www.it-ebooks.info www.it-ebooks.info Early Praise for Programming Erlang, Second Edition This second edition of Joe’s seminal Programming Erlang is a welcome update, covering not only the core language and framework fundamentals but also key community projects such as rebar and cowboy. Even experienced Erlang program- mers will find helpful tips and new insights throughout the book, and beginners to the language will appreciate the clear and methodical way Joe introduces and explains key language concepts. ➤ Alexander Gounares Former AOL CTO, advisor to Bill Gates, and founder/CEO of Concurix Corp. A gem; a sensible, practical introduction to functional programming. ➤ Gilad Bracha Coauthor of the Java language and Java Virtual Machine specifications, creator of the Newspeak language, member of the Dart language team Programming Erlang is an excellent resource for understanding how to program with Actors. It’s not just for Erlang developers, but for anyone who wants to understand why Actors matters and why they are such an important tool in building reactive, scalable, resilient, and event-driven systems. ➤ Jonas Bonér Creator of the Akka Project and the AspectWerkz Aspect-Oriented Programming (AOP) framework, co-founder and CTO of Typesafe www.it-ebooks.info Programming Erlang, Second Edition Software for a Concurrent World Joe Armstrong The Pragmatic Bookshelf Dallas, Texas • Raleigh, North Carolina www.it-ebooks.info 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: Susannah Davidson Pfalzer (editor) Potomac Indexing, LLC (indexer) Kim Wimpsett (copyeditor) David J Kelly (typesetter) Janet Furlow (producer) Juliet Benda (rights) Ellie Callahan (support) Copyright © 2013 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: 978-1-937785-53-6 Encoded using the finest acid-free high-entropy binary digits. Book version: P1.0—August 2013 www.it-ebooks.info Contents Introduction . xiii Part I — Why Erlang? 1. Introducing Concurrency . 3 1.1 Modeling Concurrency 3 1.2 Benefits of Concurrency 6 1.3 Concurrent Programs and Parallel Computers 8 1.4 Sequential vs. Concurrent Programming Languages 9 2. A Whirlwind Tour of Erlang . 11 2.1 The Shell 11 2.2 Processes, Modules, and Compilation 13 2.3 Hello, Concurrency 15 Part II — Sequential Programming 3. Basic Concepts . 25 3.1 Starting and Stopping the Erlang Shell 25 3.2 Simple Integer Arithmetic 27 3.3 Variables 28 3.4 Floating-Point Numbers 32 3.5 Atoms 33 3.6 Tuples 34 3.7 Lists 37 3.8 Strings 39 3.9 Pattern Matching Again 41 4. Modules and Functions . 43 4.1 Modules Are Where We Store Code 43 4.2 Back to Shopping 50 www.it-ebooks.info Contents • vi 4.3 Funs: The Basic Unit of Abstraction 52 4.4 Simple List Processing 57 4.5 List Comprehensions 59 4.6 BIFs 63 4.7 Guards 64 4.8 case and if Expressions 68 4.9 Building Lists in Natural Order 70 4.10 Accumulators 71 5. Records and Maps . 75 5.1 When to Use Maps or Records 75 5.2 Naming Tuple Items with Records 76 5.3 Maps: Associative Key-Value Stores 79 6. Error Handling in Sequential Programs . 87 6.1 Handling Errors in Sequential Code 88 6.2 Trapping an Exception with try…catch 89 6.3 Trapping an Exception with catch 92 6.4 Programming Style with Exceptions 93 6.5 Stack Traces 95 6.6 Fail Fast and Noisily, Fail Politely 96 7. Binaries and the Bit Syntax . 99 7.1 Binaries 99 7.2 The Bit Syntax 101 7.3 Bitstrings: Processing Bit-Level Data 110 8. The Rest of Sequential Erlang . 113 8.1 apply 115 8.2 Arithmetic Expressions 116 8.3 Arity 116 8.4 Attributes 117 8.5 Block Expressions 120 8.6 Booleans 120 8.7 Boolean Expressions 121 8.8 Character Set 122 8.9 Comments 122 8.10 Dynamic Code Loading 122 8.11 Erlang Preprocessor 126 8.12 Escape Sequences 126 8.13 Expressions and Expression Sequences 127 www.it-ebooks.info Contents • vii 8.14 Function References 128 8.15 Include Files 128 8.16 List Operations ++ and - - 129 8.17 Macros 129 8.18 Match Operator in Patterns 131 8.19 Numbers 132 8.20 Operator Precedence 133 8.21 The Process Dictionary 134 8.22 References 135 8.23 Short-Circuit Boolean Expressions 135 8.24 Term Comparisons 136 8.25 Tuple Modules 137 8.26 Underscore Variables 137 9. Types . 141 9.1 Specifying Data and Function Types 141 9.2 Erlang Type Notation 143 9.3 A Session with the Dialyzer 148 9.4 Type Inference and Success Typing 152 9.5 Limitations of the Type System 155 10. Compiling and Running Your Program . 159 10.1 Modifying the Development Environment 159 10.2 Different Ways to Run Your Program 161 10.3 Automating Compilation with Makefiles 166 10.4 When Things Go Wrong 169 10.5 Getting Help 172 10.6 Tweaking the Environment 173 Part III — Concurrent and Distributed Programs 11. Real-World Concurrency . 177 12. Concurrent Programming . 181 12.1 The Concurrency Primitives 182 12.2 Introducing Client-Server 184 12.3 Processes Are Cheap 189 12.4 Receive with a Timeout 191 12.5 Selective Receive 193 12.6 Registered Processes 194 www.it-ebooks.info Contents • viii 12.7 A Word About Tail Recursion 196 12.8 Spawning with MFAs or Funs 197 13. Errors in Concurrent Programs . 199 13.1 Error Handling Philosophy 199 13.2 Error Handling Semantics 202 13.3 Creating Links 203 13.4 Groups of Processes That All Die Together 204 13.5 Setting Up a Firewall 205 13.6 Monitors 205 13.7 Error Handling Primitives 206 13.8 Programming for Fault Tolerance 207 14. Distributed Programming . 211 14.1 Two Models for Distribution 212 14.2 Writing a Distributed Program 213 14.3 Building the Name Server 213 14.4 Libraries and BIFS for Distributed Programming 219 14.5 The Cookie Protection System 222 14.6 Socket-Based Distribution 224 Part IV — Programming Libraries and Frameworks 15. Interfacing Techniques . 231 15.1 How Erlang Communicates with External Programs 232 15.2 Interfacing an External C Program with a Port 234 15.3 Calling a Shell Script from Erlang 240 15.4 Advanced Interfacing Techniques 240 16. Programming with Files . 243 16.1 Modules for Manipulating Files 243 16.2 Ways to Read a File 244 16.3 Ways to Write a File 251 16.4 Directory and File Operations 255 16.5 Bits and Pieces 258 16.6 A Find Utility 258 17. Programming with Sockets . 263 17.1 Using TCP 263 17.2 Active and Passive Sockets 272 17.3 Error Handling with Sockets 275 www.it-ebooks.info Contents • ix 17.4 UDP 276 17.5 Broadcasting to Multiple Machines 280 17.6 A SHOUTcast Server 281 18. Browsing with Websockets and Erlang . 287 18.1 Creating a Digital Clock 288 18.2 Basic Interaction 291 18.3 An Erlang Shell in the Browser 292 18.4 Creating a Chat Widget 293 18.5 IRC Lite 295 18.6 Graphics in the Browser 299 18.7 The Browser Server Protocol 301 19. Storing Data with ETS and DETS . 305 19.1 Types of Table 306 19.2 ETS Table Efficiency Considerations 308 19.3 Creating an ETS Table 309 19.4 Example Programs with ETS 310 19.5 Storing Tuples on Disk 315 19.6 What Haven’t We Talked About? 318 20. Mnesia: The Erlang Database . 321 20.1 Creating the Initial Database 321 20.2 Database Queries 322 20.3 Adding and Removing Data in the Database 326 20.4 Mnesia Transactions 328 20.5 Storing Complex Data in Tables 332 20.6 Table Types and Location 333 20.7 The Table Viewer 336 20.8 Digging Deeper 337 21. Profiling, Debugging, and Tracing . 339 21.1 Tools for Profiling Erlang Code 340 21.2 Testing Code Coverage 341 21.3 Generating Cross-References 342 21.4 Compiler Diagnostics 343 21.5 Runtime Diagnostics 346 21.6 Debugging Techniques 347 21.7 The Erlang Debugger 350 21.8 Tracing Messages and Process Execution 352 21.9 Frameworks for Testing Erlang Code 355 www.it-ebooks.info Contents • x 22. Introducing OTP . 359 22.1 The Road to the Generic Server 360 22.2 Getting Started with gen_server 368 22.3 The gen_server Callback Structure 372 22.4 Filling in the gen_server Template 376 22.5 Digging Deeper 377 23. Making a System with OTP .