This guide covers:

• prerequisites (such as Leiningen) and installing
• running the REPL
• creating a project
• interactive development

This work is licensed under a Creative Commons Attribution 3.0 Unported License (including images & stylesheets). The source is available on Github.

Overview

Clojure is a wonderfully simple language and you are going to love it.

To quickly get started, first make sure you've got Java installed.

Then install the Leiningen project management tool.

This author (jg) recommends always installing by downloading the script directly (as described in the instructions at leiningen.org), rather than using your OS's package manager. This will ensure that you get the latest lein version 2.

Clojure programs are typically developed inside their own project directory, and Leiningen manages projects for you. Lein takes care of pulling in dependencies (including Clojure itself), running the REPL, running your program and its tests, packaging your program for distribution, and other administrative tasks. Run lein help to see the list of all the tasks in can perform.

Again, there's no need to "install" Clojure, per se. Lein will take care of fetching it for you.

Trying out the REPL

Although lein facilitates managing your projects, you can also run it on its own (outside of any particular project directory). Once you have the lein tool installed, run it from anywhere you like to get a repl:

$ lein repl

You should be greeted with a "user=>" prompt. Try it out:

user=> (+ 1 1)
;; ⇒ 2
user=> (distinct [:a :b :a :c :a :d])
;; ⇒ (:a :b :c :d)
user=> (dotimes [i 3]
  #_=>   (println (rand-nth ["Fabulous!" "Marvelous!" "Inconceivable!"])
  #_=>            i))
;; Marvelous! 0
;; Inconceivable! 1
;; Fabulous! 2
;; ⇒ nil

Your first project

Create your first Clojure program like so:

lein new app my-proj
cd my-proj
# Have a look at the "-main" function in src/my_proj/core.clj.
lein run

and see the output from that println function call in my_proj/core.clj!

Interactive Development

In your project directory, start up a repl (lein repl) and run your -main function to see its output in the repl:

$ lein repl
...
my-proj.core=> (-main)
Hello, World!
nil

(The prompt is now "my-proj.core=>" instead of "user=>" because lein has started the repl in an app project. More about that ("namespaces") in the topical guides.)

From elsewhere, open up your my-proj/src/my_proj/core.clj file in your editor. Modify the text in that println call.

Back in the repl, reload your source file and run -main again:

my-proj.core=> (require 'my-proj.core :reload)
my-proj.core=> (-main)

to see your changes.

See Also

Other getting started documentation you might find useful:

• Clojure Distilled: introduction to core concepts necessary for working with Clojure
• A Brief Beginner's Guide to Clojure: contains a bit more overview and background material for learning your way around the landscape.
• Starting Clojure screencast: an extensive getting-started screencast using Eclipse to develop a webapp project.

Next Stop

Next stop: the basic Clojure language tutorial.

Contributors

John Gabriele jmg3000@gmail.com (original author)