Odersky M. Programming in Scala
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List of Listings xxi
7.6 Finding .scala files using a for with a filter. . . . . . . . 156
7.7 Using multiple filters in a for expression. . . . . . . . . . 156
7.8 Using multiple generators in a for expression. . . . . . . . 157
7.9 Mid-stream assignment in a for expression. . . . . . . . . 158
7.10 Transforming an Array[File] to Array[Int] with a for. 159
7.11 A try-catch clause in Scala. . . . . . . . . . . . . . . . . 161
7.12 A try-finally clause in Scala. . . . . . . . . . . . . . . 162
7.13 A catch clause that yields a value. . . . . . . . . . . . . . 163
7.14 A match expression with side effects. . . . . . . . . . . . . 164
7.15 A match expression that yields a value. . . . . . . . . . . . 164
7.16 Looping without break or continue. . . . . . . . . . . . . 166
7.17 A recursive alternative to looping with vars. . . . . . . . . 166
7.18 Variable scoping when printing a multiplication table. . . . 168
7.19 A functional way to create a multiplication table. . . . . . . 171
8.1 LongLines with a private processLine method. . . . . . . 174
8.2 LongLines with a local processLine function. . . . . . . 176
9.1 Using closures to reduce code duplication. . . . . . . . . . 198
9.2 Defining and invoking a “plain old” function. . . . . . . . . 201
9.3 Defining and invoking a curried function. . . . . . . . . . . 201
9.4 Using the loan pattern to write to a file. . . . . . . . . . . . 205
9.5 Using a by-name parameter. . . . . . . . . . . . . . . . . . 206
10.1 Defining an abstract method and class. . . . . . . . . . . . 211
10.2 Defining parameterless methods width and height. . . . . 212
10.3 Defining ArrayElement as a subclass of Element. . . . . . 214
10.4 Overriding a parameterless method with a field. . . . . . . 216
10.5 Defining contents as a parametric field. . . . . . . . . . . 218
10.6 Invoking a superclass constructor. . . . . . . . . . . . . . . 219
10.7 Declaring a final method. . . . . . . . . . . . . . . . . . . 225
10.8 Declaring a final class. . . . . . . . . . . . . . . . . . . . . 225
10.9 Class Element with above, beside, and toString. . . . . 230
10.10 A factory object with factory methods. . . . . . . . . . . . 231
10.11 Class Element refactored to use factory methods. . . . . . 232
10.12 Hiding implementation with private classes. . . . . . . . . 233
10.13 Element with widen and heighten methods. . . . . . . . 234
10.14 The Spiral application. . . . . . . . . . . . . . . . . . . . 235
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List of Listings xxii
12.1 The definition of trait Philosophical. . . . . . . . . . . . 245
12.2 Mixing in a trait using extends. . . . . . . . . . . . . . . 246
12.3 Mixing in a trait using with. . . . . . . . . . . . . . . . . 247
12.4 Mixing in multiple traits. . . . . . . . . . . . . . . . . . . 247
12.5 Defining an enrichment trait. . . . . . . . . . . . . . . . . 251
12.6 Abstract class IntQueue. . . . . . . . . . . . . . . . . . . 255
12.7 A BasicIntQueue implemented with an ArrayBuffer. . . 255
12.8 The Doubling stackable modification trait. . . . . . . . . . 256
12.9 Mixing in a trait when instantiating with new. . . . . . . . . 257
12.10 Stackable modification traits Incrementing and Filtering. 257
13.1 Placing the contents of an entire file into a package. . . . . 265
13.2 Nesting multiple packages in the same file. . . . . . . . . . 265
13.3 Nesting packages with minimal indentation. . . . . . . . . 266
13.4 Scala packages truly nest. . . . . . . . . . . . . . . . . . . 267
13.5 Accessing hidden package names. . . . . . . . . . . . . . . 267
13.6 Bob’s delightful fruits, ready for import. . . . . . . . . . . 268
13.7 Importing the members of a regular (not singleton) object. . 269
13.8 Importing a package name. . . . . . . . . . . . . . . . . . 270
13.9 How private access differs in Scala and Java. . . . . . . . . 273
13.10 How protected access differs in Scala and Java. . . . . . . . 274
13.11 Flexible scope of protection with access qualifiers. . . . . . 275
13.12 Accessing private members of companion classes and objects. 277
14.1 Using an assertion. . . . . . . . . . . . . . . . . . . . . . . 280
14.2 Using ensuring to assert a function’s result. . . . . . . . . 280
14.3 Writing a test method with Suite. . . . . . . . . . . . . . 281
14.4 Writing a test function with FunSuite. . . . . . . . . . . . 282
14.5 Writing a JUnit test with JUnit3Suite. . . . . . . . . . . 285
14.6 Writing a TestNG test with TestNGSuite. . . . . . . . . . 286
14.7 Specifying and testing behavior with a ScalaTest Spec. . . 287
14.8 Specifying and testing behavior with the specs framework. . 288
14.9 Writing property-based tests with ScalaCheck. . . . . . . . 289
14.10 Checking properties from a JUnit TestCase with Checkers. 290
15.1 Defining case classes. . . . . . . . . . . . . . . . . . . . . 294
15.2 The simplifyTop function, which does a pattern match. . . 296
15.3 A pattern match with an empty “default” case. . . . . . . . 297
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15.4 A pattern match with wildcard patterns. . . . . . . . . . . . 298
15.5 A pattern match with constant patterns. . . . . . . . . . . . 299
15.6 A pattern match with a variable pattern. . . . . . . . . . . . 300
15.7 A pattern match with a constructor pattern. . . . . . . . . . 302
15.8 A sequence pattern with a fixed length. . . . . . . . . . . . 302
15.9 A sequence pattern with an arbitrary length. . . . . . . . . 303
15.10 A pattern match with a tuple pattern. . . . . . . . . . . . . 303
15.11 A pattern match with typed patterns. . . . . . . . . . . . . 303
15.12 Using isInstanceOf and asInstanceOf (poor style). . . . 305
15.13 A pattern with a variable binding (via the @ sign). . . . . . 307
15.14 A match expression with a pattern guard. . . . . . . . . . . 308
15.15 Match expression in which case order matters. . . . . . . . 309
15.16 A sealed hierarchy of case classes. . . . . . . . . . . . . . 311
15.17 Defining multiple variables with one assignment. . . . . . . 314
15.18 A for expression with a tuple pattern. . . . . . . . . . . . 318
15.19 Picking elements of a list that match a pattern. . . . . . . . 318
15.20 The top half of the expression formatter. . . . . . . . . . . 321
15.21 The bottom half of the expression formatter. . . . . . . . . 322
15.22 An application that prints formatted expressions. . . . . . . 325
16.1 A merge sort function for Lists. . . . . . . . . . . . . . . 342
17.1 Default map and set definitions in Predef. . . . . . . . . . 368
17.2 Mixing in the SynchronizedMap trait. . . . . . . . . . . . 376
18.1 A mutable bank account class. . . . . . . . . . . . . . . . . 388
18.2 A class with public vars. . . . . . . . . . . . . . . . . . . 390
18.3 How public vars are expanded into getter and setter methods. 391
18.4 Defining getter and setter methods directly. . . . . . . . . . 391
18.5 Defining a getter and setter without an associated field. . . 392
18.6 The halfAdder method. . . . . . . . . . . . . . . . . . . . 395
18.7 The fullAdder method. . . . . . . . . . . . . . . . . . . . 396
18.8 The Simulation class. . . . . . . . . . . . . . . . . . . . 398
18.9 The first half of the BasicCircuitSimulation class. . . . 402
18.10 The second half of the BasicCircuitSimulation class. . 403
18.11 The CircuitSimulation class. . . . . . . . . . . . . . . . 407
19.1 A basic functional queue. . . . . . . . . . . . . . . . . . . 413
19.2 Hiding a primary constructor by making it private. . . . . . 414
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19.3 An apply factory method in a companion object. . . . . . . 415
19.4 Type abstraction for functional queues. . . . . . . . . . . . 416
19.5 A nonvariant (rigid) Cell class. . . . . . . . . . . . . . . . 419
19.6 A type parameter with a lower bound. . . . . . . . . . . . . 425
19.7 A contravariant output channel. . . . . . . . . . . . . . . . 426
19.8 Covariance and contravariance of Function1s. . . . . . . . 427
19.9 Demonstration of function type parameter variance. . . . . 428
19.10 An optimized functional queue. . . . . . . . . . . . . . . . 430
19.11 A Person class that mixes in the Ordered trait. . . . . . . 432
19.12 A merge sort function with an upper bound. . . . . . . . . 432
20.1 Overriding abstract vals and parameterless methods. . . . 438
20.2 Declaring abstract vars. . . . . . . . . . . . . . . . . . . . 438
20.3 How abstract vars are expanded into getters and setters. . . 439
20.4 A trait that uses its abstract vals. . . . . . . . . . . . . . . 440
20.5 Pre-initialized fields in an anonymous class expression. . . 442
20.6 Pre-initialized fields in an object definition. . . . . . . . . . 442
20.7 Pre-initialized fields in a class definition. . . . . . . . . . . 443
20.8 Initializing a trait with lazy vals. . . . . . . . . . . . . . . 444
20.9 Modeling suitable food with an abstract type. . . . . . . . . 448
20.10 Implementing an abstract type in a subclass. . . . . . . . . 449
20.11 The US currency zone. . . . . . . . . . . . . . . . . . . . . 459
20.12 Currency zones for Europe and Japan. . . . . . . . . . . . 460
20.13 A converter object with an exchange rates map. . . . . . . 461
20.14 The full code of class CurrencyZone. . . . . . . . . . . . 462
21.1 An implicit parameter list with multiple parameters. . . . . 477
21.2 A function with an upper bound. . . . . . . . . . . . . . . 479
21.3 A function with an implicit parameter. . . . . . . . . . . . 480
21.4 A function that uses an implicit parameter internally. . . . . 482
21.5 A function with a view bound. . . . . . . . . . . . . . . . . 483
21.6 Sample code that uses an implicit parameter. . . . . . . . . 484
21.7 Sample code after type checking and insertion of implicits. 484
22.1 The definition of the Nil singleton object. . . . . . . . . . 489
22.2 Prepending a supertype element to a subtype list. . . . . . . 491
22.3 The definition of method :: (cons) in class List. . . . . . 491
22.4 The definition of method ::: in class List. . . . . . . . . 493
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22.5 The definition of method map in class List. . . . . . . . . 495
22.6 The definition of the :: subclass of List. . . . . . . . . . 496
24.1 The EMail string extractor object. . . . . . . . . . . . . . . 518
24.2 The Twice string extractor object. . . . . . . . . . . . . . . 521
24.3 The UpperCase string extractor object. . . . . . . . . . . . 521
24.4 The Domain string extractor object. . . . . . . . . . . . . . 523
24.5 The ExpandedEMail extractor object. . . . . . . . . . . . . 524
24.6 Hiding a primary constructor by making it private. . . . . . 525
24.7 How the r method is defined in RichString. . . . . . . . . 529
27.1 A simple Food entity class. . . . . . . . . . . . . . . . . . 555
27.2 Simple Recipe entity class. . . . . . . . . . . . . . . . . . 556
27.3 Food and Recipe examples for use in tests. . . . . . . . . . 556
27.4 Mock database and browser modules. . . . . . . . . . . . . 557
27.5 Database and browser modules with categories added. . . . 558
27.6 A Browser class with an abstract database val. . . . . . . 559
27.7 A Database class with abstract methods. . . . . . . . . . . 560
27.8 The SimpleDatabase object as a Database subclass. . . . 560
27.9 The SimpleBrowser object as a Browser subclass. . . . . 561
27.10 A student database and browser. . . . . . . . . . . . . . . . 561
27.11 A trait for food categories. . . . . . . . . . . . . . . . . . . 562
27.12 A Database class that mixes in the FoodCategories trait. 562
27.13 A SimpleDatabase object composed solely of mixins. . . 562
27.14 A SimpleFoods trait. . . . . . . . . . . . . . . . . . . . . 562
27.15 A SimpleRecipes trait with a self type. . . . . . . . . . . 563
27.16 An app that dynamically selects a module implementation. 564
27.17 Using a singleton type. . . . . . . . . . . . . . . . . . . . 566
28.1 A superclass equals method that calls canEqual. . . . . . 580
28.2 A subclass equals method that calls canEqual. . . . . . . 581
28.3 Hierarchy for binary trees. . . . . . . . . . . . . . . . . . . 583
28.4 A parameterized type with equals and hashCode. . . . . . 587
28.5 Class Rational with equals and hashCode. . . . . . . . . 588
29.1 A Scala method that declares a Java throws clause. . . . . 599
30.1 A simple actor. . . . . . . . . . . . . . . . . . . . . . . . . 609
30.2 An actor that calls receive. . . . . . . . . . . . . . . . . . 611
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30.3 An actor that calls react. . . . . . . . . . . . . . . . . . . 615
30.4 An actor’s act method that uses loop. . . . . . . . . . . . 616
30.5 An actor that uses a helper actor to avoid blocking itself. . . 618
30.6 An actor that uses case classes for messages. . . . . . . . . 623
30.7 The Simulant trait. . . . . . . . . . . . . . . . . . . . . . 631
30.8 Adder components. . . . . . . . . . . . . . . . . . . . . . 638
31.1 An arithmetic expression parser. . . . . . . . . . . . . . . . 644
31.2 A regular expression parser for Java identifiers. . . . . . . . 646
31.3 Data in JSON format. . . . . . . . . . . . . . . . . . . . . 648
31.4 A simple JSON parser. . . . . . . . . . . . . . . . . . . . . 648
31.5 A full JSON parser that returns meaningful results. . . . . . 653
31.6 The ~ combinator method. . . . . . . . . . . . . . . . . . . 661
32.1 A simple Swing application in Scala. . . . . . . . . . . . . 672
32.2 Component assembly on a panel. . . . . . . . . . . . . . . 674
32.3 Implementing a reactive Swing application. . . . . . . . . . 678
32.4 An implementation of the temperature converter. . . . . . . 680
33.1 Code for spreadsheet in Figure 33.1. . . . . . . . . . . . . 685
33.2 The main program for the spreadsheet application. . . . . . 686
33.3 A spreadsheet with a rendererComponent method. . . . . 687
33.4 First version of the Model class. . . . . . . . . . . . . . . . 688
33.5 Classes representing formulas. . . . . . . . . . . . . . . . 690
33.6 A spreadsheet that parses formulas. . . . . . . . . . . . . . 694
33.7 The evaluate method of trait Evaluator. . . . . . . . . . 697
33.8 A library for arithmetic operations. . . . . . . . . . . . . . 699
33.9 The finished spreadsheet component. . . . . . . . . . . . . 705
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Foreword
Martin Odersky made a huge impact on the Java world with his design of the
Pizza language. Although Pizza itself never became popular, it demonstrated
that object-oriented and functional language features, when combined with
skill and taste, form a natural and powerful combination. Pizza’s design be-
came the basis for generics in Java, and Martin’s GJ (Generic Java) compiler
was Sun Microsystem’s standard compiler starting in 1.3 (though with gener-
ics disabled). I had the pleasure of maintaining this compiler for a number
of years, so I can report from first-hand experience that Martin’s skill in lan-
guage design extends to language implementation.
Since that time, we at Sun tried to simplify program development by ex-
tending the language with piecemeal solutions to particular problems, like
the for-each loop, enums, and autoboxing. Meanwhile, Martin continued his
work on more powerful orthogonal language primitives that allow program-
mers to provide solutions in libraries.
Lately, there has been a backlash against statically typed languages. Ex-
perience with Java has shown that programming in a static language results
in an abundance of boilerplate. The common wisdom is that one must aban-
don static typing to eliminate the boilerplate, and there is a rising interest
in dynamic languages such as Python, Ruby, and Groovy. This common
wisdom is debunked by the existence of Martin’s latest brainchild, Scala.
Scala is a tastefully typed language: it is statically typed, but explicit
types appear in just the right places. Scala takes powerful features from
object-oriented and functional languages, and combines them with a few
novel ideas in a beautifully coherent whole. The syntax is so lightweight,
and its primitives so expressive, that APIs can be used with virtually no syn-
tactic overhead at all. Examples can be found in standard libraries such
as parser combinators and actors. In this sense Scala supports embedded
domain-specific languages.
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Foreword xxviii
Will Scala be the next great language? Only time will tell. Martin Oder-
sky’s team certainly has the taste and skill for the job. One thing is sure:
Scala sets a new standard against which future languages will be measured.
Neal Gafter
San Jose, California
September 3, 2008
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Acknowledgments
Many people have contributed to this book and to the material it covers. We
are grateful to all of them.
Scala itself has been a collective effort of many people. The design and
the implementation of version 1.0 was helped by Philippe Altherr, Vincent
Cremet, Gilles Dubochet, Burak Emir, Stéphane Micheloud, Nikolay Mi-
haylov, Michel Schinz, Erik Stenman, and Matthias Zenger. Iulian Dragos,
Gilles Dubochet, Philipp Haller, Sean McDirmid, Ingo Maier, and Adriaan
Moors joined in the effort to develop the second and current version of the
language and tools.
Gilad Bracha, Craig Chambers, Erik Ernst, Matthias Felleisen, Shriram
Krishnamurti, Gary Leavens, Sebastian Maneth, Erik Meijer, David Pol-
lak, Jon Pretty, Klaus Ostermann, Didier Rémy, Vijay Saraswat, Don Syme,
Mads Torgersen, Philip Wadler, Jamie Webb, and John Williams have shaped
the design of the language by graciously sharing their ideas with us in lively
and inspiring discussions, as well as through comments on previous versions
of this document. The contributors to the Scala mailing list have also given
very useful feedback that helped us improve the language and its tools.
George Berger has worked tremendously to make the build process and
the web presence for the book work smoothly. As a result this project has
been delightfully free of technical snafus.
Many people gave us valuable feedback on early versions of the text.
Thanks goes to Eric Armstrong, George Berger, Alex Blewitt, Gilad Bracha,
William Cook, Bruce Eckel, Stéphane Micheloud, Todd Millstein, David
Pollak, Frank Sommers, Philip Wadler, and Matthias Zenger. Thanks also to
the Silicon Valley Patterns group for their very helpful review: Dave Astels,
Tracy Bialik, John Brewer, Andrew Chase, Bradford Cross, Raoul Duke,
John P. Eurich, Steven Ganz, Phil Goodwin, Ralph Jocham, Yan-Fa Li, Tao
Ma, Jeffery Miller, Suresh Pai, Russ Rufer, Dave W. Smith, Scott Turnquest,
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Acknowledgments xxx
Walter Vannini, Darlene Wallach, and Jonathan Andrew Wolter. And we’d
like to thank Dewayne Johnson and Kim Leedy for their help with the cover
art, and Frank Sommers for his work on the index.
We’d also like to extend a special thanks to all of our readers who con-
tributed comments. Your comments were very helpful to us in shaping this
into an even better book. We couldn’t print the names of everyone who con-
tributed comments, but here are the names of readers who submitted at least
five comments during the eBook PrePrint™ stage by clicking on the Suggest
link, sorted first by the highest total number of comments submitted, then
alphabetically. Thanks goes to: David Biesack, Donn Stephan, Mats Hen-
ricson, Rob Dickens, Blair Zajac, Tony Sloane, Nigel Harrison, Javier Diaz
Soto, William Heelan, Justin Forder, Gregor Purdy, Colin Perkins, Bjarte
S. Karlsen, Ervin Varga, Eric Willigers, Mark Hayes, Martin Elwin, Calum
MacLean, Jonathan Wolter, Les Pruszynski, Seth Tisue, Andrei Formiga,
Dmitry Grigoriev, George Berger, Howard Lovatt, John P. Eurich, Marius
Scurtescu, Jeff Ervin, Jamie Webb, Kurt Zoglmann, Dean Wampler, Nikolaj
Lindberg, Peter McLain, Arkadiusz Stryjski, Shanky Surana, Craig Borde-
lon, Alexandre Patry, Filip Moens, Fred Janon, Jeff Heon, Boris Lorbeer,
Jim Menard, Tim Azzopardi, Thomas Jung, Walter Chang, Jeroen Dijkmei-
jer, Casey Bowman, Martin Smith, Richard Dallaway, Antony Stubbs, Lars
Westergren, Maarten Hazewinkel, Matt Russell, Remigiusz Michalowski,
Andrew Tolopko, Curtis Stanford, Joshua Cough, Zemian Deng, Christo-
pher Rodrigues Macias, Juan Miguel Garcia Lopez, Michel Schinz, Peter
Moore, Randolph Kahle, Vladimir Kelman, Daniel Gronau, Dirk Detering,
Hiroaki Nakamura, Ole Hougaard, Bhaskar Maddala, David Bernard, Derek
Mahar, George Kollias, Kristian Nordal, Normen Mueller, Rafael Ferreira,
Binil Thomas, John Nilsson, Jorge Ortiz, Marcus Schulte, Vadim Gerassi-
mov, Cameron Taggart, Jon-Anders Teigen, Silvestre Zabala, Will McQueen,
and Sam Owen.
Lastly, Bill would also like to thank Gary Cornell, Greg Doench, Andy
Hunt, Mike Leonard, Tyler Ortman, Bill Pollock, Dave Thomas, and Adam
Wright for providing insight and advice on book publishing.
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