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 a build tool for Clojure designed to not set your hair on fire.

Building Clojure projects with tools designed for Java can be an exercise in frustration. If you use Ant, you end up copying around a lot of the same tasks around between XML files on all your projects; there's a lot of repetition. Maven avoids repetition, but provides very little transparency into what's really going on behind the scenes and forces you to become a Maven expert to script a nontrivial build. Either way you end up writing far more XML than is necessary.

With Leiningen, your build is described using Clojure. You can put any code you like in your project.clj file; the only requirement is that it includes a call to defproject. You can define your own tasks in there if you need to, but the majority of projects should be able to get by on the tasks that are provided with Leiningen. If you do find a common task that you need to add, you can implement it as a plugin rather than copying and pasting among each of your projects.

Installation

Download the script: http://github.com/technomancy/leiningen/raw/master/bin/lein
Place it on your path and chmod it to be executable.
Run: lein self-install

Usage

$ lein deps # install dependencies in lib/

$ lein test [PRED] # run the project's tests, optionally filtered on PRED

$ lein compile # ahead-of-time compile into classes/

$ lein repl # launch a REPL with the project classpath configured

$ lein clean # remove all build artifacts

$ lein jar # create a jar of the project

$ lein uberjar # create a standalone jar that contains all dependencies

$ lein pom # output a pom.xml file for interop with Maven

$ lein install # install in local repo (currently requires mvn)

$ lein help [TASK] # show a list of tasks or help for a given TASK

$ lein new NAME # generate a new project skeleton

Configuration

Place a project.clj file in the project root that looks something like this:

(defproject leiningen "0.5.0-SNAPSHOT"
  :dependencies [[org.clojure/clojure "1.1.0-alpha-SNAPSHOT"]
                 [org.clojure/clojure-contrib "1.0-SNAPSHOT"]
                 [ant/ant-launcher "1.6.2"]
                 [org.apache.maven/maven-ant-tasks "2.0.10"]]
  :dev-dependencies [[org.clojure/swank-clojure "1.0"]])

Other keys you can set are :namespaces to compile if you don't want all of them AOT'd as well as a :main namespace for building executable jars.

Currently Leiningen can only be used to compile projects that use the same version of Clojure as it uses, though this restriction should go away soon.

FAQ

Q: How do you pronounce Leiningen?
A: It's LINE-ing-en. I think.

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 the actual task execution cycles 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're able to write it with a more pleasing syntax, it's not so bad.

Q: What happened to Corkscrew?
A: I tried, but I really couldn't make the wine metaphor work. That, and the Plexus Classworlds container was an ornery beast causing much frustration.

Q: What about Windows?
A: Patches welcome.

Publishing

If your project is a library and you would like others to be able to use it as a dependency in their projects, you will need to get it into a public repository. While it's possible to maintain your own or get it into Maven central, the easiest way is to publish it at Clojars, which is a Clojure-specific repository for open-source code. Once you have created an account there, publishing is easy:

$ lein pom
$ scp pom.xml $PROJECT.jar clojars@clojars.org:

Once that succeeds it will be available for other projects to depend on.

Hacking

Working on the Leiningen codebase has a few unique challenges since there's a bit of a chicken-and-egg bootstrap problem. To go from a clean checkout to a working environment, the following steps are necessary:

Place bin/lein on your $PATH somewhere.
Do a self-install of leiningen (from outside the checkout tree).
Place ~/.m2/repository/leiningen/leiningen/$VERSION/leiningen-$VERSION.jar in lib.
Invoke "lein compile" followed by "lein deps".
Remove .leiningen.jar from lib.
Invoke "lein uberjar", and place the jar in ~/.leiningen.jar for future use.

The mailing list and the #leiningen or #clojure channels on Freenode are the best places to bring up questions or suggestions. Contributions are preferred as either Github pull requests or using "git format-patch" as described at http://clojure.org/patches.

Please use standard indentation with no tabs or trailing whitespace.

Leiningen is extensible, you can define new tasks in plugins. Add your plugin as a dev-dependency of your project, and you'll be able to call "lein $YOUR_COMMAND". See the lein-swank directory for an example of a plugin.

License

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

Distributed under the Eclipse Public License, the same as Clojure uses. See the file COPYING.

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