Odersky M. Programming in Scala
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Cover · Overview · Contents · Discuss · Suggest · Glossary · Index
Programming in Scala
Cover · Overview · Contents · Discuss · Suggest · Glossary · Index
Programming in Scala
Martin Odersky, Lex Spoon, Bill Venners
artima
ARTIMA PRESS
MOUNTAIN VIEW, CALIFORNIA
Cover · Overview · Contents · Discuss · Suggest · Glossary · Index
iv
Programming in Scala
First Edition, Version 6
Martin Odersky is the creator of the Scala language and a professor at EPFL in
Lausanne, Switzerland. Lex Spoon worked on Scala for two years as a post-doc
with Martin Odersky. Bill Venners is president of Artima, Inc.
Artima Press is an imprint of Artima, Inc.
P.O. Box 390122, Mountain View, California 94039
Copyright © 2007, 2008 Martin Odersky, Lex Spoon, and Bill Venners.
All rights reserved.
First edition published as PrePrint™ eBook 2007
First edition published 2008
Produced in the United States of America
12 11 10 09 08 5 6 7 8 9
ISBN-10: 0-9815316-1-X
ISBN-13: 978-0-9815316-1-8
No part of this publication may be reproduced, modified, distributed, stored in a
retrieval system, republished, displayed, or performed, for commercial or
noncommercial purposes or for compensation of any kind without prior written
permission from Artima, Inc.
All information and materials in this book are provided "as is" and without
warranty of any kind.
The term “Artima” and the Artima logo are trademarks or registered trademarks of
Artima, Inc. All other company and/or product names may be trademarks or
registered trademarks of their owners.
Cover · Overview · Contents · Discuss · Suggest · Glossary · Index
to Nastaran - M.O.
to Fay - L.S.
to Siew - B.V.
Cover · Overview · Contents · Discuss · Suggest · Glossary · Index
Overview
Contents vii
List of Figures xvii
List of Tables xix
List of Listings xx
Foreword xxvii
Acknowledgments xxix
Introduction xxxi
1. A Scalable Language 39
2. First Steps in Scala 58
3. Next Steps in Scala 71
4. Classes and Objects 93
5. Basic Types and Operations 107
6. Functional Objects 129
7. Built-in Control Structures 149
8. Functions and Closures 173
9. Control Abstraction 194
10. Composition and Inheritance 209
11. Scala’s Hierarchy 237
12. Traits 245
13. Packages and Imports 264
14. Assertions and Unit Testing 279
15. Case Classes and Pattern Matching 293
16. Working with Lists 327
17. Collections 359
18. Stateful Objects 387
19. Type Parameterization 410
20. Abstract Members 435
21. Implicit Conversions and Parameters 465
22. Implementing Lists 487
23. For Expressions Revisited 500
24. Extractors 516
25. Annotations 532
26. Working with XML 539
27. Modular Programming Using Objects 553
28. Object Equality 568
29. Combining Scala and Java 594
30. Actors and Concurrency 607
31. Combinator Parsing 642
32. GUI Programming 671
33. The SCells Spreadsheet 683
A. Scala scripts on Unix and Windows 708
Glossary 709
Bibliography 725
About the Authors 728
Index 729
Cover · Overview · Contents · Discuss · Suggest · Glossary · Index
Contents
Contents vii
List of Figures xvii
List of Tables xix
List of Listings xx
Foreword xxvii
Acknowledgments xxix
Introduction xxxi
1 A Scalable Language 39
1.1 A language that grows on you . . . . . . . . . . . . . . 40
1.2 What makes Scala scalable? . . . . . . . . . . . . . . . 45
1.3 Why Scala? . . . . . . . . . . . . . . . . . . . . . . . . 48
1.4 Scala’s roots . . . . . . . . . . . . . . . . . . . . . . . 55
1.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . 57
2 First Steps in Scala 58
Step 1. Learn to use the Scala interpreter . . . . . . . . . . . . 58
Step 2. Define some variables . . . . . . . . . . . . . . . . . 60
Step 3. Define some functions . . . . . . . . . . . . . . . . . 62
Step 4. Write some Scala scripts . . . . . . . . . . . . . . . . 64
Step 5. Loop with while; decide with if . . . . . . . . . . . 65
Step 6. Iterate with foreach and for . . . . . . . . . . . . . 67
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
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Contents viii
3 Next Steps in Scala 71
Step 7. Parameterize arrays with types . . . . . . . . . . . . 71
Step 8. Use lists . . . . . . . . . . . . . . . . . . . . . . . . 75
Step 9. Use tuples . . . . . . . . . . . . . . . . . . . . . . . 80
Step 10. Use sets and maps . . . . . . . . . . . . . . . . . . . 81
Step 11. Learn to recognize the functional style . . . . . . . . 86
Step 12. Read lines from a file . . . . . . . . . . . . . . . . . 89
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
4 Classes and Objects 93
4.1 Classes, fields, and methods . . . . . . . . . . . . . . . 93
4.2 Semicolon inference . . . . . . . . . . . . . . . . . . . 98
4.3 Singleton objects . . . . . . . . . . . . . . . . . . . . . 99
4.4 A Scala application . . . . . . . . . . . . . . . . . . . 102
4.5 The Application trait . . . . . . . . . . . . . . . . . . 105
4.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . 106
5 Basic Types and Operations 107
5.1 Some basic types . . . . . . . . . . . . . . . . . . . . . 107
5.2 Literals . . . . . . . . . . . . . . . . . . . . . . . . . . 108
5.3 Operators are methods . . . . . . . . . . . . . . . . . . 115
5.4 Arithmetic operations . . . . . . . . . . . . . . . . . . 118
5.5 Relational and logical operations . . . . . . . . . . . . 119
5.6 Bitwise operations . . . . . . . . . . . . . . . . . . . . 121
5.7 Object equality . . . . . . . . . . . . . . . . . . . . . . 123
5.8 Operator precedence and associativity . . . . . . . . . . 124
5.9 Rich wrappers . . . . . . . . . . . . . . . . . . . . . . 127
5.10 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . 127
6 Functional Objects 129
6.1 A specification for class Rational . . . . . . . . . . . 129
6.2 Constructing a Rational . . . . . . . . . . . . . . . . 130
6.3 Reimplementing the toString method . . . . . . . . . 132
6.4 Checking preconditions . . . . . . . . . . . . . . . . . 133
6.5 Adding fields . . . . . . . . . . . . . . . . . . . . . . . 133
6.6 Self references . . . . . . . . . . . . . . . . . . . . . . 135
6.7 Auxiliary constructors . . . . . . . . . . . . . . . . . . 136
6.8 Private fields and methods . . . . . . . . . . . . . . . . 138
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Contents ix
6.9 Defining operators . . . . . . . . . . . . . . . . . . . . 139
6.10 Identifiers in Scala . . . . . . . . . . . . . . . . . . . . 141
6.11 Method overloading . . . . . . . . . . . . . . . . . . . 144
6.12 Implicit conversions . . . . . . . . . . . . . . . . . . . 146
6.13 A word of caution . . . . . . . . . . . . . . . . . . . . 147
6.14 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . 147
7 Built-in Control Structures 149
7.1 If expressions . . . . . . . . . . . . . . . . . . . . . . 150
7.2 While loops . . . . . . . . . . . . . . . . . . . . . . . 151
7.3 For expressions . . . . . . . . . . . . . . . . . . . . . . 154
7.4 Exception handling with try expressions . . . . . . . . 159
7.5 Match expressions . . . . . . . . . . . . . . . . . . . . 163
7.6 Living without break and continue . . . . . . . . . . 165
7.7 Variable scope . . . . . . . . . . . . . . . . . . . . . . 167
7.8 Refactoring imperative-style code . . . . . . . . . . . . 170
7.9 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . 172
8 Functions and Closures 173
8.1 Methods . . . . . . . . . . . . . . . . . . . . . . . . . 173
8.2 Local functions . . . . . . . . . . . . . . . . . . . . . . 175
8.3 First-class functions . . . . . . . . . . . . . . . . . . . 177
8.4 Short forms of function literals . . . . . . . . . . . . . 179
8.5 Placeholder syntax . . . . . . . . . . . . . . . . . . . . 180
8.6 Partially applied functions . . . . . . . . . . . . . . . . 181
8.7 Closures . . . . . . . . . . . . . . . . . . . . . . . . . 184
8.8 Repeated parameters . . . . . . . . . . . . . . . . . . . 188
8.9 Tail recursion . . . . . . . . . . . . . . . . . . . . . . . 189
8.10 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . 193
9 Control Abstraction 194
9.1 Reducing code duplication . . . . . . . . . . . . . . . . 194
9.2 Simplifying client code . . . . . . . . . . . . . . . . . 198
9.3 Currying . . . . . . . . . . . . . . . . . . . . . . . . . 200
9.4 Writing new control structures . . . . . . . . . . . . . . 202
9.5 By-name parameters . . . . . . . . . . . . . . . . . . . 205
9.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . 208
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Contents x
10 Composition and Inheritance 209
10.1 A two-dimensional layout library . . . . . . . . . . . . 209
10.2 Abstract classes . . . . . . . . . . . . . . . . . . . . . 210
10.3 Defining parameterless methods . . . . . . . . . . . . . 211
10.4 Extending classes . . . . . . . . . . . . . . . . . . . . 214
10.5 Overriding methods and fields . . . . . . . . . . . . . . 216
10.6 Defining parametric fields . . . . . . . . . . . . . . . . 217
10.7 Invoking superclass constructors . . . . . . . . . . . . . 219
10.8 Using override modifiers . . . . . . . . . . . . . . . . 220
10.9 Polymorphism and dynamic binding . . . . . . . . . . 222
10.10 Declaring final members . . . . . . . . . . . . . . . . . 224
10.11 Using composition and inheritance . . . . . . . . . . . 226
10.12 Implementing above, beside, and toString . . . . . . 227
10.13 Defining a factory object . . . . . . . . . . . . . . . . . 229
10.14 Heighten and widen . . . . . . . . . . . . . . . . . . . 231
10.15 Putting it all together . . . . . . . . . . . . . . . . . . . 235
10.16 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . 236
11 Scala’s Hierarchy 237
11.1 Scala’s class hierarchy . . . . . . . . . . . . . . . . . . 237
11.2 How primitives are implemented . . . . . . . . . . . . 241
11.3 Bottom types . . . . . . . . . . . . . . . . . . . . . . . 243
11.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . 244
12 Traits 245
12.1 How traits work . . . . . . . . . . . . . . . . . . . . . 245
12.2 Thin versus rich interfaces . . . . . . . . . . . . . . . . 248
12.3 Example: Rectangular objects . . . . . . . . . . . . . . 249
12.4 The Ordered trait . . . . . . . . . . . . . . . . . . . . 252
12.5 Traits as stackable modifications . . . . . . . . . . . . . 254
12.6 Why not multiple inheritance? . . . . . . . . . . . . . . 258
12.7 To trait, or not to trait? . . . . . . . . . . . . . . . . . . 262
12.8 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . 263
13 Packages and Imports 264
13.1 Packages . . . . . . . . . . . . . . . . . . . . . . . . . 264
13.2 Imports . . . . . . . . . . . . . . . . . . . . . . . . . . 268
13.3 Implicit imports . . . . . . . . . . . . . . . . . . . . . 272
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