Programming Clojure

284 Pages·2012·5.6 MB·English

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What Readers Are Saying About
Programming Clojure, Second Edition
Clojure is one of the most interesting languages out there right now, and the best
way of learning Clojure just got better. The second edition of Programming Clojure
adds up-to-date information, plenty of practical examples, and a ton of useful
tips on how to learn, work with, and succeed with Clojure.
➤ Ola Bini
Creator of Ioke language, developer, ThoughtWorks
Intimidated by Clojure? You won’t be after you read this book. Written in a clear
and enjoyable style, it teaches the language one small piece at a time in a very
accessible way.
➤ Tim Berglund
Founder and Principal, August Technology Group
The authors have charted the smoothest path yet to Clojure fluency with this
well-organized and easy-to-read book. They have a knack for creating simple
and effective examples that demonstrate how the language’s unique features
fit together.
➤ Chris Houser
Primary Clojure contributor and library author
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Clojure is a beautiful, elegant, and very powerful language on the JVM. It’s
like a cathedral: you could wander into it, but you’d prefer the company of a
knowledgeable guide who can give you their perspectives, to help you grasp and
appreciate the architecture and the art. In this book you can enjoy and benefit
from the company of not one, but two seasoned developers who have the depth
of knowledge and the perspective you need.
➤ Dr. Venkat Subramaniam
Award-winning author and founder, Agile Developer, Inc.
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Programming Clojure
Second Edition
Stuart Halloway
Aaron Bedra
The Pragmatic Bookshelf
Dallas, Texas • Raleigh, North Carolina
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Many of the designations used by manufacturers and sellers to distinguish their products
are claimed as trademarks. Where those designations appear in this book, and The Pragmatic
Programmers, LLC was aware of a trademark claim, the designations have been printed in
initial capital letters or in all capitals. The Pragmatic Starter Kit, The Pragmatic Programmer,
Pragmatic Programming, Pragmatic Bookshelf, PragProg and the linking g device are trade-
marks of The Pragmatic Programmers, LLC.
Every precaution was taken in the preparation of this book. However, the publisher assumes
no responsibility for errors or omissions, or for damages that may result from the use of
information (including program listings) contained herein.
Our Pragmatic courses, workshops, and other products can help you and your team create
better software and have more fun. For more information, as well as the latest Pragmatic
titles, please visit us at http://pragprog.com.
The team that produced this book includes:
Michael Swaine (editor)
Potomac Indexing, LLC (indexer)
Kim Wimpsett (copyeditor)
David J Kelly (typesetter)
Janet Furlow (producer)
Juliet Benda (rights)
Ellie Callahan (support)
Copyright © 2012 The Pragmatic Programmers, LLC.
All rights reserved.
No part of this publication may be reproduced, stored in a retrieval system, or
transmitted, in any form, or by any means, electronic, mechanical, photocopying,
recording, or otherwise, without the prior consent of the publisher.
Printed in the United States of America.
ISBN-13: 978-1-934356-86-9
Encoded using the finest acid-free high-entropy binary digits.
Book version: P1.0—April 2012
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In loving memory of my father and mentor,
Craig Bedra, who taught me the value of
learning by exploration and that there is no
such thing as magic.—Aaron
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Contents
Foreword for the Second Edition . . . . . . . . xi
Foreword for the First Edition . . . . . . . . xiii
Acknowledgments . . . . . . . . . . . xv
Preface . . . . . . . . . . . . . . xvii
1. Getting Started . . . . . . . . . . . . 1
1.1 Why Clojure? 2
1.2 Clojure Coding Quick Start 11
1.3 Exploring Clojure Libraries 16
1.4 Wrapping Up 20
2. Exploring Clojure . . . . . . . . . . . 21
2.1 Forms 21
2.2 Reader Macros 30
2.3 Functions 32
2.4 Vars, Bindings, and Namespaces 36
2.5 Calling Java 43
2.6 Flow Control 45
2.7 Where’s My for Loop? 48
2.8 Metadata 51
2.9 Wrapping Up 53
3. Unifying Data with Sequences . . . . . . . . 55
3.1 Everything Is a Sequence 56
3.2 Using the Sequence Library 60
3.3 Lazy and Infinite Sequences 69
3.4 Clojure Makes Java Seq-able 71
3.5 Calling Structure-Specific Functions 76
3.6 Wrapping Up 84
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Contents • viii
4. Functional Programming . . . . . . . . . 85
4.1 Functional Programming Concepts 85
4.2 How to Be Lazy 90
4.3 Lazier Than Lazy 98
4.4 Recursion Revisited 103
4.5 Wrapping Up 112
5. State . . . . . . . . . . . . . . 113
5.1 Concurrency, Parallelism, and Locking 114
5.2 Refs and Software Transactional Memory 115
5.3 Use Atoms for Uncoordinated, Synchronous Updates 122
5.4 Use Agents for Asynchronous Updates 123
5.5 Managing Per-Thread State with Vars 127
5.6 A Clojure Snake 132
5.7 Wrapping Up 141
6. Protocols and Datatypes . . . . . . . . . 143
6.1 Programming to Abstractions 143
6.2 Interfaces 146
6.3 Protocols 147
6.4 Datatypes 151
6.5 Records 156
6.6 reify 162
6.7 Wrapping Up 163
7. Macros . . . . . . . . . . . . . . 165
7.1 When to Use Macros 165
7.2 Writing a Control Flow Macro 166
7.3 Making Macros Simpler 172
7.4 Taxonomy of Macros 177
7.5 Wrapping Up 185
8. Multimethods . . . . . . . . . . . . 187
8.1 Living Without Multimethods 187
8.2 Defining Multimethods 189
8.3 Moving Beyond Simple Dispatch 192
8.4 Creating Ad Hoc Taxonomies 194
8.5 When Should I Use Multimethods? 198
8.6 Wrapping Up 201
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ix • Contents
9. Java Down and Dirty . . . . . . . . . . 203
9.1 Exception Handling 204
9.2 Wrestling with the Integers 207
9.3 Optimizing for Performance 209
9.4 Creating Java Classes in Clojure 214
9.5 A Real-World Example 219
9.6 Wrapping Up 226
10. Building an Application . . . . . . . . . 227
10.1 Scoring a Clojurebreaker Game 228
10.2 Testing the Scorer 231
10.3 test.generative 235
10.4 Creating an Interface 243
10.5 Deploying Your Code 248
10.6 Farewell 251
A1. Editor Support . . . . . . . . . . . . 253
A2. Bibliography . . . . . . . . . . . . 255
Index . . . . . . . . . . . . . . 257
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Foreword for the Second Edition
A lot has changed since the first edition of the book. Yes, the language has
had some enhancements, such as protocols and records. Most significant,
though, is that Clojure has seen adoption across a wide variety of domains.
People are building start-ups, analyzing large data sets, and doing communi-
cations, financial, web, and database work in Clojure. A large and supportive
community has grown up around Clojure and, with it, a ton of libraries. These
libraries are particularly exciting, not just in the facilities they provide. The
best of them embrace the Clojure approach and mechanisms and, in doing
so, reach new levels of simplicity and interoperability.
In this second edition, Stuart and Aaron make sure to cover the language
enhancements and include a taste of what it’s like to leverage some of the
community libraries, while taking care to convey the concepts that make it
all work. The book remains an exhilarating introduction to Clojure, and I
hope it inspires you to join the community and, eventually, contribute to the
library ecosystem.
—Rich Hickey
Creator of Clojure
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