Clojure Cookbook Recipes for Functional Programming (Luke VanderHart, Ryan Neufeld) (z-library.sk, 1lib.sk, z-lib.sk)

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Luke VanderHart and Ryan Neufeld Clojure Cookbook

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Clojure Cookbook by Luke VanderHart and Ryan Neufeld Copyright © 2014 Cognitect, Inc. All rights reserved. Printed in the United States of America. Published by O’Reilly Media, Inc., 1005 Gravenstein Highway North, Sebastopol, CA 95472. O’Reilly books may be purchased for educational, business, or sales promotional use. Online editions are also available for most titles (http://my.safaribooksonline.com). For more information, contact our corporate/ institutional sales department: 800-998-9938 or corporate@oreilly.com. Editor: Meghan Blanchette Production Editor: Kristen Brown Copyeditor: Amanda Kersey Proofreader: Rachel Head Indexer: Judith McConville Cover Designer: Karen Montgomery Interior Designer: David Futato Illustrator: Rebecca Demarest March 2014: First Edition Revision History for the First Edition: 2014-03-04: First release See http://oreilly.com/catalog/errata.csp?isbn=9781449366179 for release details. Nutshell Handbook, the Nutshell Handbook logo, and the O’Reilly logo are registered trademarks of O’Reilly Media, Inc. Clojure Cookbook, the image of an aardwolf, and related trade dress are trademarks of O’Reilly Media, Inc. 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 O’Reilly Media, Inc. was aware of a trademark claim, the designations have been printed in caps or initial caps. While every precaution has been taken in the preparation of this book, the publisher and authors assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. ISBN: 978-1-449-36617-9 [LSI]

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Table of Contents Preface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix 1. Primitive Data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1. Changing the Capitalization of a String 3 1.2. Cleaning Up Whitespace in a String 4 1.3. Building a String from Parts 5 1.4. Treating a String as a Sequence of Characters 7 1.5. Converting Between Characters and Integers 8 1.6. Formatting Strings 10 1.7. Searching a String by Pattern 12 1.8. Pulling Values Out of a String Using Regular Expressions 13 1.9. Performing Find and Replace on Strings 15 1.10. Splitting a String into Parts 17 1.11. Pluralizing Strings Based on a Quantity 18 1.12. Converting Between Strings, Symbols, and Keywords 20 1.13. Maintaining Accuracy with Extremely Large/Small Numbers 22 1.14. Working with Rational Numbers 24 1.15. Parsing Numbers 25 1.16. Truncating and Rounding Numbers 26 1.17. Performing Fuzzy Comparison 28 1.18. Performing Trigonometry 30 1.19. Inputting and Outputting Integers with Different Bases 31 1.20. Calculating Statistics on Collections of Numbers 32 1.21. Performing Bitwise Operations 36 1.22. Generating Random Numbers 37 1.23. Working with Currency 39 1.24. Generating Unique IDs 41 1.25. Obtaining the Current Date and Time 43 1.26. Representing Dates as Literals 44 iii

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1.27. Parsing Dates and Times Using clj-time 46 1.28. Formatting Dates Using clj-time 47 1.29. Comparing Dates 49 1.30. Calculating the Length of a Time Interval 50 1.31. Generating Ranges of Dates and Times 52 1.32. Generating Ranges of Dates and Times Using Native Java Types 53 1.33. Retrieving Dates Relative to One Another 56 1.34. Working with Time Zones 58 1.35. Converting a Unix Timestamp to a Date 59 1.36. Converting a Date to a Unix Timestamp 61 2. Composite Data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 2.1. Creating a List 65 2.2. Creating a List from an Existing Data Structure 66 2.3. “Adding” an Item to a List 68 2.4. “Removing” an Item from a List 69 2.5. Testing for a List 70 2.6. Creating a Vector 71 2.7. “Adding” an Item to a Vector 72 2.8. “Removing” an Item from a Vector 73 2.9. Getting the Value at an Index 74 2.10. Setting the Value at an Index 76 2.11. Creating a Set 77 2.12. Adding and Removing Items from Sets 79 2.13. Testing Set Membership 80 2.14. Using Set Operations 82 2.15. Creating a Map 84 2.16. Retrieving Values from a Map 86 2.17. Retrieving Multiple Keys from a Map Simultaneously 89 2.18. Setting Keys in a Map 90 2.19. Using Composite Values as Map Keys 94 2.20. Treating Maps as Sequences (and Vice Versa) 96 2.21. Applying Functions to Maps 98 2.22. Keeping Multiple Values for a Key 100 2.23. Combining Maps 103 2.24. Comparing and Sorting Values 105 2.25. Removing Duplicate Elements from a Collection 109 2.26. Determining if a Collection Holds One of Several Values 111 2.27. Implementing Custom Data Structures: Red-Black Trees—Part I 112 2.28. Implementing Custom Data Structures: Red-Black Trees—Part II 115 3. General Computing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 iv | Table of Contents

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3.1. Running a Minimal Clojure REPL 121 3.2. Interactive Documentation 123 3.3. Exploring Namespaces 125 3.4. Trying a Library Without Explicit Dependencies 126 3.5. Running Clojure Programs 127 3.6. Running Programs from the Command Line 130 3.7. Parsing Command-Line Arguments 132 3.8. Creating Custom Project Templates 135 3.9. Building Functions with Polymorphic Behavior 139 3.10. Extending a Built-In Type 145 3.11. Decoupling Consumers and Producers with core.async 146 3.12. Making a Parser for Clojure Expressions Using core.match 150 3.13. Querying Hierarchical Graphs with core.logic 153 3.14. Playing a Nursery Rhyme 159 4. Local I/O. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 4.1. Writing to STDOUT and STDERR 165 4.2. Reading a Single Keystroke from the Console 167 4.3. Executing System Commands 168 4.4. Accessing Resource Files 171 4.5. Copying Files 173 4.6. Deleting Files or Directories 175 4.7. Listing Files in a Directory 176 4.8. Memory Mapping a File 178 4.9. Reading and Writing Text Files 179 4.10. Using Temporary Files 181 4.11. Reading and Writing Files at Arbitrary Positions 182 4.12. Parallelizing File Processing 183 4.13. Parallelizing File Processing with Reducers 185 4.14. Reading and Writing Clojure Data 188 4.15. Using edn for Configuration Files 190 4.16. Emitting Records as edn Values 194 4.17. Handling Unknown Tagged Literals When Reading Clojure Data 196 4.18. Reading Properties from a File 199 4.19. Reading and Writing Binary Files 201 4.20. Reading and Writing CSV Data 203 4.21. Reading and Writing Compressed Files 204 4.22. Working with XML Data 206 4.23. Reading and Writing JSON Data 207 4.24. Generating PDF Files 209 Table of Contents | v

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4.25. Making a GUI Window with Scrollable Text 213 5. Network I/O and Web Services. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219 5.1. Making HTTP Requests 219 5.2. Performing Asynchronous HTTP Requests 221 5.3. Sending a Ping Request 223 5.4. Retrieving and Parsing RSS Data 224 5.5. Sending Email 226 5.6. Communicating over Queues Using RabbitMQ 229 5.7. Communicating with Embedded Devices via MQTT 236 5.8. Using ZeroMQ Concurrently 240 5.9. Creating a TCP Client 243 5.10. Creating a TCP Server 245 5.11. Sending and Receiving UDP Packets 248 6. Databases. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253 6.1. Connecting to an SQL Database 254 6.2. Connecting to an SQL Database with a Connection Pool 257 6.3. Manipulating an SQL Database 260 6.4. Simplifying SQL with Korma 266 6.5. Performing Full-Text Search with Lucene 270 6.6. Indexing Data with ElasticSearch 272 6.7. Working with Cassandra 277 6.8. Working with MongoDB 280 6.9. Working with Redis 284 6.10. Connecting to a Datomic Database 286 6.11. Defining a Schema for a Datomic Database 289 6.12. Writing Data to Datomic 293 6.13. Removing Data from a Datomic Database 296 6.14. Trying Datomic Transactions Without Committing Them 298 6.15. Traversing Datomic Indexes 300 7. Web Applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305 7.1. Introduction to Ring 305 7.2. Using Ring Middleware 307 7.3. Serving Static Files with Ring 309 7.4. Handling Form Data with Ring 311 7.5. Handling Cookies with Ring 312 7.6. Storing Sessions with Ring 314 7.7. Reading and Writing Request and Response Headers in Ring 316 7.8. Routing Requests with Compojure 318 7.9. Performing HTTP Redirects with Ring 320 vi | Table of Contents

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7.10. Building a RESTful Application with Liberator 321 7.11. Templating HTML with Enlive 323 7.12. Templating with Selmer 330 7.13. Templating with Hiccup 334 7.14. Rendering Markdown Documents 337 7.15. Building Applications with Luminus 339 8. Performance and Production. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343 8.1. AOT Compilation 343 8.2. Packaging a Project into a JAR File 345 8.3. Creating a WAR File 348 8.4. Running an Application as a Daemon 352 8.5. Alleviating Performance Problems with Type Hinting 358 8.6. Fast Math with Primitive Java Arrays 360 8.7. Simple Profiling with Timbre 363 8.8. Logging with Timbre 365 8.9. Releasing a Library to Clojars 367 8.10. Using Macros to Simplify API Deprecations 369 9. Distributed Computation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375 9.1. Building an Activity Feed System with Storm 376 9.2. Processing Data with an Extract Transform Load (ETL) Pipeline 385 9.3. Aggregating Large Files 389 9.4. Testing Cascalog Workflows 394 9.5. Checkpointing Cascalog Jobs 396 9.6. Explaining a Cascalog Query 398 9.7. Running a Cascalog Job on Elastic MapReduce 400 10. Testing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403 10.1. Unit Testing 404 10.2. Testing with Midje 408 10.3. Thoroughly Testing by Randomizing Inputs 411 10.4. Finding Values That Cause Failure 415 10.5. Running Browser-Based Tests 418 10.6. Tracing Code Execution 424 10.7. Avoiding Null-Pointer Exceptions with core.typed 427 10.8. Verifying Java Interop Using core.typed 429 10.9. Type Checking Higher-Order Functions with core.typed 433 Index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 437 Table of Contents | vii

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Preface The primary goal of this book is to provide mid-length examples of Clojure code that go beyond the basics, with a focus on real-world, everyday applications (as opposed to more conceptual or academic issues). Unlike many of the other books on Clojure written to date, the organizing theme of this book is not the language itself, or its features and capabilities. Instead, it focuses on specific tasks that developers face (regardless of what language they’re using) and shows an example of how to use Clojure to solve each of those specific problems. As such, this book is not and cannot be truly comprehensive; there are infinite possible example problems. However, we do hope we’ve documented some of the more common ones that most programmers encounter frequently, and that by induction readers will be able to learn some common patterns, approaches, and techniques that will serve them well as they design solutions for their own unique problems. How This Book Was Written An important thing you should understand about this book is that it is, first and fore‐ most, a group effort. It is not authored by one or two people. It isn’t even the work of a single, well-defined group. Instead, it is the collaborative product of more than 60 of the best Clojurists from all over the world, from all backgrounds. These authors use Clojure every day on real applications, ranging from aerospace to social media, banking to robotics, AI research to e-commerce. As such, you will see a lot of diversity in the recipes presented. Some are quick and to the point. Others are more deliberate, presenting digestible yet penetrating insights into the philosophy and implementation of certain aspects of Clojure. We hope that there is something in this book for readers of diverse interests. We believe that it will be useful not only as a reference for looking up solutions to specific problems, but also as a worthwhile survey of the variety and expressivity that Clojure is capable ix

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of. As we edited submissions, we were astonished by the number of concepts and tech‐ niques that were new to us, and will hopefully be new to our readers as well. Something else that we discovered while writing and editing was how difficult it was to draw a circumference around what we wanted to cover. Every single recipe is a beautiful, endless fractal, touching multiple topics, each of which deserves a recipe, a chapter, or a book of its own. But each recipe also needs to stand on its own. Each one should provide some useful nugget of information that readers can understand and take away with them. We sincerely hope that we have balanced these goals appropriately, and that you find this book useful without being tedious, and insightful without being pedantic. Audience Anyone who uses Clojure will, we hope, be able to get something out of this book. There are a lot of recipes on truly basic things that beginners will find useful, but there are also many recipes on more specialized topics that advanced users should find useful for getting a head start on implementation. That said, if you’re completely new to Clojure, this probably isn’t the book to start with —at least, not by itself. It covers a great many useful topics, but not as methodically or as thoroughly as a good introductory text. See the following section for a list of general Clojure books you may find useful as prior or supplemental texts. Other Resources One thing that this book is not, and could never be, is complete. There is too much to cover, and by presenting information in a task-oriented recipe format we have inherently precluded ourselves from methodical, narrative explanation of the features and capa‐ bilities of the whole language. For a more linear, thorough explanation of Clojure and its features, we recommend one of the following books: • Clojure Programming (O’Reilly, 2012), by Chas Emerick, Brian Carper, and Chris‐ tophe Grand. A good, comprehensive, general-purpose Clojure book focusing on the language and common tasks, oriented toward beginner Clojure programmers. • Programming Clojure, 2nd ed. (Pragmatic Bookshelf, 2012), by Stuart Halloway and Aaron Bedra. The first book on Clojure, this is a clear, comprehensive introductory tutorial on the Clojure language. • Practical Clojure (Apress, 2010), by Luke VanderHart and Stuart Sierra. This is a terse, no-nonsense explanation of what Clojure is and what its features do. x | Preface

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• The Joy of Clojure (Manning, 2011), by Michael Fogus and Chris Houser. This is a slightly more advanced text that really digs into the themes and philosophies of Clojure. • ClojureScript: Up and Running (O’Reilly, 2012), by Stuart Sierra and Luke Vander‐ Hart. While Clojure Cookbook and the other Clojure books listed here focus mainly or entirely on Clojure itself, ClojureScript (a dialect of Clojure that compiles to JavaScript) has gained considerable uptake. This book introduces ClojureScript and how to get started with it, and covers the similarities and differences between Clo‐ jureScript and Clojure. Finally, you should look at the source code for this book itself, which is freely available on GitHub. The selection of recipes available online is larger than that in the print version, and we are still accepting pull requests for new recipes that might someday make it into a future edition of this book. Structure The chapters in this book are for the most part groupings of recipes by theme, rather than strictly categorical. It is entirely possible for a recipe to be applicable to more than one chapter—in these cases, we have simply tried to place it where we think the majority of readers will likely look first. A recipe consists of three primary parts and one secondary: problem, solution, discus‐ sion, and “see also.” A recipe’s problem statement lays out a task or obstacle to be over‐ come. Its solution tackles the problem head-on, illustrating a particular technique or library that effectively accomplishes the task. The discussion rounds everything out, exploring the solution and any caveats that may come with it. Finally, we tie off each recipe with a “see also” section, pointing you, the reader, to any additional resources or recipes that will assist you in enacting the described solution. Chapter Listing The book is composed of the following chapters: • Chapter 1, Primitive Data, and Chapter 2, Composite Data, cover Clojure’s built-in primitive and composite data structures, and explain many common (and less common) ways one might want to use them. • Chapter 3, General Computing, is a grab bag of useful topics that are generally applicable in many different application areas and project domains, from Clojure features such as Protocols to alternate programming paradigms such as logic pro‐ gramming with core.logic or asynchronous coordination with core.async. Preface | xi

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• Chapter 4, Local I/O, deals with all the ways in which your program can interact with the local computer upon which it is running. This includes reading fromand writing to standard input and output streams, creating and manipulating files, se‐ rializing and deserializing files, etc. • Chapter 5, Network I/O and Web Services, contains recipes with similar themes to Chapter 4, Local I/O, but instead deals with remote communication over a network. It includes recipes on a variety of network communication protocols and libraries. • Chapter 6, Databases, demonstrates techniques and tools for connecting to and using a variety of databases. Special attention is given to Datomic, a datastore that shares and extends much of Clojure’s underlying philosophy of value, state, and identity to persistent storage. • Chapter 7, Web Applications, dives in-depth into one of the most common appli‐ cations for Clojure: building and maintaining dynamic websites. It provides com‐ prehensive treatment of Ring (the most popular HTTP server library for Clojure), as well as tools for HTML templating and rendering. • Chapter 8, Performance and Production, explains what to do with a Clojure program once you have one, going over common patterns for packaging, distributing, profil‐ ing, logging, and associated ongoing tasks over the lifetime of an application. • Chapter 9, Distributed Computation, focuses on cloud computing and using Clojure for heavyweight distributed data crunching. Special attention is given to Cascalog, a declarative Clojure interface to the Hadoop MapReduce framework. • Last but not least, Chapter 10, Testing, covers a variety of techniques for ensuring the integrity and correctness of your code and data, ranging from traditional unit and integration tests to more comprehensive generative and simulation testing, and even optional compile-time validations using static typing with core.typed. Software Prerequisites To follow along with the recipes in this book you will need valid installations of the Java Development Kit (JDK) and Clojure’s de facto build tool, Leiningen. We recommend version 7 of the JDK, but a minimum of 6 will do. For Leiningen, you should have at least version 2.2. If you don’t have Java installed (or would like to upgrade), visit the Java Download Page for instructions on downloading and installing the Java JDK. To install Leiningen, follow the installation instructions on Leiningen’s website. If you already have Leiningen installed, get the latest version by executing the command lein upgrade. If you aren’t familiar with Leiningen, visit the tutorial to learn more. xii | Preface

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The one thing you won’t need to manually install is Clojure itself; Leiningen will do this for you on an ad hoc basis. To verify your installation, run lein repl and check your Clojure version: $ lein repl # ... user=> *clojure-version* {:major 1, :minor 5, :incremental 1, :qualifier nil} Some recipes have accompanying online materials available on Git‐ Hub. If you do not have Git installed on your system, follow the setup instructions to enable you to check out a GitHub repository locally. Some recipes—such as the database recipes—require further software installations. Where this is the case, recipes will include additional information on installing those tools. Conventions Used in This Book Being a book full of solutions, you’ll find no shortage of Clojure source code in this book. Clojure source code appears in a monospace font, like this: (defn add [x y] (+ x y)) When a Clojure expression is evaluated for a return value, that value is denoted with a comment followed by an arrow, much like it would appear on the command line: (add 1 2) ;; -> 3 Where appropriate, code samples may omit or ellipsize return value comments. The two most common places you’ll see this are when defining a function/var or shortening lengthy output: ;; This would return #'user/one, but do you really care? (def one 1) (into [] (range 1 20)) ;; -> [1 2 ... 20] When an expression produces output to STDOUT or STDERR, it is denoted by a comment (*out* or *error*, respectively), followed by a comment with each line of output: (do (println "Hello!") (println "Goodbye!")) ;; -> nil Preface | xiii

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;; *out* ;; Hello! ;; Goodbye! REPL Sessions Seeing that REPL-driven development is in vogue at present, it follows that this be a REPL-driven book. REPLs (read-eval-print loops) are interactive prompts that evaluate expressions and print their results. The Bash prompt, irb, and the python prompt are examples of REPLs. Nearly every recipe in this book is designed to be run at a Clojure REPL. While Clojure REPLs are traditionally displayed as user=> ..., this book aims for readers to be able to copy and paste all of the examples in a recipe and see the indicated results. As such, samples omit user=> and comment out any output to make things easier. This is especially helpful if you’re following along on a computer: you can blindly copy and paste code samples without worrying about trying to run noncode. When an example is only relevant in the context of a REPL, we will retain the traditional REPL style (with user=>). What follows is an example of each, a REPL-only sample and its simplified version. REPL-only: user=> (+ 1 2) 3 user=> (println "Hello!") Hello! nil Simplified: (+ 1 2) ;; -> 3 (println "Hello!") ;; *out* ;; Hello! Console/Terminal Sessions Console sessions (e.g., shell commands) are denoted by monospace font, with lines beginning with a dollar sign ($) indicating a shell prompt. Output is printed without a leading $: $ lein version Leiningen 2.0.0-preview10 on Java 1.6.0_29 Java HotSpot(TM) 64-Bit Server VM A backslash (\) at the end of a command indicates to the console that the command continues on the next line. xiv | Preface

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Our Golden Boy, lein-try Clojure is not known for its extensive standard library. Unlike languages like Perl or Ruby, Clojure’s standard library is comparatively small; Clojure chose simplicity and power instead. As such, Clojure is a language full of libraries, not built-ins (well, except for Java). Since so many of the solutions in this book rely on third-party libraries, we developed lein-try. lein-try is a small plug-in for Leiningen, Clojure’s de facto project tool, that lets you quickly and easily try out various Clojure libraries. To use lein-try, ensure you have Leiningen installed, then edit your user profile (~/.lein/profiles.clj) as follows: {:user {:plugins [[lein-try "0.4.1"]]}} Now, inside of a project or out, you can use the lein try command to launch a REPL with access to whichever library you please: $ lein try clj-time #... user=> Long story short: where possible, you’ll see instructions on which lein-try command to execute above recipes that use third-party libraries. You’ll find an example of trying recipes with lein-try in Recipe 3.4, “Trying a Library Without Explicit Dependen‐ cies” on page 126. If a recipe cannot be run via lein-try, we have made efforts to include adequate in‐ structions on how to run that recipe on your local machine. Typesetting Conventions The following typographic conventions are used in this book: Italic Used for URLs, filenames, pathnames, and file extensions. New terms are also ita‐ licized when they first appear in the text, and italics are used for emphasis. Constant width Used for function and method names and their arguments; for data types, classes, and namespaces; in examples to show both input and output; and in regular text to show literal code. Constant width bold Used to indicate commands that you should enter literally at the command line. Preface | xv

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<replaceable-value> Elements of pathnames, commands, function names, etc. that should be replaced with user-supplied values are shown in angle brackets. The names of libraries follow one of two conventions: libraries with proper names are displayed in plain text (e.g., “Hiccup” or “Swing”), while libraries with names meant to mimic code symbols are displayed in constant-width text (e.g., core.async or clj- commons-exec). This element signifies a tip or suggestion. This element signifies a general note. This element indicates a warning or caution. Using Code Examples Supplemental material (code examples, exercises, etc.) is available for download at http://bit.ly/clj-ckbk. This book is here to help you get your job done. In general, if example code is offered with this book, you may use it in your programs and documentation. You do not need to contact us for permission unless you’re reproducing a significant portion of the code. For example, writing a program that uses several chunks of code from this book does not require permission. Selling or distributing a CD-ROM of examples from O’Reilly books does require permission. Answering a question by citing this book and quoting example code does not require permission. Incorporating a significant amount of ex‐ ample code from this book into your product’s documentation does require permission. We appreciate, but do not require, attribution. An attribution usually includes the title, author, publisher, and ISBN. For example: “Clojure Cookbook by Luke VanderHart and Ryan Neufeld (O’Reilly). Copyright 2014 Cognitect, Inc., 978-1-449-36617-9.” xvi | Preface

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If you feel your use of code examples falls outside fair use or the permission given above, feel free to contact us at permissions@oreilly.com. Safari® Books Online Safari Books Online is an on-demand digital library that delivers expert content in both book and video form from the world’s leading authors in technology and business. Technology professionals, software developers, web designers, and business and crea‐ tive professionals use Safari Books Online as their primary resource for research, prob‐ lem solving, learning, and certification training. Safari Books Online offers a range of product mixes and pricing programs for organi‐ zations, government agencies, and individuals. Subscribers have access to thousands of books, training videos, and prepublication manuscripts in one fully searchable database from publishers like O’Reilly Media, Prentice Hall Professional, Addison-Wesley Pro‐ fessional, Microsoft Press, Sams, Que, Peachpit Press, Focal Press, Cisco Press, John Wiley & Sons, Syngress, Morgan Kaufmann, IBM Redbooks, Packt, Adobe Press, FT Press, Apress, Manning, New Riders, McGraw-Hill, Jones & Bartlett, Course Technol‐ ogy, and dozens more. For more information about Safari Books Online, please visit us online. How to Contact Us Please address comments and questions concerning this book to the publisher: O’Reilly Media, Inc. 1005 Gravenstein Highway North Sebastopol, CA 95472 800-998-9938 (in the United States or Canada) 707-829-0515 (international or local) 707-829-0104 (fax) We have a web page for this book, where we list errata, examples, and any additional information. You can access this page at http://oreil.ly/clojure-ckbk. To comment or ask technical questions about this book, send email to bookques tions@oreilly.com. For more information about our books, courses, conferences, and news, see our website at http://www.oreilly.com. Find us on Facebook: http://facebook.com/oreilly Preface | xvii

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Follow us on Twitter: http://twitter.com/oreillymedia or https://twitter.com/clojurecook book Watch us on YouTube: http://www.youtube.com/oreillymedia Acknowledgments This book would not have been possible without the selfless contributions of many within the Clojure community. Over 65 Clojurists rose to the occasion, submitting recipes, proofreading, and offering their input on the direction of the book. Ultimately, this is the community’s book—we’re just honored to have been able to help put it to‐ gether. Those contributors are: • Adam Bard, adambard on GitHub • Alan Busby, thebusby on GitHub • Alex Miller, puredanger on GitHub • Alex Petrov, ifesdjeen on GitHub • Alex Robbins, alexrobbins on GitHub • Alex Vzorov, 0rca on GitHub • Ambrose Bonnaire-Sergeant, frenchy64 on GitHub • arosequist • Chris Allen, bitemyapp on GitHub • Chris Ford, ctford on GitHub • Chris Frisz, cjfrisz on GitHub • Clinton Begin, cbegin on GitHub • Clinton Dreisbach, cndreisbach on GitHub • Colin Jones, trptcolin on GitHub • Craig McDaniel, cpmcdaniel on GitHub • Daemian Mack, daemianmack on GitHub • Dan Allen, mojavelinux on GitHub • Daniel Gregoire, semperos on GitHub • Dmitri Sotnikov, yogthos on GitHub • Edmund Jackson, ejackson on GitHub • Eric Normand, ericnormand on GitHub • Federico Ramirez, gosukiwi on GitHub • Filippo Diotalevi, fdiotalevi on GitHub xviii | Preface