12.1 A concatenation function using transients

(defn zencat2 [x y]

(loop [src y, ret (transient x)]

Create transient

(if (seq src)

(recur (next src) (conj! ret (first src)))

Use transient conj!

(persistent! ret))))

Return persistent

(zencat2 [1 2 3] [4 5 6])

;=> [1 2 3 4 5 6]

(time (dotimes [_ 1000000] (zencat2 [1 2 3] [4 5 6])))

; "Elapsed time: 846.258 msecs"

;=> nil

Wait, what? It seems that by using transients, we’ve actually made things worse—but

have we? The answer lies in the question, “what am I actually measuring?” The timing

code is executing zencat2 in a tight loop. This type of timing isn’t likely representa-

tive of actual use, and instead highlights an important consideration: the use of

persistent! and transient, though constant time, aren’t free. By measuring the use

of transients in a tight loop, we’ve introduced a confounding measure, with the dispa-

rate cost of using transients compared to the cost of concatenating two small vectors.

A better benchmark would instead be to measure the concatenation of larger vectors,

therefore minimizing the size-relative cost of transients:

(def bv (vec (range 1e6)))

(first (time (zencat1 bv bv)))

; "Elapsed time: 181.988 msecs"

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CHAPTER 12 Performance

;=> 0

(first (time (zencat2 bv bv)))

; "Elapsed time: 39.353 msecs"

;=> 0

In the case of concatenating large vectors, the use of transients is ~4.5 times faster

than the purely functional approach. Be careful how you use transients in your own

applications, because as you saw, they’re an incredible boon in some cases, and quite

the opposite in others. Likewise, be careful designing performance measurements,

because they might not always measure what you think.

Because transients are a mutable view of a collection, you should take care when

exposing outside of localized contexts. Fortunately, Clojure doesn’t allow a transient

to be modified across threads and will throw an exception if attempted. But it’s easy

enough to forget that you’re dealing with a transient and return it from a function.

That’s not to say that you couldn’t return a transient from a function—it can be use-

ful to build a pipeline of functions that work in concert against a transient structure.

Instead, we ask that you remain mindful when doing so.

The use of transients can help to gain speed in many circumstances. But be

mindful of the trade-offs when using them, because they’re not cost-free operations.

12.3

Chunked sequences

With the release of Clojure 1.1, the granularity of Clojure’s laziness was changed from

a one-at-a-time model to a chunk-at-a-time model. Instead of walking through a

sequence one node at a time, chunked sequences provide a windowed view (Boncz

2005) on sequences some number of elements wide, as illustrated here:

(def gimme #(do (print \.) %))

(take 1 (map gimme (range 32)))

You might expect that this snippet would print (.0) because we’re only grabbing the

first element, and if you’re running Clojure 1.0, that’s exactly what you’d see. But in

later versions, the picture is different:

;=> (................................0)

If you count the dots, you’ll see exactly 32, which is what you’d expect given the state-

ment from the first paragraph. Expanding a bit further, if you increase the argument

to range to be 33 instead, you’ll see the following:

(take 1 (map gimme (range 33)))

;=> (................................0)

Again you can count 32 dots. Moving the chunky window to the right is as simple as

obtaining the 33rd element:

(take 1 (drop 32 (map gimme (range 64))))

;=> (................................................................32)

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Chunked sequences

283

Figure 12.1 Clojure’s chunked

sequences allow a windowed view

of a sequence. This model is more

efficient, in that it allows for larger

swaths of memory to be reclaimed

by the garbage collector and better

cache locality in general. There’s a

cost to total laziness, but often the

benefit gained is worth the cost.

As we showed in chapter 5, Clojure’s sequences are implemented as trees fanning out

at increments of 32 elements per node. Therefore, chunks of size 32 are a natural fit,

allowing for the garbage collection of larger chunks of memory as seen in figure 12.1.

You might be worried that chunked sequences have squashed the entire point of

lazy sequences, and for small sequences this would be correct. But the benefits of lazy

sequences are striking when dealing with cyclopean magnitudes or sequences larger

than memory. Chunked sequences in the extreme cases are an incredible boon

because not only do they make sequence functions more efficient overall, they still ful-

fill the promise of lazy sequences: avoiding full realization of interim results.

12.3.1 Regaining one-at-a-time laziness

There are legitimate concerns about this chunked model, and one such concern is

the desire for a one-at-a-time model to avoid exploding computations. Assuming that

you have such a requirement, one counterpoint against chunked sequences is that of

building an infinite sequence of Mersenne primes.3 Implicit realization of the first 32

Mersenne primes through chunked sequences will finish long after the Sun has died.

But you can use lazy-seq to create a function seq1 that can be used to restrict (or

dechunkify, if you will) a lazy sequence and enforce the one-at-a-time model, as in the

following listing.

Listing 12.2 A dechunkifying seq1 function

(defn seq1 [s]

(lazy-seq

(when-let [[x] (seq s)]

(cons x (seq1 (rest s))))))

(take 1 (map gimme (seq1 (range 32))))

;=> (.0)

(take 1 (drop 32 (map gimme (seq1 (range 64)))))

;=> (.................................32)

3 See http://en.wikipedia.org/wiki/Mersenne_Primes.

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CHAPTER 12 Performance

You can again safely generate your lazy, infinite

sequence of Mersenne primes. The world

rejoices. But seq1 eliminates the garbage-

collection efficiencies of the chunked model and

again regressed back to that shown in figure 12.2.

Clojure may one day provide an official API

for one-at-a-time lazy sequence granularity, but

for now seq1 will suffice. We advise that you

Figure 12.2 Using seq1, you can again

instead stick to the chunked model, because

reclaim the one-at-a-time sequence model.

Though not as efficient as the chunked

you’ll probably never notice its effects during

model, it does again provide total

normal usage.

sequence laziness.

12.4 Memoization

As we mentioned briefly in section 11.4, memoization (Michie 1968) refers to storing

a cache of values local to a function so that its arguments can be retrieved rather than

calculated on every call. The cache is a simple mapping of a given set of arguments to

a previously calculated result. In order for this to work, the memoized function must

be referentially transparent, which we discussed in section 7.1. Clojure comes with a

memoize function that can be used to build a memoized version of any referentially

transparent function, as shown:

(def gcd (memoize

(fn [x y]

(cond

(> x y) (recur (- x y) y)

(< x y) (recur x (- y x))

:else x))))

(gcd 1000645475 56130776629010010)

;=> 215

Defining a “greatest common denominator” function using memoize helps to speed

subsequent calculations using the arguments 1000645475 and 56130776629010010.

The function memoize wraps another function4 in a cache lookup pass-through func-

tion and returns it. This allows you to use memoize on literally any referentially trans-

parent function. The operation of the memoize is analogous to, but not exactly the

operation of Clojure’s lazy sequences that cache the results of their realized portions.

This general technique can be useful, but the indiscriminate storage provided by

memoize might not always be appropriate. Therefore, we’ll take a step back and devise

a way to generalize the operation of memoization into useful abstractions and build a

framework for employing caching strategies more appropriate to the domain at hand.

4

You might’ve noticed that we explicitly bound the Var gcd to the memoization of an anonymous function but

then used recur for implementing the function body. This approach suffers from the inability to cache the

intermediate results (Norvig 1991) of gcd. We leave the solution to this short-coming as an exercise for the

reader.

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Memoization

285

Similar to Haskell’s typeclasses, Clojure’s protocols define a set of signatures pro-

viding a framework of adherence to a given set of features. This section serves a three-

fold goal:

 Discussion of memoization

 Discussion of protocol design

 Discussion of abstraction-oriented programming

12.4.1 Re-examining memoization

As mentioned in section 11.4, memoization is a personal affair, requiring a certain

domain knowledge to perform efficiently and correctly. That’s not to say that the core

memoize function is useless, only that the base case doesn’t cover all cases. In this sec-

tion, we’ll define a memoization protocol in terms of the primitive operations:

lookup, has?, hit, and miss. Instead of providing a memoization facility that allows

the removal of individual cache items, it’s a better idea to provide one that allows for

dynamic cache-handling strategies.5

12.4.2 A memoization protocol

The protocol for a general-purpose cache feature is provided in the following listing.

Listing 12.3 A protocol for caching

(defprotocol CacheProtocol

(lookup [cache e])

(has? [cache e] )

(hit [cache e])

(miss [cache e ret]))

The function lookup retrieves the item in the cache if it exists. The function has? will

check for a cached value. The function hit is called when an item is found in the

cache, and miss is called when it’s not. If you’re familiar with creating Java interfaces,

the process of creating a protocol should be familiar. Moving on, we next implement

the core memoize functionality.

Listing 12.4 A basic cache type

(deftype BasicCache [cache]

CacheProtocol

(lookup [_ item]

(get cache item))

(has? [_ item]

(contains? cache item))

(hit [this item] this)

(miss [_ item result]

(BasicCache. (assoc cache item result))))

5 This section is motivated by the fantastic work of the brilliant Clojurians Meikel Brandmeyer, Christophe

Grand, and Eugen Dück summarized at http://kotka.de/blog/2010/03/memoize_done_right.html.

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CHAPTER 12 Performance

The BasicCache takes a cache on construction used for its internal operations. Test-

ing the basic caching protocol in isolation shows:

(def cache (BasicCache. {}))

(lookup (miss cache '(servo) :robot) '(servo))

;=> :robot

In the case of a miss, the item to be cached is added and a new instance of BasicCache

(with the cached entry added) is returned for retrieval using lookup. This is a simple

model for a basic caching protocol, but not terribly useful in isolation. We can go fur-

ther by creating an auxiliary function through, meaning in effect, “pass an element

through the cache and return its value”:

(defn through [cache f item]

(if (has? cache item)

(hit cache item)

(miss cache item (delay (apply f item)))))

With through, the value corresponding to a cache item (function arguments in this

case) would either be retrieved from the cache via the hit function, or calculated and

stored via miss. You’ll notice that the calculation (apply f item) is wrapped in a

delay call instead of performed outright or lazily through an ad hoc initialization

mechanism. The use of an explicit delay in this way helps to ensure that the value is

calculated only on first retrieval. With these pieces in place, we can then create a

PluggableMemoization type, as shown next.

Listing 12.5 A type implementing pluggable memoization

(deftype PluggableMemoization [f cache]

CacheProtocol

(has? [_ item] (has? cache item))

(hit [this item] this)

(miss [_ item result]

(PluggableMemoization. f (miss cache item result)))

(lookup [_ item]

(lookup cache item)))

The purpose of the PluggableMemoization type is to act as a delegate to an underlying

implementation of a CacheProtocol occurring in the implementations for hit, miss,

and lookup. Likewise, the PluggableMemoization delegation is interposed at the

protocol points to ensure that when utilizing the CacheProtocol, the Pluggable-

Memoization type is used and not the BasicCache. We’ve made a clear distinction

between a caching protocol fulfilled by BasicCache and a concretized memoization

fulfilled by PluggableMemoization and through. With the creation of separate abstrac-

tions, you can use the appropriate concrete realization in its proper context. Clojure

programs will be composed of various abstractions. In fact, the term abstraction-oriented

programming is used to describe Clojure’s specific philosophy of design.

The original manipulable-memoize function from section 11.4 is modified in the

following listing to conform to our memoization cache realization.

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Understanding coercion

287

Listing 12.6 A function for applying pluggable memoization to a function

(defn memoization-impl [cache-impl]

(let [cache (atom cache-impl)]

(with-meta

(fn [& args]

(let [cs (swap! cache through (.f cache-impl) args)]

@(lookup cs args)))

{:cache cache})))

If you’ll recall from the implementation of the through function, we stored delay

objects in the cache requiring they be deferenced when looked up. Returning to our

old friend the slowly function, we can exercise the new memoization technique as

shown:

(def slowly (fn [x] (Thread/sleep 3000) x))

(def sometimes-slowly (memoization-impl

(PluggableMemoization.

slowly

(BasicCache. {}))))

(time [(sometimes-slowly 108) (sometimes-slowly 108)])

; "Elapsed time: 3001.611 msecs"

;=> [108 108]

(time [(sometimes-slowly 108) (sometimes-slowly 108)])

; "Elapsed time: 0.049 msecs"

;=> [108 108]

You can now fulfill your personalized memoization needs by implementing pointed real-

izations of CacheProtocol, plugging them into instances of PluggableMemoization,

and applying them as needed via function redefinition, higher-order functions, or

dynamic binding. Countless caching strategies can be used to better support your needs,

each displaying different characteristics, or if needed your problem may call for some-

thing wholly new.

We’ve only scratched the surface of memoization in this section in favor of provid-

ing a more generic substrate on which to build your own memoization strategies.

Using Clojure’s abstraction-oriented programming techniques, your own programs

will likewise be more generic and be built largely from reusable parts.

12.5

Understanding coercion

Although Clojure is a dynamically typed language, it does provide mechanisms for

specifying value types. The first of these mechanisms, type hints, was covered in sec-

tion 12.1. The second, coercion, is the subject of this section. Although the nature of

type hints and coercion are similar, their intended purposes are quite different. In the

case of coercion, its purpose is to get at the primitive data type for a value, which we’ll

show next.

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CHAPTER 12 Performance

12.5.1 First rule of coercion: don’t

Clojure’s compiler is sophisticated enough that in many ways it’ll be unnecessary to

coerce values into primitives. It’s often better to start with a function or code block

devoid of coercions. Unless your specific application requires the utmost speed in exe-

cution, it’s better to stick with the version that favors simplicity over the alternative.

But should you decide that coercion might be the choice for you, then this section will

provide guidance.

12.5.2 Corollary: you’re probably not doing it right

If you’ve determined that coercion can help, then it’s worth stressing that you have to

be careful when going down that road. In many cases with coercion, the act of adding

it can actually slow your functions. The reason lies in the nature of Clojure. Functional

composition leads to passing arguments back and forth between pieces, and in the cir-

cumstance of coercion you’re just boxing and unboxing6 from one call to the next.

This particular circumstance is especially devious within the body of a loop, and fol-

lows the same performance degradations observed with Java. Clojure’s unboxing is an

explicit7 operation performed using the coercion functions, so there’s a speck of light

there. Unfortunately, autoboxing is still a danger and should be avoided if speed is a

concern, as we’ll explore now:

(defn occur-count [words]

(let [res (atom {})]

(doseq [w words] (swap! res assoc w (+ 1 (@res w 0))))

@res))

(defn roll [n d]

(reduce + (take n (repeatedly #(inc (rand-int d))))))

(time (dorun (occur-count (take 1000000 (repeatedly #(roll 3 6))))))

; "Elapsed time: 4055.505 msecs"

The function occur-count will return a map of the occurrence counts8 found in a

given sequence. This fairly straightforward implementation uses the function roll to

populate a sequence with a million simulated rolls of three six-sided dice. But four sec-

onds seems like a long time to wait, so perhaps we can speed things up by using coer-

cions. An initial attempt to gain speed may be to pull out the stored count from the

table and coerce it into an int:

(defn occur-count [words]

(let [res (atom {})]

(doseq [w words]

(let [v (int (@res w 0))]

(swap! res assoc w (+ 1 v))))

@res))

6

Autoboxing is the automatic conversion the Java compiler makes between the primitive types and their cor-

responding object wrapper classes.

7

Except when directly or indirectly (via inlining or a macro body) calling a method.

8

Clojure has a function frequencies that does this, so we provide occur-count for illustrative purposes only.

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Understanding coercion

289

(time (dorun (occur-count (take 1000000 (repeatedly #(roll 3 6))))))

; "Elapsed time: 4385.8 msecs"

Well, that didn’t work. The reason for a decrease in speed is that although we’re spec-

ifying the type at the outer loop, we haven’t reduced the need to box and unbox that

value further downstream in the roll function. We might then be led to try and opti-

mize the roll function too:

(defn roll [n d]

(let [p (int d)]

(reduce + (take n (repeatedly #(inc (rand-int p)))))))

(time (dorun (occur-count (take 1000000 (repeatedly #(roll 3 6))))))

; "Elapsed time: 4456.393 msecs"

;=> nil

Again we’ve made matters worse and have spread the problems over the surface of the

entire code. Being adventurous, we grasp for straws and attempt to force integer arith-

metic with roll by using the unchecked-inc function:

(defn roll [n d]

(let [p (int d)]

(reduce + (take n (repeatedly #(unchecked-inc (rand-int p)))))))

(time (dorun (occur-count (take 1000000 (repeatedly #(roll 3 6))))))

Go ahead and run that in the REPL, then go get some coffee and a bagel. Toast the

bagel. Eat the bagel. Drink the coffee. By that time, you might’ve received a result.

So what happened? In an attempt to be clever, we’ve confused the Clojure com-

piler into near unconsciousness. Instead of making direct calls to Clojure’s math func-

tions, we’re now making calls indirectly via Java reflection! You can observe this by

setting *warn-on-reflection* to true and reentering roll.

How can we speed things up? The problem isn’t with coercion itself, but instead

with the implementations of roll and occur-count. You can observe significant

speed-ups by rethinking your original implementations first and then resorting to

coercion second. The use of coercion should always be preceded by a reevaluation of

your implementation, because often by doing so you can eliminate the need for coer-

cion altogether, as shown next.

Listing 12.7 Using coercion to gain speed

(defn roll [n d]

(loop [n (int n), sum 0]

Coerce n to int

(if (zero? n)

sum

(recur (dec n) (+ sum (inc (rand-int d)))))))

(defn occur-count [words]

(reduce #(assoc %1 %2 (inc (%1 %2 0))) {} words))

(time (dorun (occur-count (take 1000000 (repeatedly #(roll 3 6))))))

; "Elapsed time: 937.346 msecs"

;=> nil

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CHAPTER 12 Performance

By refactoring the original functions, we’ve gained a five-fold increase in speed and

yet used only a single coercion. Additionally, we’ve managed to make the new imple-

mentation faster while also maintaining clarity. This should be a general goal when

writing your Clojure code, and when forced to make a trade between the two, it might

be a good idea to favor clarity.

12.5.3 Second rule of coercion: don’t

In the previous example, there’s too much noise in collection and sequence operations

for primitive coercion to help much. This goes to show that it’s important to remember

that the Clojure compiler will often do a better job at optimization than you.

12.5.4 Third rule of coercion: coerce a stable local

When coercing a local to a primitive type, it’s tempting to do so at the point of use,

but this practice should be avoided. A good rule of thumb for coercion is to coerce

only within a local context via let, binding, or loop. This provides a stable value point

for the primitive, allowing you to reuse that same local elsewhere in the same function

without having to again coerce at different points of use. This can be illustrated by the

following:

(defn mean

"Takes a sequence of integers and returns their mean value"

[sq]

(let [length (int (count sq))]

(if (zero? length)

0

(/ (int (reduce + sq)) length))))

The length value has been bound in the let, allowing it to be reused twice within the

body of the function. This allows for a cleaner implementation than the alternative,

which coerces the results of (count sq) in multiple places. Using this advice and the

fact that Clojure provides lexical scope by default, you can also avoid the need to

define a name-mangled local by instead using let to rebind original argument names

to coerced values (defn [x] (let [x (int x)] ...)).

12.5.5 Fourth rule of coercion: watch your sizes

Primitive type coercions in Clojure act the same as type truncation in Java. If a given

value is coerced into a type that can’t hold a value of its magnitude, then data loss will

occur, and in the case of integer overflow, exceptions will be thrown.

12.5.6 Fifth rule of coercion: truncate only as a goal

By default, Clojure doesn’t limit the accuracy of mathematical operations, but this can

occur when using coercion. There will be many instances in your own projects when

speed is more important than accuracy in mathematical operations. Likewise, there

will also be times when truncation is necessary, especially when dealing with Java

library methods that take primitive types:

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Summary

291

(Math/round 1.23897398798929929872987890030893796768727987138M)

; java.lang.IllegalArgumentException:

; No matching method found: round

When a method or function isn’t overloaded, the Clojure compiler can determine

whether an argument can be coerced to a primitive type and will do so if able. The

preceding issue exception arises from the fact that Math/round is overloaded, taking

either a float or double typed argument. Therefore, you have to explicitly use coer-

cion to truncate the argument:

(Math/round (float 1.23897398798929929872987890030893796768727987138M))

;=> 1

Our goal in using the truncating operation float was to get a result that we knew

wouldn’t be affected by truncation. But many instances will arise when truncation will

affect your results and will often do so to your detriment. Therefore, it’s best to be

wary when using coercion, because it propagates inaccuracies. It’s best to limit its

usage when truncation is desired and document vigorously when it’s absolutely

needed for speed.

Coercion can be an effective tool in your Clojure applications, but take care to be

sure you understand the caveats. If you take away one lesson from this section, let it be

this: do not rush to coercion.

12.6

Summary

Clojure provides numerous ways to gain speed in your applications. Using some com-

bination of type hints, transients, chunked sequences, memoization, and coercion,

you should be able to achieve noticeable performance gains. Like any powerful tool,

these performance techniques should be used cautiously and thoughtfully. But once

you’ve determined that performance can be gained, their use is minimally intrusive

and often natural to the unadorned implementation.

In the final chapter, we’ll cover a number of ways that the Clojure way of thinking

might be different from what you’re accustomed to. The discussion therein, when

explored with an open mind, will change the way that you write software.

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Clojure changes

the way you think

This chapter covers

 DSLs

 Testing

 A lack of design patterns

 Error handling and debugging

 Fare thee well

In this final chapter, we cover some tangential topics that you might already be

familiar with, but perhaps not from a Clojure perspective. Our discussion will start

with domain-specific languages (DSLs) and the unique way that Clojure applica-

tions are built from a layering of unique application-specific DSLs. Next, you’re

unlikely to be ignorant of the general push toward a test-driven development

(TDD) philosophy, with a special focus on unit testing. We’ll explore why Clojure is

especially conducive to unit testing and why it’s often unnecessary. Next, whether

you agree with the cult of design patterns or not, it’s inarguable that patterns have

changed the way that object-oriented software is designed and developed. The clas-

sical design patterns are often invisible, or at times outright nonexistent in Clojure

292

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DSLs

293

code, which we’ll discuss in this chapter. As we’ll then show, error handling in Clojure

flows in two directions: from inner functions to outer via exceptions, and from outer

functions in via dynamic bindings. Finally, we’ll explore how having the entire lan-

guage at your disposal can help to change the way that your debugging occurs. We

hope that by the time you’ve finished this chapter, you’ll agree—Clojure changes the

way you think about programming.

13.1 DSLs

Lisp is not the right language for any particular problem. Rather, Lisp encourages

one to attack a new problem by implementing new languages tailored to that

problem.

—“Lisp: A Language for Stratified Design” (Abelson 1988)

In chapter 8, we explored the notion of a domain-specific language for describing

domains. This meta-circularity, while playful, was meant to make a subtle point: Clo-

jure blurs, and often obliterates, the line between DSL and API. When a language is

built from the same data structures that the language itself manipulates, it’s known as

homoiconic (Mooers 1965). When a programming language is homoiconic, it’s simple

to mold the language into a form that bridges the gap between the problem and solu-

tion domains. When designing DSLs in Clojure, it’s important to determine when the

existing language facilities will suffice (Raymond 2003) and when it’s appropriate to

create one from whole cloth (Ghosh 2010). In this section we’ll do both and provide a

little discussion about each.

13.1.1 A ubiquitous DSL

The declarative language SQL is among the most widespread DSLs in use today. In sec-

tion 1.2 we showed a simple Clojure DSL, which provided a simple subset of the SELECT

statement that created a representational SQL string. Though that particular example

was meant to be instructive, Clojure provides a comprehensive library for relational

algebra, on which SQL is based (Date 2009). Imagine a dataset of the following:

(def artists

#{{:artist "Burial" :genre-id 1}

{:artist "Magma" :genre-id 2}

{:artist "Can" :genre-id 3}

{:artist "Faust" :genre-id 3}

{:artist "Ikonika" :genre-id 1}

{:artist "Grouper"}})

(def genres

#{{:genre-id 1 :genre-name "Dubstep"}

{:genre-id 2 :genre-name "Zeuhl"}

{:genre-id 3 :genre-name "Prog"}

{:genre-id 4 :genre-name "Drone"}})

You can try Clojure’s relational functions by entering the examples shown in the fol-

lowing listing.

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CHAPTER 13 Clojure changes the way you think

Listing 13.1 Examples of Clojure’s relational algebra functions

(require '[clojure.set :as ra])

(def ALL identity)

(ra/select ALL genres)

;=> #{{:genre-id 4, :genre-name "Drone"}

{:genre-id 3, :genre-name "Prog"}

{:genre-id 2, :genre-name "Zeuhl"}

{:genre-id 1, :genre-name "Dubstep"}}

(ra/select #(#{1 3} (:genre-id %)) genres)

;=> #{{:genre-id 3, :genre-name "Prog"}

{:genre-id 1, :genre-name "Dubstep"}}

(take 2 (ra/select ALL (ra/join artists genres)))

;=> #{{:artist "Burial", :genre-id 1, :genre-name "Dubstep"}

{:artist "Magma", :genre-id 2, :genre-name "Zeuhl"}}

The relational functions in clojure.set are a perfect example of the way that Clojure

blurs the line between API and DSL. No macro tricks are involved, but through the

process of functional composition, the library provides a highly expressive syntax

matching closely (Abiteboul 1995) that of SQL itself. Though you might be tempted to

create a custom query language for your own application(s), there are times when the

relational functions are exactly what you need. Your time might be better spent solv-

ing actual problems, one of which we’ll cover in the following section.

13.1.2 Putting parentheses around the specification

Many applications deal in measurements of differing units. For example, it’s widely

known that the U.S. works almost exclusively in English units of measure, whereas

most of the rest of the planet works in SI, or metric units. To convert1 from one to the

other isn’t an arduous task and can be handled easily with a set of functions of this

general form:

(defn meters->feet [m] (* m 3.28083989501312))

(defn meters->miles [m] (* m 0.000621))

(meters->feet 1609.344)

;=> 5279.9999999999945

(meters->miles 1609.344)

;=> 0.999402624

This approach will certainly work if only a few functions define the extent of your

conversion needs. But if your applications are like ours, then you probably need to

convert to and from differing units of measure of many different magnitudes. You

may also need to convert back and forth between units of time, dimension, orienta-

tion, and a host of others. Therefore it’d be nice to be able to write a specification of

unit conversions (Hoyte 2008) as a Clojure DSL and use its results as a low-level layer

1 A spectacular general-purpose JVM language named Frink excels at conversions of many different units. We

highly advocate exploring Frink at your next available opportunity: http://futureboy.us/frinkdocs/.

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295

for high-layer application specifics. This is precisely the nature of Lisp development in

general—each level in an application provides the primitive abstractions for the levels

above it.

In this section, we’re going to create a small specification and then convert it into a

Clojure DSL using a technique coined by Rainer Joswig as “putting parentheses

around the specification.”

DEFUNITS

An ideal representation for a unit-conversion specification language would be simple:

Our base unit of distance is the meter. There are 1,000 meters in a kilometer. There

are 100 centimeters in a meter. There are 10 millimeters in a centimeter. There are

3.28083 feet in a meter. And finally, there are 5,280 feet in a mile.

Of course, to make sense of free text is a huge task in any language, so it behooves us

to change it so that it’s easier to reason about programmatically, but not so much that

it’s cumbersome for someone attempting to describe unit conversions. As a first pass,

we’ll try to group the most obvious parts using some Clojure syntactical elements:

(Our base unit of distance is the :meter

[There are 1000 :meters in a :kilometer]

[There are 100 :centimeters in a :meter]

[There are 10 :millimeters in a :centimeter]

[There are 3.28083 :feet in a :meter]

[There are 5280 :feet in a :mile])

This specification is starting to look a little like Clojure code, but it would still be diffi-

cult to parse this into a usable form. Likewise, it’ll be difficult for the person writing

the specification to use the correct terms, avoid spelling mistakes, properly punctuate,

and so forth. In a word, this form is still not useful. It’d be ideal if we could make this

into a form that’s still recognizable to both Clojure and a conversion expert. We’ll try

one more time:

(define unit of distance

{:m 1,

:km 1000,

:cm 1/100,

:mm [1/10 of a :cm],

:ft 0.3048,

:mile [is 5280 :ft]})

This almost looks like Clojure source code, except for a few minor details. We’ve

changed the measure of feet from an “in a” relationship to a relative one with regard

to the meter base unit. Also, a vector indicates the use of a different relative unit,

keeping the DSL regular in its meaning between one conversion and the next and

providing a way to describe intermediate relative units of measure. Those definitions

look like a map, so we should write a utility function that takes a unit and a map like

the preceding one and returns the number of units it takes to compose the base unit.

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CHAPTER 13 Clojure changes the way you think

Listing 13.2 A function for calculating compositional units of a base unit

(defn relative-units [u units]

(let [spec (u units)]

Look up unit spec

(if (nil? spec)

(throw (Exception. (str "Undefined unit " u)))

(if (vector? spec)

Deconstruct relative spec

(let [[conv to] spec]

(* conv

Multiply relative units

(relative-units to units)))

spec))))

The function relative-units goes through the map units looking up units and mul-

tiplying their compositional values. When it finds an indirect specification (such as

millimeters defined in terms of centimeters), it traverse the chain of indirect refer-

ences multiplying the factors along the way, as shown:

(relative-units :m {:m 1 :cm 100 :mm [10 :cm]})

;=> 1

(relative-units :cm {:m 1 :cm 100 :mm [10 :cm]})

;=> 100

(relative-units :mm {:m 1 :cm 100 :mm [10 :cm]})

;=> 1000

We changed the unit conversions map to remove the natural language phrase “in a,”

because English isn’t good for a DSL. Natural language often lacks the precision that a

simple yet regular form has. Now that we have the auxiliary function created, we’d like

to create a macro to interpret the unit specification as shown:

(defunits-of distance :m

:km 1000

:cm 1/100

:mm [1/10 :cm]

:ft 0.3048

:mile [5280 :ft])

This is a simplification versus the original verbal form of the conversion specification.

This final form is indubitably more conducive to parsing, yet doesn’t appreciably sacri-

fice readability. The implementation of the defunits-of macro is presented in the

following listing.

Listing 13.3 A defunits-of macro

(defmacro defunits-of [name base-unit & conversions]

(let [magnitude (gensym)

unit (gensym)

Create 

units-map (intò{~base-unit 1}

units map

(map vec (partition 2 conversions)))]

`(defmacro ~(symbol (str "unit-of-" name))

[~magnitude ~unit]

`(* ~~magnitude

Multiply magnitude 

~(case ~unit

by target unit

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297

~@(mapcat

Unroll unit conversions

(fn [[u# & r#]

into case lookup

`[~u# ~(relative-units u# units-map)])

units-map))))))

The macro defunits-of is different than any macro that you’ve seen thus far, but it’s

typical for macros that expand into another macro definition. In this book you’ve yet

to see a macro that builds another macro and uses multiple levels of nested2 syntax-

quotes. You won’t likely see macros of this complexity often, but in this case we use

nested syntax-quotes so that we can feed structures from the inner layers of the nested

macros to the outer layers, processing each fully before proceeding. At this point, we

can now run a call to the defunits-of macro with the simplified metric to English

units conversion specification to define a new macro named unit-of-distance:

(unit-of-distance 1 :m)

;=> 1

(unit-of-distance 1 :mm)

;=> 1/1000

(unit-of-distance 1 :ft)

;=> 0.3048

(unit-of-distance 1 :mile)

;=> 1609.344

Perfect! Everything is relative to the base unit :m, just as we’d like (read as “how many

meters are in a _”). The generated macro unit-of-distance allows you to work in

your given system of measures relative to a standard system without loss of precision or

the need for a bevy of awkward conversion functions. To calculate the distance a home

run hit by the Orioles’ Matt Wieters travels in Canada is a simple call to (unit-of-

distance 441 :ft) away. The expansion of the distance specification given as

(defunits-of distance :m ...) looks approximately like the following:

(defmacro unit-of-distance [G__43 G__44]

`(* ~G__43

(case ~G__44

:mile 1609.344

:km 1000

:cm 1/100

:m 1

:mm 1/1000

:ft 0.3048)))

The defunits-of macro is an interpreter of the unit-conversion DSL, which generates

another macro unit-of-distance that performs a straightforward lookup of relative

unit values. Amazingly, the expansion given by (macroexpand '(unit-of-distance 1

:cm)) is that of a simple multiplication (* 1 1/100). This is an awe-inspiring revela-

tion. What we’ve managed to achieve is to fuse the notion of compilation and evalua-

2 We talked briefly about making sense out of nested syntax-quotes in section 8.1. However, you’re not likely to

need them very often.

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CHAPTER 13 Clojure changes the way you think

tion by writing a relative units of measure “mini-language” that’s interpreted into a

simple multiplication at compile time!

This is nothing new; Lisp programmers have known about this technique for

decades, but it never ceases to amaze. There’s one downside to our implementation—

it allows for circular conversion specifications (seconds defined in terms of minutes,

which are then defined in terms of seconds), but this can be identified and handled in

relative-units if you’re so inclined.

13.1.3 A note about Clojure’s approach to DSLs

DSLs and control structures implemented as macros in Common Lisp tend to be writ-

ten in a style more conducive to macro writers. But Clojure macros such as defunit-

of, cond, and case are idiomatic in their minimalism; their component parts are

paired and meant to be grouped through proper spacing. Clojure macro writers

should understand that the proliferation and placement of parentheses are legitimate

concerns for some, and as a result you should strive to reduce the number whenever

possible. Why would you explicitly group your expressions when their groupings are

only a call to partition away?

CLOJURE APHORISM If a project elicits a sense of being lost, then start from

the bottom up.

DSLs are an important part of a Clojure programmer’s toolset and stem from a long

Lisp tradition. When Paul Graham talks about “bottom-up programming” in his

perennial work On Lisp, this is what he’s referring to. In Clojure, it’s common practice

to start by defining and implementing a low-level language specifically for the levels

above. Creating complex software systems is hard, but using this approach, you can

build the complicated parts out of smaller, simpler pieces.

Clojure changes the way that you think.

13.2

Testing

Object-oriented programs can be highly complicated beasts to test properly, thanks to

mutating state coupled with the need to test across class hierarchies. Programs are a

vast tapestry of interweaving execution paths, and to test each path comprehensively is

difficult, if not impossible. In the face of unrestrained mutation, the execution paths

are overlaid with mutation paths, further adding to the chaos. Conversely, Clojure

programs tend to be compositions of pure functions with isolated pools of mutation.

The result of this approach helps to foster an environment conducive to unit testing.

But though the layers of an application are composed of numerous functions, each

individually and compositionally tested, the layers themselves and the wiring between

them must also be tested.

Test-driven development (Beck 2002) has conquered the software world, and at its

core it preaches that test development should drive the architecture of the overall

application. Unfortunately, this approach isn’t likely to bear fruit in your Clojure

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299

programs. Instead, Clojure provides the foundation for a contracts-based program

specification that’s more amenable for writing correct programs. But before we dis-

cuss contracts, we’ll touch on the ways Clojure facilitates one part of TDD, unit testing.

13.2.1 Some useful techniques

We don’t want to disparage test-driven development, because its goals are virtuous and

testing in general is essential. Because Clojure programs are organized using

namespaces, and they are themselves aggregations of functions, often pure, the act of

devising a unit-test suite at the namespace boundary is often mechanical in its direct-

ness. From a larger perspective, devising comprehensive test strategies is the subject of

numerous volumes and therefore outside of the scope of this book; but there are a few

Clojure-specific techniques that we wish to discuss.

USING WITH-VAR-ROOT TO STUB

Stubbing (Fowler 2007) is the act of supplying an imitation implementation of a func-

tion for testing purposes. One mechanism that can perform this stubbing is the

with-redefs macro implemented in the following listing. Though this exact macro

will likely be included in future versions of Clojure, it’s not in Clojure 1.2, so a defini-

tion is provided.

Listing 13.4 Macro to aid in mocking

(defn with-redefs-fn [binding-map func & args]

(let [root-bind (fn [m]

(doseq [[a-var a-val] m] (.bindRoot a-var a-val)))

old-vals (zipmap (keys binding-map)

(map deref (keys binding-map)))]

(try

(root-bind binding-map)

(apply func args)

(finally

(root-bind old-vals)))))

(defmacro with-redefs [bindings & body]

`(with-redefs-fn ~(zipmap (map #(list `var %) (take-nth 2 bindings))

(take-nth 2 (next bindings)))

(fn [] ~@body)))

The function rss-children from section 11.6 parses a Twitter RSS2 feed, returning a

sequence of the top-level feed elements. Testing functions that rely on rss-children

is futile against live Twitter feeds, so a stubbed implementation returning a known

sequence would be more prudent, as shown next.

Listing 13.5 Using with-redefs to create stubs

(defn tweetless-rss-children [s]

Create stub

'({:tag :title, :attrs nil, :content ["Stub"]}))

(defn count-rss2-children [s]

(count (rss-children s)))

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CHAPTER 13 Clojure changes the way you think

(with-redefs [rss-children tweetless-rss-children]

Dynamically

(count-rss2-children "dummy"))

bind with stub

;=> 1

The tweetless-rss-children function returns a sequence of some canned data.

Therefore, when testing the count-rss2-children function we temporarily change

the value of rss-children so that it resolves to tweetless-rss-children instead. This

change is made at the root of the rss-children Var and so is visible to all threads. As

long as all the test calls to it are made before control leaves the with-redefs form, the

stub will be invoked every time. Because tweet-occurrences doesn’t return until it col-

lects results from all the futures it creates, it will use the redef given by with-redefs:

(with-redefs [rss-children tweetless-rss-children]

(tweet-occurrences "dummy" "test-url"))

;=> 0

Another option that is sometimes suggested is to use binding in place of with-redefs.

This would push a thread-local binding for rss-children, which might seem attrac-

tive in that it could allow other threads to bind the same Var to a different stub func-

tion, potentially for simultaneously running different tests. But because tweet-

occurrences uses futures, the other threads will be calling rss-children and will see

the root binding rather than the stub,3 causing an error:

(binding [rss-children tweetless-rss-children]

(tweet-occurrences "dummy" "test-url"))

; java.util.concurrent.ExecutionException:

; java.io.FileNotFoundException: test-url

When the root binding of rss-children runs, it tries to actually load “test-url” and

fails, instead of calling our stub and succeeding. The with-redefs macro is a better

solution for mocking.

CLOJURE.TEST AS SPECIFICATION

Clojure ships with a testing library in the clojure.test namespace used to create test

suites that can further serve as partial system specifications. We won’t provide a com-

prehensive survey of the clojure.test functionality, but you should get a feel for how

it works. Unit-test specifications in Clojure are declarative in nature, as shown next.

Listing 13.6 clojure.test as a partial specification

(require '[clojure.test :as test])

(test/deftest feed-tests

(with-redefs [rss-children tweetless-rss-children]

(test/testing "RSS2 Child Counting"

(test/is (= 1000 (count-rss2-children "dummy"))))

3

Alpha versions for Clojure 1.3 handle binding’s interaction with future and Agent send differently, passing

dynamic bindings through to code executed in these other thread contexts. But because these are not the

only kinds of threads that can be spawned, with-redefs (which may be included in Clojure 1.3) is still rec-

ommended for mocking out functions during tests.

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301

(test/testing "Twitter Occurrence Counting"

(test/is (= 0 (count-tweet-text-task "#clojure" ""))))))

(defn test-ns-hook []

(feed-tests))

Clojure’s test library provides a DSL for describing unit test cases. If you’ll notice, we

added a failing test to the RSS2 Child Counting test so that when run, the test will fail

as expected:

(test/run-tests 'user)

; Testing user

;

; FAIL in (feed-tests) (NO_SOURCE_FILE:101)

; RSS2 Child Counting

; expected: (= 1000 (count-rss2-children "dummy"))

; actual: (not (= 1000 1))

;

; Ran 1 tests containing 2 assertions.

; 1 failures, 0 errors.

;=> {:type :summary, :test 1, :pass 1, :fail 1, :error 0}

Though tests are a good way to find some errors, they make few guarantees that the sys-

tem works properly. The ideal approach is the design and implementation of a frame-

work corresponding closely with the domain of the application itself. This framework

would ideally take the literal form of a domain DSL built incrementally through an

interaction with domain experts and must come before testing begins. No amount of

testing can substitute for thoroughly thinking through the fundamental design

details. That’s not to say that the domain DSLs should be fully realized from the start;

instead, the form of the DSL and its constituent parts should be reflective of the actual

domain. In our experience, there are no languages comparable to Clojure for this

kind of domain modeling, save for perhaps Haskell, Factor, and Scala. Having said

that, the domain isn’t simply defined by the shape of its language; it also includes its

expectations, which we’ll discuss presently.

13.2.2 Contracts programming

Test-driven development is in many ways a heuristic affair. People tend to only test the

error conditions and expectations that they can conceptualize. Surely there’s no such

thing as an exhaustive test suite, but in many cases test suites tend toward a local max-

ima. There’s a better way to define semantic expectations within your application:

using Clojure pre- and postconditions.

REVISITING PRE- AND POSTCONDITIONS

In section 7.1, we explored Clojure’s pre- and postcondition facility. Function con-

straint specification is a conceptually simple model for declaring the expectations for

any given function. Function constraints can cover the full range of expected condi-

tions imposed on the function’s inputs, its outputs, and their relative natures. The

beauty of specifying constraints is that they can augment a testing regimen with the

application of random values. The reason this works is that you can effectively throw out

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CHAPTER 13 Clojure changes the way you think

the values that fail the preconditions and instead focus on the values that cause error

in the postconditions. We’ll try this approach for a simple function to square a number:

(def sqr (partial

(contract sqr-contract

[n]

(require (number? n))

(ensure (pos? %)))

#(* % %)))

[(sqr 10) (sqr -9)]

;=> [100 81]

The contract for sqr states simply: require a number and ensure that its return is pos-

itive. Now we can create a simple test driver4 that throws many random values at it to

see if it breaks:

(doseq [n (range Short/MIN_VALUE Short/MAX_VALUE)]

(try

(sqr n)

(catch AssertionError e

(println "Error on input" n)

(throw e))))

; Error on input 0

;=> java.lang.AssertionError: Assert failed: (pos? %)

Even when adhering to the tenets of the preconditions, we’ve uncovered an error in

the sqr function at the postcondition end. Postconditions should be viewed as the

guarantee of the return value given that the preconditions are met. The reason for

the postcondition error is that the function’s contract doesn’t specify that the number

n should be nonzero. By adding a check for zero (not= 0 n) in the preconditions, we

can guarantee that the sqr function acts as expected. To perform this same verifica-

tion using unit testing is trivial in this case, but what if the edge condition wasn’t as

obvious? In such a case, it’s probable that the error might not be caught until it’s too

late. Of course, there’s no guarantee that your contracts are comprehensive, but that’s

why domain expertise is often critical when defining them.

ADVANTAGES OF PRE- AND POSTCONDITIONS

Function constraints aren’t code. They take the form of code, but that fact is only a mat-

ter of representation. Instead, constraints should be viewed as a specification language

describing expectations and result assurances. On the other hand, unit tests are code,

and code has bugs. Contracts, on the other hand, are essential semantic coupling.

Another potential advantage of contracts over tests is that in some cases, tests can

be generated from the contracts themselves. Also, pre- and postconditions are amena-

ble to being expressed as an overall description of the system itself, which can thus be

4 For the sake of highlighting this technique, we’ve simplified our test driver. Testing a limited range of input

values might not be an appropriate approach in all circumstances.

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303

fed into a rule base for query and verification. Both of these cases are outside of the

scope of this book, but you shouldn’t be surprised if they make their way into future

versions of Clojure. There’s tremendous potential in Clojure’s pre- and postcondi-

tions. Though they’re currently low-level constructs, they can be used to express full-

blown design by contract facilities for your own applications.

Clojure changes the way that you think.

13.3 A lack of design patterns

Any sufficiently complicated C or Fortran program contains an ad hoc, informally-

specified, bug-ridden, slow implementation of half of Common Lisp.

—Greenspun’s Tenth Rule

The book Design Patterns: Elements of Reusable Object-Oriented Software (Gamma et al

1995) was a seminal work of software design and development. You’d be hard pressed

to find a software programmer in this day and age who’s not familiar with this work.

The book describes 24 software best practices encountered throughout the course of

experience in developing software projects of varying sizes.

Design patterns have obtained a bad reputation in some circles, whereas in others

they’re considered indispensable. From our perspective, design patterns are a way to

express software best practices in a language-neutral way. But where patterns fall short

is that they don’t represent pure abstraction. Instead, design patterns have come to be

viewed as goals in and of themselves, which is likely the source of the antagonism

aimed at them. The ability to think in abstractions is an invaluable skill for a software

programmer to strengthen. In this section, we’ll attempt to dissuade you from viewing

Clojure features as design patterns (Norvig 1998) and instead as an inherent nameless

quality.

13.3.1 Clojure’s first-class design patterns

Most if not all of the patterns listed in the book are applicable to functional program-

ming languages in general, and to Clojure in particular. But at its most pragmatic, the

patterns described are aimed at patching deficiencies in popular object-oriented pro-

gramming languages. This practical view of design patterns isn’t directly relevant to

Clojure, because in many ways the patterns are ever-present and are first-class citizens

of the language itself. We won’t provide a comprehensive survey of the ways that Clo-

jure implements or eliminates popular design patterns but will provide enough to

make our point.

OBSERVER

Clojure’s add-watch and remove-watch functions provide the underpinnings of an

observer (publisher/subscriber) capability based on reference types. We can illustrate

this through the implementation of the simple defformula macro shown in listing 13.7.

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CHAPTER 13 Clojure changes the way you think

Listing 13.7 A macro to create spreadsheet-cell-like formulas

(defmacro defformula [nm bindings & formula]

`(let ~bindings

(let [formula# (agent ~@formula)

Create formula as Agent

update-fn# (fn [key# ref# o# n#]

(send formula# (fn [_#] ~@formula)))]

(doseq [r# ~(vec (take-nth 2 bindings))]

(add-watch r# :update-formula update-fn#))

Add watch to

(def ~nm formula#))))

each reference

(def h (ref 25))

(def ab (ref 100))

Create 

(defformula avg [at-bats ab hits h]

baseball formula

(float (/ @hits @at-bats)))

@avg

;=> 0.25

Update

(dosync (ref-set h 33))

dependent Ref

;=> 33

@avg

Observe 

;=> 0.33

formula change

By using watchers on references, you can use defformula to provide an abstract value

that changes when any of its parts change. A more traditional Lisp approach is to pro-

vide predefined hooks (Glickstein 1997) that are called at certain times within the

execution cycle. In addition, using proxy or gen-class to extend java.util.

Observable is the most straightforward way to wire into existing source code using the

Observer pattern.

STRATEGY

Algorithm strategies selected at runtime are common practice in Clojure, and there

are a number of ways to implement them. One such way is via continuation-passing

style, as we explored in section 7.3. A more general solution is to pass the desired

function as an argument to a higher-order function, such as you’d see in the ubiqui-

tous map, reduce, and filter functions. Further, we’ll provide a case of dynamic error

functions in the next section illustrating how Clojure’s multimethods are a more pow-

erful substitute for the classic strategy pattern.

VISITOR

The Visitor pattern is designed to describe a way to decouple operations on a struc-

ture from the structure itself. Even casual observers will see the parallel to Clojure’s

multimethods, protocols, types, proxies, and reify features.

ABSTRACT FACTORY

The Abstract Factory pattern is used to describe a way to create related objects without

having to name explicit types at the point of creation. Clojure’s types avoid the cre-

ation of explicit hierarchies (although ad hoc hierarchies can be created, as seen in

section 9.2). Therefore, in Clojure this particular usage scenario is relegated to use

within Java interoperability contexts. But the use of factory functions to abstract the

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A lack of design patterns

305

call to the constructors of types and records is idiomatic and in fact actively promoted.

The reasons for a Clojure-style factory are to simplify the importing requirements of a

type or record, and also to add additional project-specific functionality to the con-

structor (keyword arguments, default values, and so on).

INTERPRETER

The Interpreter pattern is in every way Greenspun’s Tenth Rule formalized. Many

projects of sufficient size can be well served by the inclusion of a specialized grammar

describing parts of the system itself. Clojure macros make the matter of creating spe-

cialized grammars a first-class member of the language.

BUILDER

The creation of complex structures from representation is central to Clojure pro-

gramming, although it’s viewed differently from a similar object-oriented approach—

the Builder pattern. In section 8.4, we used a simple data representation as the input

to Clojure’s clojure.xml/emit function to produce an analogous XML representa-

tion. If you preferred a different output representation, then you could write another

conversion function. If you preferred finer control over the constituent parts, then

you could write functions or multimethods for each and specialize at runtime.

FAÇADE

The use of Clojure namespaces, as seen in section 9.1, is the most obvious way to pro-

vide a simplified façade for a more complex API. You can also use the varying levels of

encapsulation (as outlined in section 2.4) for more localized façades.

ITERATOR

Iteration in Clojure is defined through an adherence to the seq protocol, as outlined

in section 5.1 and later elaborated on in sections 9.3 about types and protocols.

DEPENDENCY INJECTION

Though not a classical pattern in the Design Patterns sense, dependency injection has

become a de facto pattern for object-oriented languages that don’t allow overridable

class constructors. This condition requires that separate factory methods and/or

classes create concrete instances conforming to a given interface. In forsaking the

ability to define classes, Clojure completely avoids the problem that DI solves. Instead,

Clojure’s closest analogue to this “pattern” is the use of functions returning closures

that are specialized based on the original arguments. Likewise, you could use partial

application and composition similarly.

We could go further with this survey, but to do so would belabor the point: most of

what are known as design patterns are either invisible or trivial to implement in Clo-

jure. But what about the Prototype pattern, you ask? We implemented the UDP in sec-

tion 9.2. Decorators or chain of responsibility? Why not use a macro that returns a

function built from a list of forms spliced into the -> or ->> macro? Proxies would

likely be implemented as closures and so would commands. The list goes on and on,

and in the end you must face the inevitable—Clojure changes the way that you think.

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CHAPTER 13 Clojure changes the way you think

13.4 Error handling and debugging

Our goal throughout this book was to show the proper way to write Clojure code, with

mostly deferral and hand-waving regarding error handling and debugging. In this sec-

tion, we’ll cover these topics with what you might view as a unique twist, depending on

your programming background.

13.4.1 Error handling

As we showed in figure 10.7, there are two directions for handling errors. The first,

and likely most familiar, refers to the passive handling of exceptions bubbling outward

from inner functions. But built on Clojure’s dynamic Var binding is a more active

mode of error handling, where handlers are pushed into inner functions. In section

11.10, we mentioned that the binding form is used to create thread-local bindings,

but its utility isn’t limited to this use case. In its purest form, dynamic scope is a struc-

tured form of a side effect (Steele 1978). You can use it to push Vars down a call stack

from the outer layers of a function nesting into the inner layers, a technique that we’ll

demonstrate next.

DYNAMIC TREE TRAVERSAL

In section 8.4, we built a simple tree structure for a domain model where each node

was of this form:

{:tag <node form>, :attrs {}, :content [<nodes>]}

As it turns out, the traversal of a tree built from such nodes is straightforward using

mundane recursion, as shown:

(defn traverse [node f]

(when node

(f node)

(doseq [child (:content node)]

(traverse child f))))

For each node in the tree, the function f is called with the node itself, and then each

of the node’s children is traversed in turn. Observe how traverse works for a single

root node:

(traverse {:tag :flower :attrs {:name "Tanpopo"} :content []}

println)

; {:tag :flower, :attrs {:name Tanpopo}, :content []}

But it’s much more interesting if we traverse trees larger than a single node. There-

fore, we can build a quick tree from an XML representation using Clojure’s clojure.

xml/parse function:

(use '[clojure.xml :as xml])

(def DB

(-> "<zoo>

<pongo>

<animal>orangutan</animal>

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Error handling and debugging

307

</pongo>

<panthera>

<animal>Spot</animal>

<animal>lion</animal>

<animal>Lopshire</animal>

</panthera>

</zoo>"

.getBytes

(java.io.ByteArrayInputStream.)

xml/parse))

The DB Var contains an animal listing for a small zoo. Note that two of the animals

listed have the elements Spot and Lopshire; both are seemingly out of order for a

zoo. Therefore, we can write a function to handle these nefarious intruders.

Listing 13.8 Handling nefarious tree nodes with exceptions

(defn ^{:dynamic true} handle-weird-animal

[{[name] :content}]

Define error handler

(throw (Exception. (str name " must be 'dealt with'"))))

(defmulti visit :tag)

(defmethod visit :animal [{[name] :content :as animal}]

(case name

"Spot" (handle-weird-animal animal)

"Lopshire" (handle-weird-animal animal)

(println name)))

(defmethod visit :default [node] nil)

The multimethod visit can be used as the input function to the traverse function

and will only trigger when a node with the :tag attribute of :animal is encountered.

When the method triggered on :animal is executed, the node :content is destruc-

tured and checked against the offending Spot and Lopshire values. When found, the

devious node is then passed along to an error handler handle-weird-animal for

reporting.5 By default, the error handler throws an exception. This model of error

handling is the inside-out model of exceptions. But handling errors in this way stops

the processing:

(traverse DB visit)

; orangutan

; java.lang.Exception: Spot must be 'dealt with'

We’ve managed to identify Spot, but the equally repugnant Lopshire escapes our

grasp. It’d be nice to instead use a different version of handle-weird-animal that

allows us to both identify and deal with every such weird creature. We could pass

handle-weird-animal along as an argument to be used as an error continuation,6 but

5

The metadata {:dynamic true} attached to handle-weird-animal isn’t really used in Clojure 1.2, but it

may be required in future versions of Clojure starting with 1.3 to allow the dynamic binding we’re about to

demonstrate.

6

See section 7.3 for more information on continuation-passing style.

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CHAPTER 13 Clojure changes the way you think

that pollutes the argument list of every function along the way. Likewise, we could

inject catch blocks at a point further down the call chain, say within visit, but we

might not be able to change the source, and if we could it makes for a more insidious

pollution. Instead, using a dynamic binding is a perfect solution, because it allows us

to attach specific error handlers at any depth in the stack according to their appropri-

ate context:

(defmulti handle-weird (fn [{[name] :content}] name))

(defmethod handle-weird "Spot" [_]

(println "Transporting Spot to the circus."))

(defmethod handle-weird "Lopshire" [_]

(println "Signing Lopshire to a book deal."))

(binding [handle-weird-animal handle-weird]

(traverse DB visit))

; orangutan

; Transporting Spot to the circus.

; lion

; Signing Lopshire to a book deal.

As you might expect, this approach works across threads to allow for thread-specific

handlers:

(def _ (future

(binding [handle-weird-animal #(println (:content %))]

(traverse DB visit))))

; orangutan

; [Spot]

; lion

; [Lopshire]

What we’ve outlined here is a simplistic model for a grander error-handling scheme.

Using dynamic scope via binding is the preferred way to handle recoverable errors in

a context-sensitive manner.

13.4.2 Debugging

The natural progression of debugging techniques as discovered by a newcomer to

Clojure follows a fairly standard progression:

1

(println)

2

A macro to make (println) inclusion simpler

3

Some variation on debugging as discussed in this section

4

IDEs, monitoring, and profiling tools

Many Clojure programmers stay at step 1, because it’s simple to understand and also

highly useful, but there are better ways. After all, you’re dealing with Clojure—a

highly dynamic programming environment. Observe the following function:

(defn div [n d] (int (/ n d)))

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Error handling and debugging

309

The function div simply divides two numbers and returns an integer value. You can

break div in a number of ways, but the most obvious is to call it with zero as the

denominator: (div 10 0). Such an example would likely not give you cause for con-

cern should it fail, because the conditions under which it fails are fairly limited, well

known, and easily identified. But not all errors are this simple, and the use of println

is fairly limited. Instead, a better tool would likely be a generic breakpoint7 that could

be inserted at will and used to provide a debug console for the current valid execution

context. Imagine it would work as follows:

(defn div [n d] (break) (int (/ n d)))

(div 10 0)

debug=>

At this prompt, you can query the current lexical environment, experiment with dif-

ferent code, and then resume the previous execution as before. As it turns out, such a

tool is within your grasp.

A BREAKPOINT MACRO

We hope that by the end of this section, you’ll understand that Lisps in general, and

Clojure in particular, provide an environment where the whole of the language truly is

“always available” (Graham 1993). First of all, an interesting fact to note is that the

Clojure REPL is available and extensible via the Clojure REPL itself, via the clojure.

main/repl function. By accessing the REPL implementation directly, you can custom-

ize it as you see fit for application-specific tasks.

Typing (clojure.main/repl) at the REPL seemingly does nothing, but rest

assured you’ve started a sub-REPL. What use is this? To start, the repl function takes a

number of named parameters, each used to customize the launched REPL in different

ways. We’ll utilize three such hooks—:prompt, :eval, and :read—to fulfill a break-

point functionality.

OVERRIDING THE REPL’S READER

The repl function’s :read hook takes a function of two arguments: the first corre-

sponding to a desired display prompt, and the second to a desired exit form. We want

the debug console to provide convenience functions—we’d like it to show all of the

available lexical bindings and also to resume execution. It also needs to be able to

read valid Clojure forms, but because that’s too complex a task, we’ll instead farm that

functionality out to Clojure’s default REPL reader.

Listing 13.9 A modest debug console reader

(defn readr [prompt exit-code]

(let [input (clojure.main/repl-read prompt exit-code)]

(if (= input ::tl)

exit-code

input)))

7 The code in this section is based on debug-repl created by the amazing George Jahad, extended by Alex

Osborne, and integrated into Swank-Clojure by Hugo Duncan.

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CHAPTER 13 Clojure changes the way you think

We can start testing the reader immediately:

(readr #(print "invisible=> ") ::exit)

[1 2 3]

;=> [1 2 3]

(readr #(print "invisible=> ") ::exit)

::tl

;=> :user/exit

The prompt that we specified was of course not printed, and typing ::tl at the

prompt did nothing because the readr function isn’t yet provided to the repl as its

:read hook. But before we do that, we need to provide a function for the :eval hook.

Needless to say, this is a more complex task.

OVERRIDING THE REPL’S EVALUATOR

In order to evaluate things in context, we first need a function cab to garner the bind-

ings in the current context. Fortunately for us, Clojure macros provide an implicit

argument &env that’s a map of the local bindings available at macro-expansion time.

We can then extract from &env the values associated with the bindings and zip them

up with their names into a map for the local context, as shown next.

Listing 13.10 Creating a map of the local context using &env

(defmacro local-context []

(let [symbols (keys &env)]

(zipmap (map (fn [sym] `(quote ~sym)) symbols) symbols)))

(local-context)

;=> {}

(let [a 1, b 2, c 3]

(let [b 200]

(local-context)))

;=> {a 1, b 200, c 3}

The local-context macro provides a map to the most immediate lexical bindings,

which is what we want. But what we really want to do is to provide a way to evaluate

expressions with this contextual bindings map. Wouldn’t you know it, the contextual-

eval function from section 8.1 fits the bill. So now that we have the bulk of the imple-

mentation complete, we’ll now hook into the repl function to provide a breakpoint

facility.

PUTTING IT ALL TOGETHER

The hard parts are done, so to wire them into a usable debugging console is relatively

easy, as shown next.

Listing 13.11 The implementation of a breakpoint macro

(defmacro break []

`(clojure.main/repl

:prompt #(print "debug=> ")

:read readr

:eval (partial contextual-eval (local-context))))

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Error handling and debugging

311

Using this macro, we can now debug the original div function:

(defn div [n d] (break) (int (/ n d)))

(div 10 0)

debug=>

Querying locals to find the “problem” is simple:

debug=> n

;=> 10

debug=> d

;=> 0

debug=> (local-context)

;=> {div #<user$div__155 user$div__155@51e67ac>, n 10, d 0}

debug=> ::tl

; java.lang.ArithmeticException: Divide by zero

So there’s the problem! We passed in a zero as the denominator. We should fix that.

MULTIPLE BREAKPOINTS AND BREAKPOINTS IN MACROS

What would be the point if you couldn’t set multiple breakpoints? Fortunately, you

can, as we show in the following listing.

Listing 13.12 Using multiple breakpoints in function keys-apply

(defn keys-apply [f ks m]

(break)

(let [only (select-keys m ks)]

(break)

(zipmap (keys only) (map f (vals only)))))

(keys-apply inc [:a :b] {:a 1, :b 2, :c 3})

debug=> only

; java.lang.Exception: Unable to resolve symbol: only in this context

debug=> ks

;=> [:a :b]

debug=> m

;=> {:a 1, :b 2, :c 3}

debug=> ::tl

debug=> only

;=> {:b 2, :a 1}

debug=> ::tl

;=> {:a 2, :b 3}

And finally, you can use breakpoints within the body of a macro (in its expansion, not

its logic), as shown next.

Listing 13.13 Using a breakpoint in a macro awhen

(defmacro awhen [expr & body]

(break)

`(let [~'it ~expr]

(if ~'it

(do (break) ~@body))))

(awhen [1 2 3] (it 2))

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312

CHAPTER 13 Clojure changes the way you think

debug=> it

; java.lang.Exception: Unable to resolve symbol: it in this context

debug=> expr

;=> [1 2 3]

debug=> body

;=> ((it 2))

debug=> ::tl

debug=> it

;=> [1 2 3]

debug=> (it 1)

;=> 2

debug=> ::tl

;=> 3

There’s much room for improvement, but we believe that the point has been made.

Having access to the underpinnings of the language allows you to create a powerful

debugging environment with little code. We’ve run out of ideas by now, so we’ll say

our credo only once more, and we hope by now you believe us.

Clojure changes the way that you think.

13.5

Fare thee well

This book possess many lacunae, but it’s this way by design. In many cases, we’ve

skipped approaches to solving problems via a certain route to avoid presenting non-

idiomatic code. In many examples, we’ve left exposed wiring. For example, the

defcontract macro requires that you partially apply the contract to the function

under constraint instead of providing a comprehensive contract overlay façade. It was

our goal to leave wiring exposed because exposed wiring can be explored, tampered

with, and ultimately enhanced—which we hope you’ll find the motivation to do.

We’ve worked hard to provide a vast array of relevant references should you choose

to further enhance your understanding of the workings and motivations for Clojure.

But it’s likely that we’ve missed some excellent resources, and we hope that you

instead are able to uncover them in time. Finally, this wasn’t a survey of Clojure, and

many of the functions available to you weren’t used in this book. We provide some

pointers in the resource list, but there’s no way that we could do justice to the librar-

ies and applications mentioned and those unmentioned. We implore you to look

deeper into the functionality of not only Clojure, but the rich ecology of libraries and

applications that have sprung up in its relatively short life span.

Thank you for taking the time to read this book; we hope it was as much a pleasure

to read as it was for us to write. Likewise, we hope that you’ll continue your journey

with Clojure. Should you choose to diverge from this path, then we hope that some of

what you’ve learned has helped you to view the art of programming in a new light.

Clojure is an opinionated language, but it and most of its community believe that

these opinions can work to enhance the overall state of affairs in our software indus-

try. The onus is on us to make our software robust, performant, and extensible. We

believe that the path toward these goals lies with Clojure.

Do you?

—FOGUS AND HOUSER 2010

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resources

Miscellaneous resources

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Ungar, David, and Randal Smith. 1987. “ SELF: The power of simplicity.” Presented at the Conference on

Object-Oriented Programming Systems, Languages, and Applications (OOPSLA), Orlando. The

Self programming language is likely the greatest influence on prototypal inheritance.

Van Roy, Peter, and Seif Haridi. 2004. Concepts, Techniques, and Models of Computer Programming. 

Cambridge, MA: MIT Press.

Wadler, Philip. 1989. “Theorems for Free!” Presented at the fourth International Conference on Func-

tional Programming and Computer Architecture.

Wampler, Dean, and Alex Payne. 2009. Programming Scala. Sebastopol, CA: O’Reilly.

Download from Wow! eBook <www.wowebook.com>

RESOURCES

317

Whitehead, Alfred North. 1929. Process and Reality: An Essay in Cosmology. Cambridge University Press. For

a general overview of Whitehead, see The Wit And Wisdom of Alfred North Whitehead by A.H. Johnson

(Boston, Beacon Press, 1947).

Williams, Laurie. 2002. Pair Programming Illuminated. Boston: Addison-Wesley Professional. The limita-

tions of the book format only shadow the idealistic model of pair programming.

Online resources

Braithwaite, Reginald. 2007. “Why Why Functional Programming Matters Matters.” http://mng.bz/

2pZP. This column discusses a language-level separation of concerns.

Clementson, Bill. 2008. “Clojure could be to Concurrency-Oriented Programming what Java was to

OOP.” http://bc.tech.coop/blog/081201.html. A much deeper discussion concerning Erlang actors and Clojure agents.

Dekorte, Steve. Io . http://iolanguage.com.

Fogus, Michael. Lithp. http://github.com/fogus/lithp.

Fowler, Martin. 2005. “Fluent Interface.” http://mng.bz/e2r5.

———. 2007. “Mocks Aren’t Stubs.” http://mng.bz/mq95.

Graham, Paul. Arc. www.paulgraham.com/arc.html.

———. 2001. “What Made Lisp Different.” www.paulgraham.com/diff.html. As Paul Graham states,

“The whole language always available” appears as a theme throughout this book and as a finale in

section 13.5.

Houser, Chris. error-kit API . http://mng.bz/07FF. The clojure.contrib.error-kit namespace contains an open error system similar to CL conditions that don’t require recompilation when defining new

error types.

Krukow, Karl. 2009. “Understanding Clojure’s PersistentVector Implementation.” http://mng.bz/tmjv.

Lindholm, Tim, and Frank Yellin. 1999. Java Virtual Machine Specification. http://java.sun.com/docs/

books/jvms/.

Peter. 1998. “ Design Patterns in Dynamic Programming .” http://norvig.com/design-patterns/. 

The section on design patterns was inspired by this presentation.

Tarver Mark. 2008. Functional Programming in Qi. www.lambdassociates.org/Book/page000.htm. Some programming languages perform partial application automatically when a function is 

supplied with fewer than the expected number of arguments. One such language is Qi.

———. 2009. “The Next Lisp: Back to the Future .” http://mng.bz/8wA9. The notion of Lisp as a programming language genotype is explored.

_why. Shoes. http://github.com/shoes/shoes.

Yegge, Steve. 2006. “ Execution in the Kingdom of Nouns . ” http://mng.bz/9ApS.

———. 2008. “The Universal Design Pattern.” http://mng.bz/6531. Like many programmers of our generation, we were in many ways inspired and influenced by Steve Yegge’s work—which is why we

asked him to write this book’s foreword.

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index

Keywords and symbols

ACID 238

arglists 164

ad-hoc hierarchies 16, 70, 184,

argument 68

, 25, 158

186, 188, 304

arrays 74, 77, 79, 218–220

; 21

derivation hierarchy 186–187

amap 220

:as 40–41, 48–49, 182, 264

make-hierarchy 187

areduce 220

:exclude 41, 182, 258

adornment 278

multidimensional arrays 222

:import 42, 182

Agents 235, 240, 247, 249–255,

primitive arrays 218–219

:load 182

274

reference arrays 218

:only 182–183

agent-error 253

seq of an array 220

:post 133–134

await 249–250, 255

as-futures 263–265, 268

:pre 133–134

await-for 250–251

as-is 48

:private 181

queue 247, 251

aspects 135

:refer 41–42

restart-agent 253–254

assertions 173, 203

:refer-clojure 182, 258

send 248, 251–253

assert 134

:require 41, 182

send-off 248, 251–253

assoc 84–86, 191

:strs 49

serialize access 249

associative 66

:test 128

agile 202, 278

assoc-in 85

:use 41, 182–183

Ahead of Time Compilation

asum-sq 220, 278–280

naked :use 183

(AOT) 218, 227–228

asynchronous 240, 247

.. 37

alert 216

atomicity 238, 255

*read-eval* 270

algorithm 81

*warn-on-reflection* 278, 289

Atoms 235, 240, 255–257

alias 183

& 48, 50

compare-and-set! 255

anaphora 170–171, 211

autoboxing 288

anaphoric 173

auto-gensym 36, 165

A

anaphoric macros 170, 211

ancestors 186

A* pathfinding 149

and (logical) 114, 175

B

astar 149–154

anonymous inner classes 126

candidate paths 150

Apache Ant 181

backslash 73

cost-estimate function 150

Application Programming

bad-make-move 243

path-finding 149

Interface (API) 204, 293

beget 184

abstraction-oriented

public-facing API 181

benchmark 281

programming 16, 177,

apply 98, 127

best 72

184, 189, 206, 285–287

aquarium 69

best practices 303

abstractions 166, 188, 208, 227,

Arc (programming

best-case 81

284, 286

language) 170

best-first 149

accessors 202

awhen 170

BigDecimal 273–274

319

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320

INDEX

Big-O 77, 80–82

expressiveness of 5

compile 141, 213–214

algorithmic complexity 77

succinctness 72

compiler 117, 213, 218, 277,

asymptotic complexity 80–81,

clojure.contrib.json 168

288–291

124

clojure.contrib.repl-utils 52–53

Compiler.java 55

linear time 81

clojure.core 41, 179, 183, 271

compile-time 141, 231

logarithmic complexity 81

clojure.lang.IMeta 212

complement 128, 130

bind 47

clojure.lang.IPersistentStack 86

complexity 103, 238, 273

binding 47, 300

clojure.lang.PersistentQueue/

essential complexity 5

bit-xor 51

EMPTY 92

incidental complexities 164

blocks 29, 31

clojure.main/repl 309

composition 305

boilerplate 162, 164–165

clojure.set 40, 94, 96, 183, 294

concat 281

Booleans 279

clojure.string 41

concrescence 13

Boolean (Java class) 44, 83

clojure.test 128, 183, 300

concrete classes 16, 193, 195,

Boolean context 45

clojure.test/run-tests 128

208

bot object 139–140

clojure.xml 183, 262

concrete realization 286

bottom-up programming 298

clojure.xml/emit 305

concurrency 3, 14, 109, 234–

bound 272

clojure.xml/parse 263, 306

237, 245, 249, 258, 261

boxing 288

clojure.zip 183, 262

concurrent modification

autoboxing 288

clojure.zip/xml-zip 263

220, 259

unboxing 288

closures 17, 135–141, 197, 230,

concurrent modification

breakpoint 309–310

258, 305

errors 109

build-contract 174, 181

closing over parameters 137

concurrent

build-move 205–206

functions returning

programming 234–235,

byte 83

closures 136

248, 259

bytecode 141, 208, 210, 222

lexical closures 17

lost wakeup 236

bytes 279

passing closures as

model, distributed 248–249

functions 137

priority inversion 238

C

sharing closure context 138

vs. parallelism 235

code

cond 69, 162, 298

C (programming language) 8,

avoid repeating 196

confounding 281

21, 194, 198

is data 8, 11, 157–158, 161,

conj 86, 88, 90–92, 110, 196

calculations, precise 66

169

cons 32–33, 90

call stack 71

repetitive 47, 49

consistency 7–8, 238, 242, 261

callbacks 138, 266

coercion 277, 287–291

reporting problem 238

call-by-need 119

fifth rule of coercion 291

constant factor 81

canvas 51

first rule of coercion 288

constant time 281

capitalize 41

fourth rule of coercion 290

constructors 37, 55, 198, 204,

capturing group 74

second rule of coercion 290

212, 305

car 80, 90

third rule of coercion 290

contains? 89, 91, 95

case 147, 298

collect-bodies 173–174

continuation-passing style

chaining 202

collection 46, 280

(CPS) 148–149, 229, 304

change-message 209, 211

colon 23

accept function 148

char 83

columns 130

return continuation 148

character classes 73

commenting 21, 74

continuations 266, 307

characters, in a string 23

commits 248

continuation function 148

chars 279

Common Lisp (programming

continue 252, 254

chess 16, 202, 204–205, 241–242

language) 46, 72, 169, 298

contracts programming 173,

choose-move 242

cdr 80, 90

175–176, 299, 302–303, 312

chunked sequences 283

Common Lisp Object System

constraints 133, 173, 175,

clarity 7, 290

(CLOS) 187

301–302

clear 55

funcall 72

defcontract 312

clear-actions 253

Lisp-2 72

contrib 168, 271

Clojure

comp 127, 130

control structures 157, 159,

as opinionated language 312

comparator 95

161, 163

Clojure 1.3 271, 307

compare-and-swap 255

conversion specification 296

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INDEX

321

coordination 241, 269

subscriber patterns 129

dynamic scope 273–274, 306,

copies, defensive 226

Visitor pattern 304

308

counted 90

Design Patterns: Elements of 

dynamic type systems 278, 287

counter-optimizations 280

Reusable Object-Oriented

count-tweet-text-task 263–264,

Software 303

E

266

destructuring 8, 47–48, 54, 57,

create-ns 179–180

133, 242

elegance 13, 45

crypto 69

associative destructuring 50

embedding 73

in function parameters 50

empowerment 6

D

nested 49

empty sequence 78–79

versus accessor methods 50

empty? 46

data structures, immutable

with a map 49

encapsulation 16–17, 305

108–109

with a vector 48

block-level encapsulation 17

database 236

determinacy 235

local encapsulation 17

deadlock 236, 238, 248–250,

directive 48–49, 52

namespace encapsulation 17

259, 267

directory structure 180–181

Enlive 168

deterministic 267

disappear 53

enumeration values 68

debug 309

dispatch 187

enumerator 78

debug console 309

display 212–213

env 310

debugging 293, 310, 312

dissoc 86, 190–191, 257

ephemeral 280

debug-repl 309

distributive 66

equality 68, 70–71, 109, 187

decimal 22

do 29, 163, 262

equality partitions 79, 102,

declarative 118, 293

doall 274

224

declare 173

documentation, viewing 53

equality semantics 79

def 27–28, 135, 137, 181, 271,

domain 166

Erlang (programming

273

domain-specific language

language) 248–249

actor 248

default-handler 209–210

(DSL) 10–11, 70, 164, 167,

Actor model 143

defformula 303–304

173, 204, 292–295, 298,

in-process versus distributed

definterface 227–229

301

concurrency models 248

defmacro 162, 181, 271

domain expertise 302

share-nothing 248

defmulti 188, 271

putting parentheses around

error handling 229, 254, 

defn 17, 27, 136, 164, 181, 271

the specification 295

306–308

defonce 271

unit conversion DSL 297

escaped 73

defprotocol 229

don’t panic 55

evaluation 32–33

defrecord 189–191, 199–200

doseq 7, 46, 55

contextual-eval 160–161, 310

defstruct 182, 190

dosync 242, 248

eval 159–161, 309–310

downfall of defstructs 190

dothreads! 240, 255

meta-circular 293

defunits-of 296–297

doto 38

exceptions 38, 55, 205, 229,

def-watched 165

double 63, 83

236, 252–254, 282, 293,

delay 113, 119, 121, 149, 287

double-array 222

306–307

delegate 215, 286

double-backslash 73

catch 38, 308

dependency injection (DI) 305

double-quotes 73

checked 229–230

deref 240, 255, 270, 272

doubles 65, 279

compile-time 55, 231

derive 186–187

do-until 162

ConcurrentModification-

descendants 186

drawing 53

Exception 259

design patterns 292, 303

duck typing 189

finally 172, 261

Abstract Factory pattern 304

dummy write 239

handling 229

Builder pattern 305

durability 238

java.lang.ClassCastException

chain of responsibility 305

DynaFrame.clj 213

231

Façade pattern 305

dynamic binding 287, 293, 306,

java.lang.Exception 230

Interpreter pattern 305

308

java.lang.NullPointer-

Observer pattern 303–304

binding 117, 271, 273, 290,

Exception 114

Prototype pattern 305

306, 308

java.lang.RuntimeException

Strategy pattern 304

bound-fn 274

230

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322

INDEX

exceptions (continued)

free variables 136

get-in 85–86

runtime 230

freedom to focus 5

getter 204

runtime vs. compile-time 230

frequencies 288

global hierarchy map 188

throw 38, 54–55, 231

Frink (programming

goal 247

expand-clause 11

language) 207, 294

Gödel, Escher, Bach: An Eternal

expansion 297

frustrating 246

Golden Braid 183

expected case 81

fully qualified 258, 270, 272

good-move 242

experimentation 179

fun 51

Graham, Paul 298, 309

expression problem 16

functional programming 12,

graphic 54

extend 15, 192, 195–197, 199

19, 108, 303

graphical user interface (GUI)

extend-protocol 192, 197

currying 127

138, 212, 214–216, 218

extend-type 192–193, 197

first-class functions 72, 126,

graphics context 53

Extensible Markup Language

130

greatest common

(XML) 168, 262–264,

functional composition 169,

denominator 144, 284

305–306

288, 294

green thread 254

higher-order functions 115,

Greenspun’s Tenth Rule 303,

F

129–131, 137, 142, 151,

305

154, 161, 173–174, 230,

Groovy (programming

Factor (programming

287, 304

language) 207

language) 12, 301

partial application 127, 305

factory methods 305

pure functions 5, 118, 

H

131–132, 149, 241, 298

fail 252, 254

referentially

false 44–45, 56

Halloway, Stuart 101, 208

transparent 131–132, 256,

evil-false 44

has 285

284

Fantom (programming

functions 25, 68

hash maps 99

language) 207

anonymous 28, 129, 138, 210

hash-map 80, 88, 97, 190

fence post errors 203

arity 26

Haskell (programming

filter 78, 119, 129, 138, 304

Calling Functions 25

language) 12, 47, 113,

find-doc 51

dangerous 75

118, 149, 161, 198, 285, 301

find-ns 179

function signatures 192

out of order execution 149

finite state machines 146

local 147

Template Haskell 161

first 78, 80, 90, 121, 127

multiple function bodies 50

typeclasses 198, 285

First In, First Out (FIFO) 91,

named arguments 132

heuristic 151

192

futures 235, 261–263, 265, 269

Hickey, Rich 4–5, 206

First In, Last Out (FILO) 192

as callbacks 262

hidden 52

first-class 12, 27, 128

future 149, 255, 274

hierarchy 205

fixed-size pool 252

future-cancel 264

history 246

FIXO 192–193, 195, 197

future-cancelled? 264

homoiconicity 12, 293

fixo-peek 199

future-done? 264

hooks 304, 309

fixo-push 193, 195, 200

hops 81

flexibility 9

G

host semantics 62

float 83, 291

Hoyte, Doug 294

floating point 22, 62, 65–67

Gang of Four 303

hyphens 181

overflow 83

garbage collection 207, 280,

rounding error 64

283

I

underflow 64, 83

gcd 145

floats 279

gen-class 38, 212, 214–215,

I/O 239, 249, 252

fluent builder 202–203

218, 233, 304

idempotent 239, 255–256

FluentMove 204–205

generalized tail-call

identical? 70–71

fn 26, 28, 82, 135–136, 146, 164

optimization 143–144, 149

identifiers 70

for 7, 51

generic 46

identity 3, 13–14, 71, 202, 211,

force 113, 119, 121

genotype 9

227, 235, 247, 254

forever 68

gensym 169

IDEs 308

form 24

get 84, 88, 94, 98

idiom 190

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INDEX

323

idiomatic 34, 36, 46, 62, 82, 87,

into-array 218–219, 222

java.lang.Thread 255

98, 114, 154, 158, 169, 179,

ints 279

Java.next 208

198, 277, 280, 298

invariants 203, 246

Java.next Mantra 208

if 31, 114, 162

inversion of control 164

java.util.ArrayList 79

if-let 120, 171

invoke 214

java.util.Collection 225

image 51, 56

Io (programming

java.util.Collections/sort 223

immutability 7, 14, 30, 77–78,

language) 185

java.util.Comparator 222–223

86, 91, 93, 100, 107–109,

Ioke (programming

java.util.concurrent 258, 261

136, 143, 202, 224, 248, 280

language) 207

java.util.concurrent

imperative programming 14,

isa? 186–187

FutureTask 224

30, 108

ISeq 121

java.util.concurrent.atomic.

implementation details 182

ISeqable 200

AtomicInteger 136, 256

implicit do 30

ISliceable 228–229

java.util.concurrent.Blocking-

inc 243

isolation 238

Queue 91

inconsistency 245

iteration 46, 305

java.util.concurrent.Callable 

index numbers 47

iterator 78, 88

222

infinite lazy range 68

java.util.concurrent.locks 260

infinite loop 46

J

java.util.concurrent.locks.

infinite sequence 113, 118, 283

ReentrantLock 260

infix notation 25

Jahad, George 160

java.util.concurrent.locks.

inheritance 5, 183, 185–186

Java 37, 208, 290

ReentrantReadWriteLock

differential 185

Java (programming

261

implementation

language) 4, 6, 8, 16, 18,

java.util.List 15, 79, 223, 225

inheritance 196

21, 32, 36, 38, 67, 91, 108–

java.util.Observable 304

interface inheritance 16

109, 129, 179, 192, 194,

java.util.RandomAccess 48

prototype chain 184, 187

198, 207, 209, 211, 215,

java.util.regex.Matcher 48, 75

prototype maps 183, 186

java.util.regex.Pattern 73

inherited behaviors 186

224, 235, 255, 260, 290

javadoc 53, 73

init 55

Java 7 126

JavaScript (programming

init-proxy 212

Java Collections

language) 16, 135, 141, 184

in-process programming

Framework 224, 226

javax.swing.JFrame 212

model 249

Java libraries 52

Jess (programming

instance 55, 71

polymorphic print

language) 207

int 83, 146

facility 204–205

Joswig, Ranier 295

integers 22

variadic constructor 222

joy.gui.DynaFrame 212

overflow 64

Java Virtual Machine (JVM) 4,

JRuby (programming

interactive command

6, 36, 62, 65, 144, 146, 

language) 207

prompt 51

207–208, 220, 229, 234,

just-in-time (JIT) 207

interfaces 8, 16, 192–193, 195,

251, 280

juxt 188

200, 224, 285

HotSpot 207

Jython (programming

intern 179–180, 270, 272

java.awt 52

language) 207

internal reduce 280

java.awt.Container 212, 215

interop 73

java.awt.Frame 52

interoperability 24, 62, 196,

java.io.FilterOutputStream 211

K

200, 208, 218, 222, 224,

java.io.OutputStream 211

227, 277, 304

java.lang 42, 179

kapow 143

accessing Java instance

java.lang.Iterable 225

keep-indexed 102

members 37

java.lang.Math/round 291

key 49, 80, 88

accessing static class

java.lang.Object 128, 205, 220,

keys-apply 131–132

members 36

279

keyword arguments 278

creating Java class

java.lang.Runnable 222

keywords 23, 68–70, 241

instances 36

java.lang.StackOverflowError 

plumbing, separating from

setting Java instance

142, 230

domain 69

properties 37

java.lang.String 15

qualified 69

interpreted 298

java.lang.String/format 222

ubiquity of 75

into 83, 89, 196

java.lang.StringBuilder 218

Kingdom of Nouns 18

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324

INDEX

L

locking 14, 234, 236, 238, 250,

make-move 246

259–261

make-safe-array 260

lambda calculus 12

blocking 224, 248, 250–251,

make-smart-array 261

Lambda Papers 143

261, 265–266

manip-map 131–132

language, eager 113

contention 260–261

map 25, 68, 71, 78–79, 87, 

last 86

explicit locks 261

97–98, 100, 102, 119, 

laziness 113, 117, 119, 122,

fairness 250

129, 189, 204, 222, 269,

124, 274, 282

orphaning 238, 259

273–274, 295, 304

combinatorially exploding

reentrant 260

array map 98, 100, 190

computations 113

striping 261

PersistentHashMap 80

full realization 283

total ordering 238

thinking in maps 97

full realization of interim

log-agent 249

mapped file 74

results 117, 122

logarithmic 81

math-context 273–274

lazy evaluation 149

logging 249, 251

Maven 181

lazy sequences 68, 87, 131,

long 83, 279

max-history 247

268–269, 274, 283–284

look-around clauses 73

McCarthy, John 9

lazy-seq 115–116, 143

lookup 17–18, 68, 285–286

memoization 132, 256–257,

lazy-seq recipe 115, 117, 

loops 30–31, 146, 290

277, 284–287, 291

143

loop invariants 134

BasicCache 286

short-circuiting laziness 114

loop locals 31

cache 284–285

Leiningen 181

loop termination 46

caching protocol 286

let 29, 36, 52, 82, 117, 135, 137,

lowercase 74

hit 285–286

170, 273, 290

manipulable-memoize 

letfn 18, 147

M

256–257, 286

lexical scope 135, 273–274, 

memoization protocol 

290

M literal 21, 62–63

285–286

lexical bindings 309–310

macroexpand 161

memoize 256–257, 284–285

lexical context 138, 169

macroexpand-1 161

miss 285–286

lexical contour 170

macros 9, 24, 34, 55, 72, 

PluggableMemoization 

lexical environment 138,

119, 160–161, 164, 166, 

286–287

140, 309

169–170, 173, 231, 263,

through 286–287

line number 55

296–298, 303, 305, 308,

Mersenne primes 283–284

line terminators 74

310–311

metadata 69, 71, 128, 164, 181,

linear search 100

combining forms 164

191, 256

Lisp (programming language

compile-time 31, 157, 161,

attaching 71

family) 4, 8–9, 22, 25, 30,

169, 230

meta 71, 191

36, 68, 79–80, 87, 90, 126,

hygienic 171

with-meta 71, 191

172, 202, 295, 298, 304

macro that builds another

methods 52, 55

beauty of 9

macro 297

metric units 294

cons-cell 79

macro-definition time 171

min-by 151, 154

Lisp-1 62, 72

macro-expansion time 169,

min-history 247

lists 24–25, 33, 79, 90–91, 122,

310

mini-language 8, 47, 298

159

returning functions 173

mixins 193

as stacks 91

rules of thumb 161

ML (programming

empty 44

selective Name

language) 12

PersistentList 90

Capturings 171

monitors 236, 259

singly linked 90

using to change forms 165

monitor context 259

literal 73

using to control symbolic 

monkey-patching 16, 195

literal syntax 23–25, 83, 89, 97,

resolution time 169

Montoya, Inigo 77

189

using to manage

more 48

live-lock 239

resources 171

Move 202, 205

local-context 138, 310

magical formula 269

multi-line mode 74

locals 29–30, 72, 117, 136, 143,

main 203

multimethods 15, 68, 70, 185,

272–273, 290

make-array 219

187–188, 202, 304–305, 307

local bindings 310

make-dumb-array 258

multimethod dispatch 221

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INDEX

325

multimethods (continued)

non-termination 113

pcalls 235, 268–269

prefer-method 187

non-thread-safe 75

peek 86, 91, 93, 192

remove-method 187

nouns 13

performance 190, 202, 277,

multiple transactions 246

ns-unmap 179

291

multiversion concurrency

nth 84, 91

measurements 282

control (MVCC) 236, 239

nthnext 116

Perl (programming

snapshot isolation 245

null 203

language) 135

write skew 239, 246

numbers 21–22

persistent data structures 7, 77,

mutability 78, 108–109, 136,

binary 22

81, 90, 110, 113, 280–281

220, 224, 235, 239, 255,

distribution of represented

persistent hash trie 81

259, 261, 280–282

numbers 65

pixel 51

isolated pools of mutation

hexadecimal 21–22

pmap 235, 268–269

points 298

octal 22

polling 255

mutable fields 202

promotion 63–64

polymorphism 3, 16, 139–141,

mutable state 14

radix notation 22

184–185, 188–189, 192–

mutation 109, 235, 239, 257,

radix-32 22

194, 202, 206

298

scientific notation 22

pool 251

mutators 202

numerical precision 75

pop 86, 91–93, 192

pos 101–103

N

positionally 47

O

postconditions 133, 173, 175,

name resolution 72

302

object-oriented

name shadowing 72, 169

pow 142–143

programming 3, 5, 8,

named arguments 133

precision 62–63

13, 15, 18–19, 32, 97, 164,

named structures 50

arbitrary precision 21

205–206, 292, 303, 305

namespaces 17, 21, 36, 39,

preconditions 173, 206, 302

conflict resolution

42, 69, 71, 94, 163, 169,

predicates 103, 138

strategy 187

178, 180–182, 189, 198,

type-based 79, 102

hierarchies 5, 18, 103, 164,

212–213, 215, 262, 270,

prefix notation 9, 25

204, 298, 304

272, 305

primitives 21, 64, 83, 200, 229,

objects 279

as class specifications 212

279, 287, 290–291

obscure 47

compilation 214

println 30, 114, 160, 308–309

occur-count 288–289

in-ns 179

print-method 92

On Lisp 298

name-mangled local 290

programmer efficiency 278

one-at-a-time realization

namespace compilation 214

Programming Clojure 101

282–284

namespace qualification 69

promises 235, 261, 266–267,

operator precedence 5, 25

ns 39, 70, 179, 182, 212

269

privacy 181

optimizations 277, 280, 289

callback API to blocking

qualification 40, 69, 170,

option flags 74

API 266

179, 183

or (logical) 49, 305

deliver 265

remove-ns 180

original map 49

promise 149, 265–266

two-level mapping 178, 180

with-promises 268

user 21

P

protocols 15–16, 121, 189, 192,

natural language 296

195, 197, 227, 258, 285,

neighbors 85–86, 150, 242

pair 49

305

nest 236

parallelism 149, 235, 261,

design of 285

nested syntax-quotes 297

264–266, 269

Prototype Principle 183

new 36

dataflow 267

prototyping 214

next 46, 57, 79, 89–91, 110,

parentheses 9

proxies 305

115–116

parents 186

construct-proxy 212

nil 24–25, 44–45, 49, 56, 78,

partial 127–128, 130, 242

proxy 38, 171, 208, 210–212,

80, 94, 101, 110, 117,

partition 298

261, 304

163, 195, 253

Patriot missile 64–65

proxy-mappings 210

nil pun 57

pattern matching 47, 249

proxy-super 211

nondeterminism 235

patterns 54, 73

update-proxy 210, 212

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326

INDEX

pure virtual classes 192

tail-call optimization

reuse 127

purely functional 131, 282

(TCO) 6, 145–146

reverse 41, 87

push 192–193

tail-recursive 124, 142, 154

roll 288–289

pvalues 235, 268–269

tail-recursive self-call 144

root binding 28, 270–271

Python (programming

reduce 119, 129, 180, 280, 304

root cause 55

language) 6, 44, 126, 132

refactoring 290

rounding error 64

reference types 14, 108, 110,

rseq 80, 84

Q

165, 202, 235–236, 240,

RSS2 262, 299

242, 245, 247, 252, 255,

rss-children 266, 299

Qi (programming language) 12

257–259, 261, 270, 272,

rsubseq 99

queues 91, 93, 252–253

303–304

Ruby (programming

PersistentQueue 91–92

add-watch 164, 303

language) 16, 135, 163

priority queues 192

coordinated 240–244, 247

unless 163

queue-fish 92

remove-watch 303

runtime 141

quicksort 121–123

set-validator 241

qsort 122–123

synchronous 241, 247, 252,

S

quote 32–34, 70, 159, 171

255

quoting forms 161

uncoordinated 255

SafeArray protocol 258–259

nested syntax-quotes 160

uniform state change

Scala (programming

syntax-quote 34, 163

model 242

language) 47, 126, 145,

watchers 304

170, 207, 301

R

referential transparency 12, 132

scalar 61

reflection 279–280, 289

scalar types 75

range 51, 68, 83, 90, 117, 282

Refs 235, 240–241, 243–246,

duality of 48, 62

rationals 22, 66

255

Scheme (programming

denominator 22, 67

alter 242, 244–245

language) 87, 143–145

numerator 22, 67

commutative 245

actors 143

ratio 66

commute 244–245

screaming-filter 211

rational? 65–67

ensure 239, 246

self 185

rationalize 66–67

ref-set 245

semantic coupling 302

read 309–310

regular expressions 52, 61, 75

separation of concerns 7

reader feature 28

case insensitivity 74

seq 46, 74, 77–80, 83–84, 98,

Read-Eval-Print Loop

re-find 75

200

(REPL) 21, 26, 32, 39, 

regex 73–74

seq protocol 305

54–55, 57, 97, 117, 123,

re-groups 75

seq1 283–284

179, 213–214, 221, 267,

reluctant quantifiers 73

sequence abstraction 77–78,

270, 273, 278, 289, 309

re-matcher 75

80, 82, 97, 100–101, 204

read-time 73

re-seq 74–75

rest vs. next 116

recompiled 73

reify 38, 141, 198, 208, 211, 304

sequences 78–80, 290

records 189, 191, 195, 197–198,

factory function 198

chunked 277, 282–283, 291

205–206

relational functions 293–294

chunk-at-a-time model 282

recur 30–31, 142, 144–147, 200

SELECT 10, 293

sequentials 48, 78–79

recursion 30, 141–142

relative-units 296, 298

server 209

accumulator 143

rem 55

set 37

explicit tail-call

remainder 55

sets 25, 79, 94

optimization 145

remote-procedure call

difference 97

freeing the recursive call 142

(RPC) 262, 266–267

intersection 96

generalized tail-call

rename 42

relative complement 97

optimization 145

reordering 245

union 97

mundane 142–143, 145, 306

replace 85

setter 204

mutual 142, 145–147

reset 255

shared structure 112

recursive call trapped 142

resolve 72, 272

shared-state concurrency 234

stack consumption 142

rest 46, 78, 80, 89, 91, 115–116,

short 83

tail 30, 145

121, 127

shorts 279

tail position 30–31, 143, 146

reusability 234

shuffle 226

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INDEX

327

side effects 29, 131–132, 154,

subseq 95, 99

transactions 235–236, 239, 241,

163, 180, 248, 255, 306

subvec 88–89

243–248, 255–257

simplicity 4, 103, 288

superclass constructor 212

embedded transactions 236

simplification 204

Sussman, Gerald 143

retry 236, 243, 247–248, 255,

single quote 33

Swank-Clojure 309

258

sleep 246, 252

swap 255, 257

size of 247

slice 228

Swing 126, 216

transients 108, 277, 280–282,

Sliceable 229

Sybil 62

291

sliceCount 228

symbols 23, 26, 28, 70–71, 159,

Rule of Transients 280

slope 133–134

171, 178, 264

TreeNode 189, 193, 195, 201

slowly 257, 287

auto-qualification 34

trees 110–111, 166, 168, 195,

snapshot 235–236

symbolic mappings 179

262, 283, 306

isolation 236

syntax 5, 7, 9

binary 113

software transactional memory

syntactic sugar 222

persistent binary 191

(STM) 3, 235–238, 243,

syntax-quote 32, 34–35, 72,

245–246, 248, 255

160–161, 169, 171

red-black 113

barging 239

nested 160

traversing 111, 306–307

commit 236, 243, 245–246

unbalanced 113

commit-time 245

T

triangle 118

in-transaction 243, 245–246

truncation 62, 290–291

retry 236

tail position 142

truthiness 43–44, 56

some 94

and recur targets 145

truthy 52, 69, 94, 119

sometimes-slowly 257

tail recursion 143

tuning 247

sort 47, 225

tail recursive 151

tweet-items 266

sort-by 129–130

tail-call 144

tweetless-rss-children 300

sorted-map 98–99, 113

take 124

tweet-occurrences 264

sorted-map-by 95, 99

terminating condition 46

Twitter 262–264, 299

sorted-set 94–95, 113

test-driven development (TDD)

type conversion, automatic 63

sorted-set-by 95

109, 292, 298–301

type hints 228, 277–280, 

sort-parts 123

specification language 302

287, 291

special form 24, 26, 33

unit tests 128

types 189, 227, 304–305

split 74–75

there ain’t no such thing as a

deftype 38, 200–202

spot optimizations 280

free lunch 237

spreadsheets 130, 139, 268

third-party libraries 171, 194,

U

SQL 10–11, 293–294

198

this 108, 171, 185, 198, 210–211

sqr 302

thread-bound 272

unbox 289

stack overflow 115

thread-local 28, 240, 270, 273

unchecked 64

errors 145

thread-local bindings 271–273,

unchecked-inc 289

stack trace 55

306

underscores 181

stacks 86, 91

threads 5, 28, 220, 224, 236,

Unicode 73

IPersistentStack 201

240–241, 247, 249–252,

case insensitivity 74

state 3, 13–14, 186, 202, 

254–255, 258, 260–261,

234–235, 239, 242, 

unit conversions 294–295

265, 267–268, 274, 282,

245, 247, 249, 255

unit testing 128, 292, 298, 302

308

managing 238

unit-of-distance 297

starved 250

static type system 278

Universal Design Pattern

thread-safe 211, 240

static versus dynamic 278

(UDP) 183–186, 188

time 13, 235, 247

Steele, Guy Lewis 47, 143

timelines 235, 243

Unix line terminator 74

straight-line path 150

to-array 220

unmodifiable 226

stress 246

to-array-2d 220

unquote 35, 160, 163–164

strings 23, 71, 73, 79

toString 204–205

unquote-splice 35, 163

zero-length strings 44

trampoline 147

up-arrow key 51

structural sharing 110

trampoline for mutual

update-in 85

stubbing 299

recursion 147

uppercase 74

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328

INDEX

V

verbs 13

wrapping 195

very bad things 259

write-once 265

validation 205

visible 52

validator 241

void 212

X

variable arguments 26

volatile 202

variables 14, 29, 255

xconj 110, 112, 191, 193, 

Vars 25, 27, 32–34, 40, 72, 128,

W

195–196

137, 163–164, 178, 181,

xor 51

235, 240, 270, 307

when 8, 31, 52, 162–163, 170

xs 48

anonymous 272

when-let 120, 171

xseq 191, 200

root binding 164

when-not 163

var 270, 272

where-am-i 72

Y

var-get 272

while 8

with-local-vars 272–273

Yegge, Steve 183

vectors 24, 33, 80, 82, 98, 193

Whitehead, Alfred North 61

as stacks 86

whitespace, ignoring 74

IPersistentVector 193, 198

wit 73

Z

of names 49

with-binding 273

of primitives 83

with-open 82, 172, 274

Zawinski, Jamie (JWZ) 73

subvectors 88

with-out-str 273

zencat 281

vec 83

with-precision 273

zencat2 281

vector 29, 32, 83–84, 126,

with-redefs 300

zero 44

245, 295

with-resource 172

zipmap 98

vector-of 83

workflow 91

zipper 263

walking in reverse order 84

world 150–152

verbosity 140, 202

worst-case 81

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FUNCTIONAL PROGRAMMING

THE Joy OF Clojure

SEE INSERT

Fogus Houser

If you’ve seen how dozens of lines of Java or Ruby can dissolve

into just a few lines of Clojure, you’ll know why the authors

of this book call it a “joyful language.” Clojure is a dialect

“You’ll learn fast!”

of Lisp that runs on the JVM. It combines the nice features of a

—From the foreword

scripting language with the powerful features of a production

by Steve Yegge, Google

environment—features like persistent data structures and clean

multithreading that you’ll need for industrial-strength

“Simply unputdownable!”

application development.

—Baishampayan Ghose (BG)

Qotd, Inc .

The Joy of Clojure goes beyond just syntax to show you how to

write fl uent and idiomatic Clojure code. You’ll learn a functional

“Discover the why, not just

approach to programming and will master Lisp techniques

the how of Clojure.”

that make Clojure so elegant and effi

cient. Th

e book gives

—Federico Tomassetti

you easy access to hard soft ware areas like concurrency,

Politecnico di Torino

interoperability, and performance. And it shows you how

great it can be to think about problems the Clojure way.

“What Irma Rombauer

did for cooking, Fogus

What’s Inside

and Houser have done

Th

e what and why of Clojure

for Clojure.”

How to work with macros

—Phil Hagelberg, Sonian

How to do elegant application design

Functional programming idioms

Written for programmers coming to Clojure from another

programming background—no prior experience with

Clojure or Lisp is required.

Michael Fogus is a member of Clojure/core with experience in

distributed simulation, machine vision, and expert systems.

Chris Houser is a key contributor to Clojure who has

implemented several of its features.

For online access to the authors and a free ebook for owners

of this book, go to manning.com/TheJoyofClojure

M A N N I N G

$44.99 / Can $51.99 [INCLUDING eBOOK]

Document Outline

- JoyOfClojure

- brief contents

- contents

- foreword

- preface

- acknowledgments

- Fogus

- Chouser

- about this book

- Why learn Clojure?

- Who should read this book?

- Roadmap

- Foundations

- Data types

- Functional programming

- Large-scale design

- Tangential considerations

- Code conventions

- Getting Clojure

- Prerequisites

- Instructions

- REPL

- Downloading code examples

- Author Online

- About the cover illustration

- Foundations

- Chapter 1 Clojure philosophy

- 1.1 The Clojure way

- 1.1.1 Simplicity

- 1.1.2 Freedom to focus

- 1.1.3 Empowerment

- 1.1.4 Clarity

- 1.1.5 Consistency

- 1.2 Why a(nother) Lisp?

- 1.2.1 Beauty

- 1.2.2 Extreme flexibility

- 1.2.3 Code is data

- 1.3 Functional programming

- 1.3.1 A workable definition of functional programming

- 1.3.2 The implications of functional programming

- 1.4 Why Clojure isn’t especially object-oriented

- 1.4.1 Defining terms

- 1.4.2 Imperative “baked in”

- 1.4.3 Most of what OOP gives you, Clojure provides

- Polymorphism and the expression problem

- Subtyping and interface-oriented programming

- Encapsulation

- Not everything is an object

- 1.5 Summary

- Chapter 2 Drinking from the Clojure firehose

- 2.1 Scalars

- 2.1.1 Numbers

- 2.1.2 Integers

- 2.1.3 Floating-point numbers

- 2.1.4 Rationals

- 2.1.5 Symbols

- 2.1.6 Keywords

- 2.1.7 Strings

- 2.1.8 Characters

- 2.2 Putting things together: collections

- 2.2.1 Lists

- 2.2.2 Vectors

- 2.2.3 Maps

- 2.2.4 Sets

- 2.3 Making things happen: functions

- 2.3.1 Calling functions

- 2.3.2 Defining functions

- 2.3.3 Simplifying function definitions with def and defn

- 2.3.4 In-place functions with #()

- 2.4 Vars

- 2.4.1 Declaring bindings using def

- 2.5 Locals, loops, and blocks

- 2.5.1 Blocks

- 2.5.2 Locals

- 2.5.3 Loops

- recur

- loop

- Tail position

- 2.6 Preventing things from happening: quoting

- 2.6.1 Evaluation

- 2.6.2 Quoting

- Syntax-quote

- Symbol auto-qualification

- 2.6.3 Unquote

- 2.6.4 Unquote-splicing

- 2.6.5 Auto-gensym

- 2.7 Leveraging Java via interop

- 2.7.1 Accessing static class members

- 2.7.2 Creating Java class instances

- 2.7.3 Accessing Java instance members with the . operator

- 2.7.4 Setting Java instance properties

- 2.7.5 The .. macro

- 2.7.6 The doto macro

- 2.7.7 Defining classes

- 2.8 Exceptional circumstances

- 2.8.1 A little pitch and catch

- 2.9 Namespaces

- 2.9.1 Creating namespaces using ns

- 2.9.2 Loading other namespaces with :require

- 2.9.3 Loading and creating mappings with :use

- 2.9.4 Creating mappings with :refer

- 2.9.5 Loading Java classes with :import

- 2.10 Summary

- Chapter 3 Dipping our toes in the pool

- 3.1 Truthiness

- 3.1.1 What’s truth?

- 3.1.2 Don’t create Boolean objects

- 3.1.3 nil versus false

- 3.2 Nil pun with care

- 3.3 Destructuring

- 3.3.1 Your assignment, should you choose to accept it

- 3.3.2 Destructuring with a vector

- 3.3.3 Destructuring with a map

- Associative destructuring

- 3.3.4 Destructuring in function parameters

- 3.3.5 Destructuring versus accessor methods

- 3.4 Using the REPL to experiment

- 3.4.1 Experimenting with seqs

- 3.4.2 Experimenting with graphics

- 3.4.3 Putting it all together

- 3.4.4 When things go wrong

- 3.4.5 Just for fun

- 3.5 Summary

- Data types

- Chapter 4 On scalars

- 4.1 Understanding precision

- 4.1.1 Truncation

- 4.1.2 Promotion

- 4.1.3 Overflow

- 4.1.4 Underflow

- 4.1.5 Rounding errors

- 4.2 Trying to be rational

- 4.2.1 Why be rational?

- 4.2.2 How to be rational

- 4.2.3 Caveats of rationality

- 4.3 When to use keywords

- 4.3.1 How are keywords different from symbols?

- As Keys

- As Enumerations

- As Multimethod Dispatch Values

- As Directives

- 4.3.2 Qualifying your keywords

- 4.4 Symbolic resolution

- 4.4.1 Metadata

- 4.4.2 Symbols and namespaces

- 4.4.3 Lisp-1

- 4.5 Regular expressions—the second problem

- 4.5.1 Syntax

- 4.5.2 Functions

- 4.5.3 Beware of mutable matchers

- 4.6 Summary

- Chapter 5 Composite data types

- 5.1 Persistence, sequences, and complexity

- 5.1.1 “You keep using that word. I do not think it means what you think it means.”

- 5.1.2 Sequence terms and what they mean

- Terms

- Equality partitions

- The sequence abstraction

- 5.1.3 Big-O

- 5.2 Vectors: creating and using them in all their varieties

- 5.2.1 Building vectors

- Primitive vectors

- 5.2.2 Large vectors

- 5.2.3 Vectors as stacks

- 5.2.4 Using vectors instead of reverse

- 5.2.5 Subvectors

- 5.2.6 Vectors as MapEntries

- 5.2.7 What vectors aren’t

- Vectors aren’t sparse

- Vectors aren’t queues

- Vectors aren’t sets

- 5.3 Lists: Clojure’s code form data structure

- 5.3.1 Lists like Lisps like

- 5.3.2 Lists as stacks

- 5.3.3 What lists aren’t

- 5.4 How to use persistent queues

- 5.4.1 A queue about nothing

- 5.4.2 Putting things on

- 5.4.3 Getting things

- 5.4.4 Taking things off

- 5.5 Persistent sets

- 5.5.1 Basic properties of Clojure sets

- How Clojure populates sets

- 5.5.2 Keeping your sets in order with sorted-set

- 5.5.3 contains?

- 5.5.4 clojure.set

- Intersection

- Union

- Difference

- 5.6 Thinking in maps

- 5.6.1 Hash maps

- 5.6.2 Keeping your keys in order with sorted maps

- 5.6.3 Keeping your insertions in order with array maps

- 5.7 Putting it all together: finding the position of items in a sequence

- 5.7.1 Implementation

- 5.8 Summary

- Functional programming

- Chapter 6 Being lazy and set in your ways

- 6.1 On immutability

- 6.1.1 Defining immutability

- Every day is like Sunday

- Immutability through convention

- 6.1.2 Being set in your ways—immutability

- Invariants

- Reasoning

- Equality has meaning

- Sharing is cheap

- Flattening the levels of indirection

- Immutability fosters concurrent programming

- 6.2 Designing a persistent toy

- 6.3 Laziness

- 6.3.1 Familiar laziness with logical-and

- 6.3.2 Understanding the lazy-seq recipe

- Utilizing lazy-seq and rest

- 6.3.3 Losing your head

- 6.3.4 Employing infinite sequences

- 6.3.5 The delay and force macros

- 6.4 Putting it all together: a lazy quicksort

- The implementation

- 6.5 Summary

- Chapter 7 Functional programming

- 7.1 Functions in all their forms

- 7.1.1 First-class functions

- Creating functions on demand using composition

- Creating functions on demand using partial functions

- Reversing truth with complement

- Using functions as data

- 7.1.2 Higher-order functions

- Functions as arguments

- Functions as return values

- 7.1.3 Pure functions

- Referential transparency

- Optimization

- Testability

- 7.1.4 Named arguments

- 7.1.5 Constraining functions with pre- and postconditions

- Decoupling assertions from functions

- 7.2 Closures

- Functions returning closures

- Closing over parameters

- Passing closures as functions

- Sharing closure context

- Compile-time versus run-time

- 7.3 Thinking recursively

- 7.3.1 Mundane recursion

- Regular recursion is fun again with lazy-seq

- 7.3.2 Tail calls and recur

- Generalized tail-call optimization

- Tail recursion

- Why recur?

- 7.3.3 Don’t forget your trampoline

- 7.3.4 Continuation-passing style

- 7.4 Putting it all together: A* pathfinding

- The world

- Neighbors

- 7.4.1 The A* implementation

- 7.4.2 Notes about the A* implementation

- 7.5 Summary

- Large-scale design

- Chapter 8 Macros

- 8.1 Data is code is data

- 8.1.1 Syntax-quote, unquote, and splicing

- 8.1.2 Macro rules of thumb

- 8.2 Defining control structures

- 8.2.1 Defining control structures without syntax-quote

- 8.2.2 Defining control structures using syntax-quote and unquoting

- 8.3 Macros combining forms

- 8.4 Using macros to change forms

- 8.5 Using macros to control symbolic resolution time

- 8.5.1 Anaphora

- 8.5.2 (Arguably) useful selective name capturing

- 8.6 Using macros to manage resources

- 8.7 Putting it all together: macros returning functions

- 8.8 Summary

- Chapter 9 Combining data and code

- 9.1 Namespaces

- 9.1.1 Creating namespaces

- ns

- in-ns

- create-ns

- 9.1.2 Expose only what’s needed

- 9.1.3 Declarative inclusions and exclusions

- 9.2 Exploring Clojure multimethods with the Universal Design Pattern

- 9.2.1 The parts

- beget

- put

- get

- 9.2.2 Usage

- No notion of self

- 9.2.3 Multimethods to the rescue

- 9.2.4 Ad hoc hierarchies for inherited behaviors

- 9.2.5 Resolving conflict in hierarchies

- 9.2.6 Arbitrary dispatch for true maximum power

- 9.3 Types, protocols, and records

- 9.3.1 Records

- 9.3.2 Protocols

- Sharing method implementations

- Reify

- Namespaced methods

- Method implementations in defrecord

- 9.3.3 Building from a more primitive base with deftype

- 9.4 Putting it all together: a fluent builder for chess moves

- 9.4.1 Java implementation

- 9.4.2 Clojure implementation

- Using records

- Separation of concerns

- 9.5 Summary

- Chapter 10 Java.next

- 10.1 Generating objects on the fly with proxy

- 10.1.1 A simple dynamic web service

- It’s called proxy for a reason

- Proxies for true power dynamism

- Proxies as proper citizens

- Final points about proxy

- 10.2 Clojure gen-class and GUI programming

- 10.2.1 Namespaces as class specifications

- The guts of namespace compilation

- 10.2.2 Exploring user interface design and development with Clojure

- 10.3 Clojure’s relationship to Java arrays

- 10.3.1 Types of arrays: primitive and reference

- Creating primitive arrays

- Creating reference arrays

- 10.3.2 Array mutability

- 10.3.3 That unfortunate naming convention

- 10.3.4 Multidimensional arrays

- 10.3.5 Variadic method/constructor calls

- 10.4 All Clojure functions implement...

- 10.4.1 java.util.Comparator

- 10.4.2 java.lang.Runnable

- 10.4.3 java.util.concurrent.Callable

- 10.5 Using Clojure data structures in Java APIs

- 10.5.1 java.util.List

- 10.5.2 java.lang.Comparable

- 10.5.3 java.util.RandomAccess

- 10.5.4 java.util.Collection

- java.util.Map

- 10.5.5 java.util.Set

- 10.6 definterface

- 10.6.1 Generating interfaces on the fly

- 10.7 Be wary of exceptions

- 10.7.1 A bit of background regarding exceptions

- 10.7.2 Runtime versus compile-time exceptions

- Runtime exceptions

- Compile-time exceptions

- 10.7.3 Handling exceptions

- 10.7.4 Custom exceptions

- 10.8 Summary

- Chapter 11 Mutation

- 11.1 Software transactional memory with multiversion concurrency control and snapshot isolation

- Software transactional memory

- 11.1.1 Transactions

- 11.1.2 Embedded transactions

- 11.1.3 The things that STM makes easy

- Consistent information

- No need for locks

- ACI

- 11.1.4 Potential downsides

- Write skew

- Live-lock

- 11.1.5 The things that make STM unhappy

- I/O

- Class instance mutation

- Large transactions

- 11.2 When to use Refs

- 11.2.1 Coordinated, synchronous change using alter

- 11.2.2 Commutative change with commute

- 11.2.3 Vulgar change with ref-set

- 11.2.4 Fixing write-skew with ensure

- 11.2.5 Refs under stress

- 11.3 When to use Agents

- 11.3.1 In-process versus distributed concurrency models

- 11.3.2 Controlling I/O with an Agent

- 11.3.3 The difference between send and send-off

- 11.3.4 Error handling

- :fail

- :continue

- 11.3.5 When not to use Agents

- 11.4 When to use Atoms

- 11.4.1 Sharing across threads

- 11.4.2 Using Atoms in transactions

- Atomic memoization

- 11.5 When to use locks

- 11.5.1 Safe mutation through locking

- References around evil mutable things

- 11.5.2 Using Java’s explicit locks

- 11.6 When to use futures

- 11.6.1 Futures as callbacks

- Counting word occurrences in a set of Twitter feeds

- 11.7 When to use promises

- 11.7.1 Parallel tasks with promises

- 11.7.2 Callback API to blocking API

- 11.7.3 Deterministic deadlocks

- 11.8 Parallelism

- 11.8.1 pvalues

- 11.8.2 pmap

- 11.8.3 pcalls

- 11.9 Vars and dynamic binding

- 11.9.1 The binding macro

- 11.9.2 Creating a named Var

- 11.9.3 Creating anonymous Vars

- 11.9.4 Dynamic scope

- 11.10 Summary

- Tangential considerations

- Chapter 12 Performance

- 12.1 Type hints

- 12.1.1 Advantages of type adornment

- 12.1.2 Type-hinting arguments and returns

- 12.1.3 Type-hinting objects

- 12.2 Transients

- 12.2.1 Ephemeral garbage

- 12.2.2 Transients compare in efficiency to mutable collections

- 12.3 Chunked sequences

- 12.3.1 Regaining one-at-a-time laziness

- 12.4 Memoization

- 12.4.1 Re-examining memoization

- 12.4.2 A memoization protocol

- 12.5 Understanding coercion

- 12.5.1 First rule of coercion: don’t

- 12.5.2 Corollary: you’re probably not doing it right

- 12.5.3 Second rule of coercion: don’t

- 12.5.4 Third rule of coercion: coerce a stable local

- 12.5.5 Fourth rule of coercion: watch your sizes

- 12.5.6 Fifth rule of coercion: truncate only as a goal

- 12.6 Summary

- Chapter 13 Clojure changes the way you think

- 13.1 DSLs

- 13.1.1 A ubiquitous DSL

- 13.1.2 Putting parentheses around the specification

- defunits

- 13.1.3 A note about Clojure’s approach to DSLs

- 13.2 Testing

- 13.2.1 Some useful techniques

- Using with-var-root to stub

- clojure.test as specification

- 13.2.2 Contracts programming

- Revisiting pre- and postconditions

- Advantages of pre- and postconditions

- 13.3 A lack of design patterns

- 13.3.1 Clojure’s first-class design patterns

- Observer

- Strategy

- Visitor

- Abstract Factory

- Interpreter

- Builder

- Façade

- Iterator

- Dependency injection

- 13.4 Error handling and debugging

- 13.4.1 Error handling

- Dynamic tree traversal

- 13.4.2 Debugging

- A breakpoint macro

- Overriding the REPL’s reader

- Overriding the REPL’s evaluator

- Putting it all together

- Multiple breakpoints and breakpoints in macros

- 13.5 Fare thee well

- resources

- Miscellaneous resources

- Online resources

- index

- Keywords and symbols

- A

- B

- C

- D

- E

- F

- G

- H

- I

- J

- K

- L

- M

- N

- O

- P

- Q

- R

- S

- T

- U

- V

- W

- X

- Y

- Z

Table of Contents

PART 1 FOUNDATIONS ......................................................................1

■

Drinking from the Clojure firehose 20

Dipping our toes in the pool 43

PART 2 DATA TYPES . .......................................................................59

4 ■

5 ■

PART 3 FUNCTIONAL PROGRAMMING............................................. 105

6 ■

7 ■

PART 4 LARGE-SCALE DESIGN ........................................................ 155

Macros 157

Combining data and code 177

0

Mutation 234

PART 5 TANGENTIAL CONSIDERATIONS.......................................... 275

2 ■

3 ■

foreword xvii

preface xix

acknowledgments xxi

about this book xxiii

Simplicity 4

Freedom to focus 5

Clarity 6

Consistency 7

1.2 Why a(nother) Lisp? 8

Beauty 9

Code is data 11

1.3 Functional programming 12

1.4 Why Clojure isn’t especially object-oriented 13

Imperative “baked in” 14 ■

Most of what

Summary 19

Numbers 21 ■

Integers 22 ■ Floating-point numbers 22

Symbols 23 ■ Keywords 23

2.2 Putting things together: collections 24

Maps 25

Defining functions 26

Simplifying

In-place functions

2.5 Locals, loops, and blocks 29

Loops 30

2.6 Preventing things from happening: quoting 32

Quoting 33

Unquote 34

Unquote-splicing 35

Auto-gensym 36

Accessing Java instance members with the .