Leiningen

"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.

Installation

If your preferred package manager has a relatively recent version of Leiningen, try that first. Otherwise you can install by hand:

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. (I like to use ~/bin)
3. Set it to be executable. (chmod 755 ~/bin/lein)

The instructions above will install the stable release. The Leiningen 2 preview release has some great new features, but not all projects and plugins have been upgraded to work with it yet. Please see the upgrade guide for instructions on migrating to version 2.

On Windows most users can get the batch file. If you have wget.exe or curl.exe already installed and in PATH, you can just run lein self-install, otherwise get the standalone jar from the downloads page. If you have Cygwin you should be able to use the shell script above rather than the batch file.

Usage

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

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

$ lein install [NAME VERSION] # install a project

$ lein search ... # find jars for your project.clj dependencies

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

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, search, version, repl, 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.

Configuration

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 "http://github.com/technomancy/myproject"
  :dependencies [[org.clojure/clojure "1.2.1"]]
  :plugins [[lein-ring "0.4.5"]])

To find specific versions of a dependency, use lein search.

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.

Profiles

You can change the configuration of your project by applying various profiles. For instance, you may want to have a few extra test data directories on the classpath during development without including them in the jar, or you may want to have Swank Clojure available in every project you hack on without modifying every single project.clj you use.

By default the :dev, :user, and :default profiles are activated for each task. Each profile is defined as a map which gets merged into your project map. To add resources directories during development, add a :profiles key to project.clj like so:

(defproject myproject "0.5.0-SNAPSHOT"
  :description "A project for doing things."
  :dependencies [[org.clojure/clojure "1.2.1"]]
  :profiles {:dev {:resources-path ["dummy-data"]}})

You can place any arbitrary defproject entries into a given profile and they will be merged into the project map when that profile is active. In addition to project.clj, profiles specified in ~/.lein/profiles.clj will be available in all projects.

{:user {:plugins [[lein-swank "1.4.0"]
                  [lein-pprint "1.1.1"]]}}

Another use of profiles is to test against various sets of dependencies:

(defproject swank-clojure "1.5.0-SNAPSHOT"
  :description "Swank server connecting Clojure to Emacs SLIME"
  :dependencies [[org.clojure/clojure "1.2.1"]
                 [clj-stacktrace "0.2.4"]
                 [cdt "1.2.6.2"]]
  :profiles {:1.3 {:dependencies [[org.clojure/clojure "1.3.0"]]}
             :1.4 {:dependencies [[org.clojure/clojure "1.4.0-beta1"]]}})

To activate other profiles for a given run, use the with-profile higher-order task:

$ lein with-profile qa,1.3 test :database

Multiple profiles may be specified with commas:

$ lein with-profile qa,user test :database

A single with-profile call does not apply across task comma-chains.

To see how a given profile affects your project map, use the lein-pprint plugin:

$ lein pprint
{:compile-path "/home/phil/src/leiningen/lein-pprint/classes",
 :group "lein-pprint",
 :source-path ("/home/phil/src/leiningen/lein-pprint/src"),
 :dependencies
 ([org.clojure/tools.nrepl "0.0.5" :exclusions [org.clojure/clojure]]
  [clojure-complete "0.1.4" :exclusions [org.clojure/clojure]]
  [org.thnetos/cd-client "0.3.3" :exclusions [org.clojure/clojure]]),
 :target-path "/home/phil/src/leiningen/lein-pprint/target",
 :name "lein-pprint",
 [...]
 :description "Pretty-print a representation of the project map."}

Leiningen Plugins

Leiningen supports plugins which may contain both new tasks and hooks that modify behaivour of existing tasks. See the plugins wiki page for a full list. If a plugin is needed for successful test or build runs, (such as lein-tar) then it should be added to :plugins in project.clj, but if it's for your own convenience (such as swank-clojure) then it should be added to the :plugins list in the :user profile from ~/.lein/profiles.clj. The plugin guide explains how to write plugins.

FAQ

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

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: 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: I want to hack two projects in parallel, but it's annoying to switch between them.
A: 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, and the copy in checkouts will take precedence over the dependency declared in project.clj. Note that this is not a replacement for listing the project in :dependencies; it simply supplements that for convenience.

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 lein help sample for details.

Q: What does java.lang.NoSuchMethodError: clojure.lang.RestFn.<init>(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: Set the $http_proxy environment variable in Leiningen 2.x. For Leiningen 1.x versions, see the instructions for configuring a Maven proxy.

Q: What can be done to speed up launch?
A: The main delay involved in Leiningen comes from starting the JVM. Most people use a development cycle that involves keeping a single process running for as long as you're working on that project. Depending on your editor you may be able to do this via its Clojure integration. (See swank-clojure or VimClojure, for example.) Otherwise you can use the basic lein repl.

Q: Still too slow; what else can make startup faster?
A: If you are running an older version of Leiningen (before 1.7) you can export LEIN_JVM_OPTS=-XX:+TieredCompilation to improve boot time. This requires Hotspot version 20 or newer. On newer versions of Leiningen it is enabled automatically.

Q: I don't have access to stdin inside my project.
A: This is a limitation of the JVM's process-handling methods; none of them expose stdin correctly. 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.

Contributing

Please report issues on the Github issue tracker or the mailing list. Personal email addresses are not appropriate for bug reports. See the readme for the leiningen-core library and doc/PLUGINS.md for more details on how Leiningen's codebase is structured. Design discussions also occur in the #leiningen channel on Freenode.

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 use your judgment regarding potentially-destabilizing work and 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.

Building

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.

For the master branch, use Leiningen 1.x to run lein install in the leiningen-core subproject directory. When the dependencies change you will also have to do rm .lein-classpath in the project root.

Once you've done that, symlink bin/lein to somewhere on your $PATH, usually as lein2 in order to keep it distinct from your existing installation.

If you want to develop on the 1.x branch, this should be unnecessary.

License

Source Copyright © 2009-2012 Phil Hagelberg, Alex Osborne, Dan Larkin, and 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.