"Leiningen!" he shouted. "You're insane! They're not creatures you can fight--they're an elemental--an 'act of God!' Ten miles long, two miles wide--ants, nothing but ants! And every single one of them a fiend from hell..." -- from Leiningen Versus the Ants by Carl Stephenson

Leiningen is for automating Clojure projects without setting your hair on fire.

Working on Clojure projects with tools designed for Java can be an exercise in frustration. With Leiningen, you just write Clojure.

Leiningen bootstraps itself using the lein shell script; there is no separate 'install script'. It installs its dependencies upon the first run on unix, so the first run will take longer.

1. Download the script.
2. Place it on your path and chmod it to be executable.

I like to place it in ~/bin, but it can go anywhere on the $PATH.

On Windows most users can

1. Download the Windows distribution leiningen-1.5.2-win.zip
2. Unzip in a folder of choice.
3. Include the "lein" directory in PATH.

If you have wget.exe or curl.exe already installed and in PATH, you can download either the stable version lein.bat, or the development version and use self-install.

The tutorial has a detailed walk-through of the steps involved in creating a new project, but here are the commonly-used tasks:

$ lein new NAME # generate a new project skeleton

$ lein test [TESTS] # run the tests in the TESTS namespaces, or all tests

$ lein repl # launch an interactive REPL session and socket server

$ lein jar # package up the whole project as a .jar file

$ lein install [NAME VERSION] # install a project

Use lein help to see a complete list. lein help $TASK shows the usage for a specific one.

You can also chain tasks together in a single command by using commas:

$ lein clean, test foo.test-core, jar

Most tasks need to be run from somewhere inside a project directory to work, but some (new, help, version, plugin, and the two-argument version of install) may run from anywhere.

The install task places shell scripts in the ~/.lein/bin directory for projects that include them, so if you want to take advantage of this, you should put it on your $PATH.

The project.clj file in the project root should look like this:

(defproject myproject "0.5.0-SNAPSHOT"
  :description "A project for doing things."
  :url "https://github.com/technomancy/myproject"
  :dependencies [[org.clojure/clojure "1.2.1"]
                 [org.clojure/clojure-contrib "1.2.0"]]
  :dev-dependencies [[lein-ring "0.4.5"]])

The lein new task generates a project skeleton with an appropriate starting point from which you can work. See the sample.project.clj file for a detailed listing of configuration options.

You can also have user-level configuration that applies for all projects. The ~/.lein/init.clj file will be loaded every time Leiningen launches; any arbitrary code may go there. This code is executed inside Leiningen itself, not in your project. Set the :repl-init key in project.clj to point to a namespace if you want code executed inside your project.

You can also manage your plugins with the plugin task. Use the same arguments you would put in the Leiningen :dev-dependencies if you were only using the plugin on a single project.

$ lein plugin install lein-clojars "0.6.0"

See the plugin task's help for more information.

$ lein plugin help

Q: How do you pronounce Leiningen?
A: It's LINE-ing-en. ['laɪnɪŋən]

Q: What does this offer over Lancet?
A: Lancet is more of a library than a build tool. It doesn't predefine any tasks apart from what Ant itself offers, so there is nothing Clojure-specific in it. Leiningen builds on Lancet, but takes things further. In addition, it includes some Maven functionality for dependencies.

Q: But Maven is terrifying!
A: That's not a question. Anyway, Leiningen only uses the dependency resolution parts of Maven, which are quite tame. For some other build-related functionality it uses Ant under the covers via Lancet.

Q: But Ant is terrifying!
A: That's true. Ant is an interpreter for a procedural language with a regrettable syntax. But if you treat it as a standard library of build-related functions and are able to write it with a more pleasing syntax, it's not bad.

Q: What's a group ID? How do snapshots work?
A: See the tutorial for background.

Q: How should I pick my version numbers?
A: Use semantic versioning.

Q: It says a required artifact is missing for "super-pom". What's that?
A: The Maven API that Leiningen uses refers to your project as "super-pom". It's just a quirk of the API. It probably means there is a typo in your :dependency declaration in project.clj.

Q: What if my project depends on jars that aren't in any repository?
A: The deploy guide explains how to set up a private repository. If you are not sharing them with a team you could also just install locally..

Q: How do I write my own tasks?
A: If it's a task that may be useful to more than just your project, you should make it into a plugin. You can also include one-off tasks in your src/leiningen/ directory if they're not worth spinning off; the plugin guide shows how.

Q: I want to hack two projects in parallel, but it's annoying to switch between them.
A: Use a feature called checkout dependencies. If you create a directory called checkouts in your project root and symlink some other project roots into it, Leiningen will allow you to hack on them in parallel. That means changes in the dependency will be visible in the main project without having to go through the whole install/switch-projects/deps/restart-repl cycle. Note that this is not a replacement for listing the project in :dependencies; it simply supplements that for tighter change cycles.

Q: Is it possible to exclude indirect dependencies?
A: Yes. Some libraries, such as log4j, depend on projects that are not included in public repositories and unnecessary for basic functionality. Projects listed as :dependencies may exclude any of their dependencies by using the :exclusions key. See sample.project.clj for details.

Q: What does java.lang.NoSuchMethodError: clojure.lang.RestFn.(I)V mean?
A: It means you have some code that was AOT (ahead-of-time) compiled with a different version of Clojure than the one you're currently using. If it persists after running lein clean then it is a problem with your dependencies. Note that for your own project that AOT compilation in Clojure is much less important than it is in other languages. There are a few language-level features that must be AOT-compiled to work, generally for Java interop. If you are not using any of these features, you should not AOT-compile your project if other projects may depend upon it.

Q: I'm behind an HTTP proxy; how can I fetch my dependencies?
A: Currently you need to configure the underlying Maven library by creating ~/.m2/settings.xml as explained in the Maven guide.

Q: What can be done to speed up launch?
A: The main delay involved in Leiningen comes from starting the JVM. Launching "lein interactive" will give you an interactive session so you can run many tasks against the same process instead of launching a new one every time. Depending on your editor you may also be able to take advantage of its Clojure integration. (See swank-clojure or VimClojure, for example.)

Q: Still too slow; what else can make startup faster?
A: There are two flavours of Hotspot (Oracle/OpenJDK's JVM), client and server. The server is optimized for long-running processes and has quite a poor startup time. Leiningen will try to launch a client JVM, but this only works on 32-bit Hotspot. If you are on a 64-bit machine you can still use a client JVM if you install 32-bit packages; on Debian try ia32-sun-java6-bin. Once you've installed it, run sudo update-java-alternatives -s ia32-java-6-sun.

Q: I don't have access to stdin inside my project.
A: There's a bug in the Ant library that Leiningen uses to spawn new processes that blocks access to console input. This means that functions like read-line will not work as expected in most contexts, though the repl task necessarily includes a workaround. You can also use the trampoline task to launch your project's JVM after Leiningen's has exited rather than launching it as a subprocess

Please report issues on the Github issue tracker or the mailing list. Personal email addresses are not appropriate for bug reports. See the file HACKING.md for more details on how Leiningen's codebase is structured.

Patches are preferred as Github pull requests, though patches from git format-patch are also welcome on the mailing list. Please use topic branches when sending pull requests rather than committing directly to master in order to minimize unnecessary merge commit clutter.

Contributors who have had a single patch accepted may request commit rights on the mailing list or in IRC. Please be careful with the master branch and keep any potentially-destabilizing work on topic branches. Other contributors will usually be glad to review topic branches before merging if you ask on IRC or the mailing list.

Contributors are also welcome to request a free Leiningen sticker by asking on the mailing list and mailing a SASE.

You don't need to "build" Leiningen per se, but when you're using a checkout you will need to get its dependencies in place. In most cases a lein self-install will usually get you what you need. However, this will occasionally fail for very new SNAPSHOT versions since the standalone jar will not have been uploaded yet.

Alternatively if you have a copy of an older Leiningen version around (at least 1.1.0, installed as lein-stable, for example), then you can run "lein-stable deps" in your checkout. If Leiningen's dependencies change it will be necessary to remove the lib/ directory entirely before running "lein deps" again. (This is not necessary for most projects, but Leiningen has unique bootstrapping issues when working on itself.)

You can also use Maven, just for variety's sake:

$ mvn dependency:copy-dependencies
$ mv target/dependency lib

Symlink bin/lein from your checkout into a location on the $PATH. The script can figure out when it's being called from inside a checkout and use the checkout rather than the self-install uberjar if necessary.

Source Copyright © 2009-2011 Phil Hagelberg, Alex Osborne, Dan Larkin, and other contributors. Distributed under the Eclipse Public License, the same as Clojure uses. See the file COPYING.

Thanks to Stuart Halloway for Lancet and Tim Dysinger for convincing me that good builds are important.

Images Copyright © 2010 Phil Hagelberg. Distributed under the Creative Commons Attribution + ShareAlike License. Full-size version available.