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Scala in Depth IN DEPTH Joshua D. Suereth FOREWORD BY Martin Odersky MANNING www.finebook.ir Scala in Depth www.finebook.ir www.finebook.ir Scala in Depth JOSHUA D. SUERETH MANNING SHELTER ISLAND www.finebook.ir For online information and ordering of this and other Manning books, please visit www.manning.com. The publisher offers discounts on this book when ordered in quantity. For more information, please contact Special Sales Department Manning Publications Co. 20 Baldwin Road PO Box 261 Shelter Island, NY 11964 Email: [email protected] ©2012 by Manning Publications Co. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by means electronic, mechanical, photocopying, or otherwise, without prior written permission of the publisher. Many of the designations used by manufacturers and sellers to distinguish their products are claimed as trademarks. Where those designations appear in the book, and Manning Publications was aware of a trademark claim, the designations have been printed in initial caps or all caps. Recognizing the importance of preserving what has been written, it is Manning’s policy to have the books we publish printed on acid-free paper, and we exert our best efforts to that end. Recognizing also our responsibility to conserve the resources of our planet, Manning books are printed on paper that is at least 15 percent recycled and processed without the use of elemental chlorine. Manning Publications Co. Development editor: Katharine Osborne 20 Baldwin Road Technical proofreader: Justin Wick PO Box 261 Copyeditors: Linda Kern, Benjamin Berg Shelter Island, NY 11964 Proofreader: Elizabeth Martin Typesetter:Dottie Marsico Cover designer: Marija Tudor ISBN 9781935182702 Printed in the United States of America 1 2 3 4 5 6 7 8 9 10 – MAL – 17 16 15 14 13 12 www.finebook.ir contents foreword xi preface xiii acknowledgments xiv about this book xvi about the cover illustration xix Scala—a blended language 1 1 1.1 Functional programming meets object orientation 2 Discovering existing functional concepts 4 ■ Examining functional concepts in Google Collections 6 1.2 Static typing and expressiveness 8 Changing sides 8 ■ Type inference 9 ■ Dropping verbose syntax 9 ■ Implicits are an old concept 10 ■ Using Scala’s implicit keyword 11 1.3 Transparently working with the JVM 12 Java in Scala 12 ■ Scala in Java 13 ■ The benefits of a JVM 14 1.4 Summary 15 v www.finebook.ir vi CONTENTS The core rules 16 2 2.1 Learn to use the Read Eval Print Loop (REPL) 16 Experiment-driven development 18 ■ Working around eager parsing 19 ■ Inexpressible language features 20 2.2 Think in expressions 21 Don’t use return 22 ■ Mutability 24 2.3 Prefer immutability 26 Object equality 27 ■ Concurrency 31 2.4 Use None instead of null 34 Advanced Option techniques 35 2.5 Polymorphic equality 38 Example: A timeline library 38 ■ Polymorphic equals implementation 40 2.6 Summary 42 Modicum of style—coding conventions 43 3 3.1 Avoid coding conventions from other languages 44 The block debacle 45 3.2 Dangling operators and parenthetical expressions 48 3.3 Use meaningful variable names 49 Avoid $ in names 50 ■ Working with named and default parameters 53 3.4 Always mark overridden methods 55 3.5 Annotate for expected optimizations 60 Using the tableswitch optimization 61 ■ Using the tail recursion optimization 64 3.6 Summary 66 Utilizing object orientation 68 4 4.1 Limit code inside an object or trait’s body to initialization logic 69 Delayed construction 69 ■ And then there’s multiple inheritance 70 4.2 Provide empty implementations for abstract methods on traits 72 www.finebook.ir CONTENTS vii 4.3 Composition can include inheritance 76 Member composition by inheritance 78 ■ Classic constructors with a twist 80 4.4 Promote abstract interface into its own trait 82 Interfaces you can talk to 84 ■ Learning from the past 85 4.5 Provide return types in your public APIs 86 4.6 Summary 88 Using implicits to write expressive code 89 5 5.1 Introduction to implicits 90 Identifiers: A digression 91 ■ Scope and bindings 92 Implicit resolution 96 5.2 Enhancing existing classes with implicit views 101 5.3 Utilize implicit parameters with defaults 106 5.4 Limiting the scope of implicits 112 Creating implicits for import 113 ■ Implicits without the import tax 115 5.5 Summary 119 The type system 120 6 6.1 Types 121 Types and paths 122 ■ The type keyword 124 ■ Structural types 125 6.2 Type constraints 131 6.3 Type parameters and higher-kinded types 134 Type parameter constraints 134 ■ Higher-kinded types 135 6.4 Variance 137 Advanced variance annotations 141 6.5 Existential types 144 The formal syntax of existential types 146 6.6 Summary 149 Using implicits and types together 150 7 7.1 Context bounds and view bounds 151 When to use implicit type constraints 152 www.finebook.ir viii CONTENTS 7.2 Capturing types with implicits 153 Manifests 153 ■ Using Manifests 154 ■ Capturing type constraints 156 ■ Specialized methods 158 7.3 Use type classes 159 FileLike as a type class 163 ■ The benefits of type classes 166 7.4 Conditional execution using the type system 167 Heterogeneous typed list 169 ■ IndexedView 172 7.5 Summary 178 Using the right collection 179 8 8.1 Use the right collection 180 The collection hierarchy 180 ■ Traversable 182 Iterable 185 ■ Seq 187 ■ LinearSeq 187 IndexedSeq 189 ■ Set 190 ■ Map 191 8.2 Immutable collections 192 Vector 192 ■ List 194 ■ Stream 195 8.3 Mutable collections 198 ArrayBuffer 198 ■ Mixin mutation event publishing 199 ■ Mixin synchronization 200 8.4 Changing evaluation with views and parallel collections 200 Views 201 ■ Parallel collections 203 8.5 Writing methods to use with all collection types 205 Optimizing algorithms for each collections type 209 8.6 Summary 211 Actors 212 9 9.1 Know when to use actors 213 Using actors to search 213 9.2 Use typed, transparent references 216 Scatter-Gather with OutputChannel 217 9.3 Limit failures to zones 221 Scatter-Gather failure zones 221 ■ General failure handling practices 224 9.4 Limit overload using scheduler zones 225 Scheduling zones 227 www.finebook.ir CONTENTS ix 9.5 Dynamic actor topology 228 9.6 Summary 233 Integrating Scala with Java 234 10 10.1 The language mismatch between Scala and Java 235 Differences in primitive boxing 236 ■ Differences in visibility 240 ■ Inexpressible language features 241 10.2 Be wary of implicit conversions 244 Object identity and equality 245 ■ Chaining implicits 246 10.3 Be wary of Java serialization 248 Serializing anonymous classes 250 10.4 Annotate your annotations 252 Annotation targets 254 ■ Scala and static fields 255 10.5 Summary 256 Patterns in functional programming 257 11 11.1 Category theory for computer science 258 11.2 Functors and monads, and how they relate to categories 262 Monads 264 11.3 Currying and applicative style 266 Currying 266 ■ Applicative style 268 11.4 Monads as workflows 272 11.5 Summary 276 index 277 www.finebook.ir www.finebook.ir foreword Joshua Suereth is one of the most complete programmers I know. Familiar with a whole gamut of programming languages and techniques, he is an expert in high- performance systems, build tools, type theory, and many other areas. He is also a gifted teacher, and all that combined is what makes Scala in Depth special. This book provides in-depth coverage of several of the more intricate areas of Scala, including advanced aspects of its type system, implicits, composition techniques with traits, collections, actors, functional categories. But this is not a dry recollection of language and library concepts. The book is full of practical advice on how to apply these lesser known parts of Scala in useful ways, and what the best practices are. The explanations and examples demonstrate Joshua’s great experience constructing large- scale systems in Scala. Scala in Depth is not a beginner’s introduction; it should primarily appeal to compe- tent Scala programmers who want to become experts. The techniques that are taught are handy for constructing flexible and type-safe library abstractions. Many of these techniques were folklore until now; they have been, for the first time, written-up here. I am particularly happy about one other thing: The book fills a gap in that it explains key parts of the formal Scala specification to programmers who are not language law- yers. Scala is one of few languages that actually has a specification. That specification consists mainly of definitions written in highly stylized prose and mathematical formu- las; so it’s not everybody’s piece of cake. Joshua’s book manages to be both authorative and understandable as it explains these concepts. MARTIN ODERSKY CREATOR OF SCALA HEAD OF PROGRAMMING RESEARCH GROUP, EPFL xi www.finebook.ir www.finebook.ir preface In fall 2010 Michael Stephens from Manning contacted me about writing a Scala Book. I was working for a small virtualization/security startup where I had been learning Scala and applying it to our codebase. During that first conversation Michael and I dis- cussed the Scala ecosystem and what kind of a book would best serve the community. I believed Scala needed a “practical Scala” book to help guide those new to the lan- guage. Scala is a beautiful language, but it brings many new concepts to the table. I had watched as the community slowly discovered best practices and a code style that was wholly “Scala.” But I wasn’t sure whether I was the right person to write such a book. When it came down to it, I was passionate about the topic, had enough free time to do the research, and had the support of the magnates of the community to help achieve what you are reading today—so I decided to go ahead.
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