Clojure is a dynamically and strongly typed programming language hosted on the Java Virtual Machine (JVM), now in its fifth year. It has seen enthusiastic adoption by programmers from a variety of backgrounds, working in essentially all problem domains. Clojure offers a compelling mix of features and characteristics applicable to solving modern programming challenges:

  • Functional programming foundations, including a suite of persistent data structures with performance characteristics approaching typical mutable data structures

  • A mature, efficient runtime environment, as provided by the host JVM

  • JVM/Java interoperability capabilities suited for a wide variety of architectural and operational requirements

  • A set of mechanisms providing reliable concurrency and parallelism semantics

  • A Lisp pedigree, thereby providing remarkably flexible and powerful metaprogramming facilities

Clojure offers a compelling practical alternative to many who strain against the limitations of typical programming languages and environments. We aim to demonstrate this by showing Clojure seamlessly interoperating with existing technologies, libraries, and services that many working programmers already use on a day-to-day basis. Throughout, we’ll provide a solid grounding in Clojure fundamentals, starting from places of common expertise and familiarity rather than from (often foreign) computer science first principles.

Who Is This Book For?

We wrote this book with a couple of audiences in mind. Hopefully, you consider yourself a part of one of them.

Clojure matches and often exceeds your current favorite language’s expressivity, concision, and flexibility while allowing you to effortlessly leverage the performance, libraries, community, and operational stability of the JVM. This makes it a natural next step for Java developers (and even JVM developers using interpreted or otherwise not particularly fast non-Java languages), who simply will not accept a performance hit or who do not want to give up their JVM platform investment. Clojure is also a natural step for Ruby and Python developers who refuse to compromise on language expressivity, but wish they had a more reliable, efficient execution platform and a larger selection of quality libraries.

Engaged Java Developers

There are millions of Java developers in the world, but some fewer number are working in demanding environments solving nontrivial, often domain-specific problems. If this describes you, you’re probably always on the hunt for better tools, techniques, and practices that will boost your productivity and value to your team, organization, and community. In addition, you’re probably at least somewhat frustrated with the constraints of Java compared to other languages, but you continue to find the JVM ecosystem compelling: its process maturity, massive third-party library selection, vendor support, and large skilled workforce is hard to walk away from, no matter how shiny and appealing alternative languages are.

You’ll find Clojure to be a welcome relief. It runs on the JVM with excellent performance characteristics, interoperates with all of your existing libraries, tools, and applications, and is simpler than Java, yet is demonstrably more expressive and less verbose.

Ruby, Python, and Other Developers

Ruby and Python are not new languages by any means, but they have garnered significant (dare we say, “mainstream”?) traction over recent years. It’s not hard to see why: both are expressive, dynamic languages that, along with their thriving communities, encourage maximal developer productivity in many domains.

Clojure is a natural next step for you. As a Ruby or Python programmer, you’re probably unwilling to compromise on their strengths, but you may wish for a more capable execution platform, better runtime performance, and a larger selection of libraries. The fact that Clojure is efficiently hosted on the JVM fulfills those desires—and it matches or exceeds the degrees of language sophistication and developer productivity that you’ve come to expect.


We will frequently compare and contrast Clojure with Java, Ruby, and Python to help you translate your existing expertise to Clojure. In such comparisons, we will always refer to the canonical implementations of these other languages:

  • Ruby MRI (also called CRuby)

  • CPython

  • Java 6/7

How to Read This Book

In formulating our approach to this book, we wanted to provide a fair bit of concrete detail and practical examples that you could relate to, but stay clear of what we thought were generally unsuccessful approaches for doing so. In particular, we’ve been frustrated in the past by books that attempted to thread the implementation of a single program or application through their pages. Such approaches seem to result in a disjointed narrative, as well as the dominance of a tortured “practical” example that may or may not apply or appeal to readers.

With that in mind, we split the book in two, starting with foundational, instructional narrative that occupies roughly two-thirds of the book, followed in Part IV by a number of discrete, practical examples from real-world domains. This clear segmentation of content with decidedly distinct objectives may qualify this book as a “duplex book.” (This term may have been coined by Martin Fowler in In any case, we can conceive of two obvious approaches to reading it.

Start with Practical Applications of Clojure

Often the best way to learn is to dig straight into the nitty-gritty of how a language is used in the real world. If that sounds appealing, the hope is that you will find that at least a couple of the practicums resonate with what you do on a day-to-day basis, so that you can readily draw parallels between how you solve certain categories of problems in your current language(s) and how they may be solved using Clojure. You’re going to bump into a lot of potentially foreign concepts and language constructs in those chapters—when you do, use that context within the domain in question as your entry point for understanding those concepts using the relevant instructional material in the first part of the book.

Start from the Ground Up with Clojure’s Foundational Concepts

Sometimes the only way to truly understand something is to learn it inside-out, starting with the fundamentals. If you prefer that approach, then you will likely find that digesting this book starting from the first page of Chapter 1 will be best. We have attempted to provide a comprehensive treatment of all of Clojure’s foundational principles and constructs in a narrative that progresses such that it will be very rare for you to need to look ahead in the book to understand concepts in earlier sections. As you begin to get a handle on Clojure’s fundamentals, feel free to jump ahead into the practicums you find most interesting and relevant to your work.

Who’s “We”?

We are three software developers who have each taken different paths in coming to use and appreciate Clojure. In writing this book, we have attempted to distill all that we’ve learned about why and how you should use Clojure so that you can be successful in your use of it as well.

Chas Emerick

Chas has been a consistent presence in the Clojure community since early 2008. He has made contributions to the core language, been involved in dozens of Clojure open source projects, and frequently writes and speaks about Clojure and software development generally.

Chas maintains the Clojure Atlas (, an interactive visualization of and learning aid for the Clojure language and its standard libraries.

The founder of Snowtide (, a small software company in Western Massachusetts, Chas’s primary domain is unstructured data extraction, with a particular specialty around PDF documents. He writes about Clojure, software development, entrepreneurship, and other passions at

Brian Carper

Brian is a Ruby programmer turned Clojure devotee. He’s been programming Clojure since 2008, using it at home and at work for everything from web development to data analysis to GUI apps.

Brian is the author of Gaka (, a Clojure-to-CSS compiler, and Oyako (, an Object-Relational Mapping library. He writes about Clojure and other topics at

Christophe Grand

Christophe was a long-time enthusiast of functional programming lost in Java-land when he encountered Clojure in early 2008, and it was love at first sight! He authored Enlive (, an HTML/XML transformation, extraction, and templating library; Parsley (, an incremental parser generator; and Moustache (, a routing and middleware application DSL for Ring.

As an independent consultant, he develops, coaches, and offers training in Clojure. He also writes about Clojure at


Like any sizable piece of work, this book would not exist without the tireless efforts of dozens, probably hundreds of people.

First, Rich Hickey, the creator of Clojure. In just a few short years, he has designed, implemented, and shepherded a new programming language into the world that, for so many, has been not just another tool, but a reinvigoration of our love of programming. Beyond that, he’s personally taught us a great deal—certainly about programming, but also about patience, humility, and perspective. Thanks, Rich.

Dave Fayram and Mike Loukides were essential in helping to formulate the initial concept and approach of the book. Of course, you likely wouldn’t be reading this book right now if it weren’t for Julie Steele, our editor, and all of the fine people at O’Reilly who took care of the logistics and minutiae that go along with publishing.

The quality of this book would be far less than it is were it not for the efforts of our technical reviewers, including Sam Aaron, Antoni Batchelli, Tom Faulhaber, Chris Granger, Anthony Grimes, Phil Hagelberg, Tom Hicks, Alex Miller, William Morgan, Laurent Petit, and Dean Wampler. We’d also like to thank all of those who provided feedback and comments on the early releases and Rough Cuts of the book, both on the O’Reilly forums and via email, Twitter, and so on.

Michael Fogus and Chris Houser have inspired us in many ways large and small. One of the smaller ways was the style and presentation of the REPL interactions in their Clojure book, The Joy of Clojure, which we shamelessly copied and iterated.

If we’ve neglected to mention anyone, please accept our implicit thanks and our apologies; at the end of this endeavor, we are quite lucky to be upright and coherent at all!

And Last, but Certainly Far from Least

The Clojure community has been my home away from home for a number of years. The hospitality and positive, helpful energy I see anywhere Clojure programmers congregate continues to be an inspiration and example to me. In particular, many of the regular denizens of #clojure on Freenode IRC—in addition to becoming good friends—have guided me toward learning things I never would have otherwise.

To my coauthors, Christophe and Brian: working with you has been a great honor for me. There is absolutely no way that I would have been able to complete this work without you.

To my parents, Charley and Darleen: my compulsive curiosity about how things work, my love of language and rhetoric, and my interest in business—all of these can be traced back over the years to your consistent influence. Without it, I am certain I would not have found my unique path, started a software company, or written this book, each done against all odds.

Finally, to my wife, Krissy: the sacrifices you’ve made to enable me to chase my ambitions are legion. It is likely that I’ll never be able to thank you sufficiently. So, I’ll just say: I love you.

—Chas Emerick, February 2012

To everyone in the community who helped create Clojure: thank you for your tireless hard work, for making my professional and personal coding life so much more enjoyable, and for opening my eyes to what’s possible.

To my coauthors, Christophe and Chas: I’ve never worked with a smarter group of people. It’s been an honor and a privilege.

To my wife Nicole: sorry I kept you awake all night with my typing.

—Brian Carper, February 2012

To Rich Hickey for creating Clojure and fostering such a friendly community.

To this community for having brought me to higher standards.

To my coauthors, Brian and Chas: it has been a great honor to work with you.

A mon professeur Daniel Goffinet, et à ses exercices improbables, qui a radicalement changé mon approche de la programmation et de l’informatique—sur ces sujets je lui suis plus redevable qu’à nul autre.

(To Pr. Daniel Goffinet, and his meta mind twisters, who radically altered the way I think about programming and computing—on those subjects there is no one I’m more indebted to.)

A mes parents pour votre amour bien sûr mais aussi pour tout le temps à s’inquiéter que je passais trop de temps sur l’Amstrad.

(To my parents: for your love obviously and for buying me that 8-bit computer you worried I was spending too much time on.)

A ma compagne Emilie, et mon fils Gaël, merci d’être là et de m’avoir supporté pendant l’écriture de ce livre.

(To my wife Emilie and to my son Gaël: thank you for being there and having supported me throughout the writing of this book.)

—Christophe Grand, February 2012

Conventions Used in This Book

The following typographical conventions are used in this book:


Indicates new terms, URLs, email addresses, filenames, and file extensions.

Constant width

Used for program listings, as well as within paragraphs to refer to program elements such as variable or function names, databases, data types, environment variables, statements, and keywords.

; listing lines prefixed with a semicolon

Used to indicate content printed (i.e., to standard out/err) by code evaluated in the REPL.

;= listing lines prefixed with a semicolon + equal sign

Used to indicate the result/return value of a REPL evaluation.

Constant width bold

Shows commands or other text that should be typed literally by the user.

Constant width italic

Shows text that should be replaced with user-supplied values or by values determined by context.


This icon signifies a tip, suggestion, or general note.


This icon indicates a warning or caution.

Using Code Examples

This book is here to help you get your job done. In general, you may use the code in this book 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 example 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 Programming by Chas Emerick, Brian Carper, and Christophe Grand (O’Reilly). Copyright 2012 Chas Emerick, Brian Carper, and Christophe Grand, 978-1-449-39470-7.”

If you feel your use of code examples falls outside fair use or the permission given above, feel free to contact us at .

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