The Clojure Tutorial for Clojure Newbies

If, like so many other developers, you suffer from a mild addition to Hacker News, then you've surely run across a reference to the Clojure language (pronounced "closure") more than once while perusing the latest headlines. A Lisp dialect targeting the JVM runtime, Clojure users stand to gain from the portability and stability of the JVM, the rich syntax of a functional language, and the ability to integrate with the enormous Java ecosystem. These advantages have clearly resonated with the programming community, as a swarm of activity has amassed around the language in the four years since its inception.

If you've been wondering what all the buzz is about, this article offers a practical, hands-on overview to Clojure, covering the installation process, basic syntax, and potential for Web development.

Installing Clojure

Clojure is available on all major platforms, including Linux/Unix, OS X, and Windows. You will, however, need to make sure Java is installed before proceeding. (See this link to determine whether Java is already installed on your computer). Linux/Unix users will almost certainly be able to install Clojure via their package manager. For instance on a Debian-based system such as Ubuntu, you can install Clojure by executing the following command:

$ sudo apt-get install clojure

OS X users can install Clojure via MacPorts:

$ sudo port install clojure +rlwrap

Windows users can install Clojure and several other useful Clojure-related tools using Clojure Box.

Barring any of the aforementioned solutions, or if you want to be certain you've installed the very latest stable version, download the code by cloning Clojure's GitHub repository:

$ git clone git://github.com/clojure/clojure.git

Additionally you'll need to install either Ant or Maven. Once installed, see the Clojure README file for the latest build instructions.

Using the REPL

The easiest way to begin experimenting with Clojure is via the REPL (read-eval-print-loop) command-line interface. If you're not familiar with functional programming syntax, the REPL is a great place to begin learning because it offers an easy way to execute commands repeatedly and view the outcome.

Windows users can interact with the REPL via SLIME, which is bundled with Clojure Box. See this video for more information. Linux/Unix and OS X users can start the REPL by executing clojure (clj on OS X) via the command line:

$ clojure
Clojure 1.1.0
user=>

In my opinion, Clojure's prefix notation is the one feature which requires the most adjustment on the part of the programmer. Prefix notation dictates that the function is identified before the arguments. Consider the following addition operation:

user=> (+ 2 5)
7

The first thing you've undoubtedly noticed is the parentheses. Anything provided to Clojure must come in the form of a list, with lists consisting of code and data. In this particular example the "code" is the addition function, which happens to be named +, and the code, or arguments, are 2 and 5. So what about variable (known as a binding in Clojure) assignment? Indeed, you'll use a function (def) to perform the assignment:

user=> (def minutes 60)
#'user/minutes
user=> (def hours 24)
#'user/hours
user=> (println "Minutes in day: " (* minutes hours))
Minutes in day:  1440
nil
user=>

So far so good. But other than a different syntactical approach we really haven't seen anything which separates Clojure from other languages. Clojure's unique (relative to non-functional languages) capabilities begin to grow apparent when considering the succinctness with which it can execute manipulation tasks such as map, filter and reduce. The following example demonstrates how Clojure can convert a collection of values representing Fahrenheit-oriented temperatures to Celsius:

user=> (map #(* 0.556 (- % 32)) [32 98.6 150])
(0.0 37.0296 65.608)

I think this simple example provides a pretty good idea of just how succinct yet readable Clojure programs can be!

Creating a Clojure Script

While the REPL is great for syntax experimentation, you are, of course, going to want the luxury of saving and executing your Clojure scripts. The following example will convert any temperature in Fahrenheit passed into the script as an argument into the associated Celsius-based value:

#! /usr/bin/env clojure

(def fahrenheit (first *command-line-args*))

(println (* 0.556 (- (read-string fahrenheit) 32)))

Save this file as convert.clj, change its permissions so the script is executable, and execute it like this:

$ ./convert.clj 98.6
37.0296

Introducing Compojure

It might come as a surprise to you that a Lisp-dialect such as Clojure can be used for Web development. In fact, Hacker News' founder Paul Graham created and sold the world's first e-commerce startup to Yahoo!, a project written entirely in Lisp. If you'd like to explore the idea of building Web applications using Clojure, check out Compojure, a Clojure-based Web framework.

Further Reading

Hopefully this short introduction to Clojure piqued your interest. If so, you're in luck because a great number of online learning resources are at your disposal. Consider starting with these five sites:

• Clojure.org: The official Clojure website is the logical starting point, providing the latest information about the language, in addition to pointers to tutorials, books and other learning resources.
• Learn Clojure: The Learn Clojure website has compiled a great list of freely available video-based Clojure tutorials.
• Clojure Wikibook: Although still very much a work in progress, the Clojure Wikibook already offers some pretty useful instruction.
• Casting Spells in Lisp: Despite the title, this fantastic tutorial really does talk specifically about Clojure, introducing Clojure topics by way of creating a simple text adventure game.
• Clojure for the Non-Lisp Programmer: Moxley Stratton published a great tutorial which introduces Clojure newcomers to language fundamentals.

About the Author

Jason Gilmore--Contributing Editor, PHP--is the founder of EasyPHPWebsites.com, and author of the popular book, "Easy PHP Websites with the Zend Framework". Jason is a cofounder and speaker chair of CodeMash, a nonprofit organization tasked with hosting an annual namesake developer's conference, and was a member of the 2008 MySQL Conference speaker selection board.