Clojure in action

434 Pages·2012·7.587 MB·English

by  Amit Rathore

Elegant applications on the JVM
IN ACTION
Amit Rathore
M A N N I N G
Clojure in Action

Clojure in Action
AMIT RATHORE
MANNING
SHELTER ISLAND
To my parents, my son, and my wonderful wife
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ISBN: 9781935182597
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1 2 3 4 5 6 7 8 9 10 – MAL – 17 16 15 14 13 12 11
brief contents
PART 1 GETTING STARTED........................................................1
1 ■ Introduction to Clojure 3
2 ■ A whirlwind tour 30
3 ■ Building blocks of Clojure 60
4 ■ Polymorphism with multimethods 90
5 ■ Clojure and Java interop 106
6 ■ State and the concurrent world 122
7 ■ Evolving Clojure through macros 148
PART 2 GETTING REAL..........................................................167
8 ■ Test-driven development and more 169
9 ■ Data storage with Clojure 189
10 ■ Clojure and the web 221
11 ■ Scaling through messaging 240
12 ■ Data processing with Clojure 273
13 ■ More on functional programming 307
14 ■ Protocols, records, and types 339
15 ■ More macros and DSLs 367
v

contents
preface xiii
acknowledgments xvi
about this book xviii
PART 1 GETTING STARTED.............................................1
1 Introduction to Clojure 3
1.1 What is Clojure? 4
Clojure—the reincarnation of Lisp 4 ■ How we got here 5
How this book teaches Clojure 5
1.2 Understanding Clojure syntax 6
XML and parentheses 7 ■ Lists, vectors, and hashes 9
1.3 The sources of Clojure’s power 10
Clojure and Lisp 10 ■ Clojure and functional programming 11
Clojure and the JVM 11 ■ Clojure as a Lisp 11
More advantages of Clojure 18 ■ Clojure as a
functional language 18 ■ Clojure as a JVM-based language 23
1.4 Clojure—beyond object orientation 26
1.5 Summary 28
vii
viii CONTENTS
2 A whirlwind tour 30
2.1 Getting started 30
Installing Clojure 31 ■ The Clojure REPL 31
Hello, world 32 ■ doc and find-doc 33 ■ A few more
points on Clojure syntax 34
2.2 Program structure 36
Functions 36 ■ The let form 37 ■ Side effects with do 39
try/catch/finally and throw 40 ■ Reader macros 41
2.3 Program flow 42
Conditionals 43 ■ Functional iteration 45
The threading macros 50
2.4 Clojure data structures 52
nil, truth, and falsehood 52 ■ Chars, strings, and numbers 52
Keywords and symbols 53 ■ Sequences 53
2.5 Summary 58
3 Building blocks of Clojure 60
3.1 Functions 61
Defining functions 61 ■ Calling functions 67
Higher-order functions 67 ■ Anonymous functions 70
Keywords and symbols 71
3.2 Scope 73
Vars and binding 74 ■ The let form revisited 78
Lexical closures 79
3.3 Namespaces 80
ns 80 ■ Working with namespaces 83
3.4 Destructuring 83
Vector bindings 84 ■ Map bindings 86
3.5 Metadata 88
3.6 Summary 89
4 Polymorphism with multimethods 90
4.1 Polymorphism 91
Subtype polymorphism 91 ■ Duck typing 92
4.2 Method dispatch 92
Single and double dispatch 93 ■ The visitor pattern
(and simulating double dispatch) 95 ■ Multiple dispatch 96
CONTENTS ix
4.3 Multimethods 97
Without multimethods 97 ■ Using multimethods 97
Multiple dispatch 99 ■ Ad hoc hierarchies 100
Redis-clojure 103
4.4 Summary 104
5 Clojure and Java interop 106
5.1 Calling Java from Clojure 107
Importing Java classes into Clojure 107 ■ Creating instances
and accessing methods and fields 108 ■ memfn 112
bean 113 ■ Arrays 113 ■ Implementing interfaces and
extending classes 114
5.2 Compiling Clojure code to Java byte code 115
Example–a tale of two calculators 115 ■ Creating Java classes
and interfaces using gen-class and gen-interface 117
5.3 Calling Clojure from Java 120
5.4 Summary 121
6 State and the concurrent world 122
6.1 The problem with state 123
Common problems with shared state 123
The traditional solution 124
6.2 Identities and values 125
Immutable values 126 ■ Objects and time 127
Immutability and concurrency 128
6.3 The Clojure way 129
Requirements for immutability 130 ■ Managed references 131
6.4 Refs 131
Mutating refs 132 ■ Software transactional memory 134
6.5 Agents 136
Mutating agents 136 ■ Working with agents 137
Side effects in STM transactions 139
6.6 Atoms 140
Mutating atoms 141
6.7 Vars 142
6.8 State and its unified access model 143
6.9 Watching for mutation 144