"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.
Leiningen bootstraps itself using the 'lein' shell script you download, there is no separate 'install script'. It installs its dependencies in $HOME/.m2/repository.
- Download the script.
- Place it on your path and chmod it to be executable.
- Run: lein self-install
This only works with stable versions of Leiningen; for development versions see "Hacking" below.
On Windows you can download lein.bat, instead, though support on that platform is still experimental.
$ lein new NAME # generate a new project skeleton $ lein deps # install dependencies in lib/ $ lein test [TESTS] # run the tests in the TESTS namespaces, or all tests $ 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 install # install in local repository
These are the most commonly-used tasks; you can use "lein help" to see a complete list. "lein help $TASK" will show the usage for a specific one.
Place a project.clj file in the project root that looks something like this:
(defproject leiningen "0.5.0-SNAPSHOT" :description "A build tool designed not to set your hair on fire." :url "http://github.com/technomancy/leiningen" :dependencies [[org.clojure/clojure "1.1.0] [org.clojure/clojure-contrib "1.1.0"] [ant/ant-launcher "1.6.2"] [org.apache.maven/maven-ant-tasks "2.0.10"]] :dev-dependencies [[org.clojure/swank-clojure "1.2.1"]])
The "lein new" task will generate a project skeleton with an appropriate starting point from which you can work. See the file sample.project.clj for a detailed listing of all the configuration options that Leiningen knows about.
Q: How do you pronounce Leiningen?
A: It's LINE-ing-en. I think.
Q: What does this offer over Lancet?
A: Lancet is more of a library than a build tool. It doesn't predefine any tasks apart from what Ant itself offers, so there is nothing Clojure-specific in it. Leiningen builds on Lancet, but takes things further. In addition, it includes some Maven functionality for dependencies.
Q: But Maven is terrifying!
A: That's not a question. Anyway, Leiningen only uses the dependency resolution parts of Maven, which are quite tame. For the actual task execution cycles it uses Ant under the covers via Lancet.
Q: But Ant is terrifying!
A: That's true. Ant is an interpreter for a procedural language with a regrettable syntax. But if you treat it as a standard library of build-related functions and are able to write it with a more pleasing syntax, it's not bad.
Q: What's a group ID? How do snapshots work?
A: See the intro for background on JVM dependency concepts.
Q: What if my project depends on jars that aren't in any repository?
A: Open-source jars can be uploaded to Clojars (see "Publishing" below), though be sure to use the group-id of "org.clojars.$USERNAME" in order to avoid conflicts and to allow the original authors to claim it in the future once they get around to uploading. Alternatively you can do a one-off install into your local repository in ~/.m2 with Maven for Java libs or "lein install" for Clojure libs.
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. If you depend on contrib, make sure the contrib version matches the Clojure version. Also note 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: 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: 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: How should I pick my version numbers?
A: Use semantic versioning.
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 jar && lein pom $ scp pom.xml $PROJECT.jar firstname.lastname@example.org:
Once that succeeds it will be available for other projects to depend on. Leiningen adds Clojars and the Clojure nightly build snapshots to the default repositories.
You'll need to bootstrap using a stable release before you can hack on Leiningen. Grab the stable bin script (linked under "Installation" above), put it on your $PATH as "lein-stable", and do "lein-stable self-install". Then run "lein-stable deps" in your checkout. When that finishes, symlink bin/lein from your checkout to your path. This will make "lein" run from your checkout while "lein-stable" uses the jar self-installed in ~/.m2.
The mailing list and the leiningen or clojure channels on Freenode are the best places to bring up questions or suggestions. If you're planning on adding a feature or fixing a nontrivial bug, please discuss it first to avoid duplicating effort. If you haven't discussed it on the mailing list, please include in your pull request details of what problem your patch intendeds to solve as well as the approach you took.
Contributions are preferred as either Github pull requests or using "git format-patch" and the mailing list as is requested for contributing to Clojure itself. Please use standard indentation with no tabs, trailing whitespace, or lines longer than 80 columns. See this post on submitting good patches for some tips. If you've got some time on your hands, reading this style guide wouldn't hurt either.
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 file PLUGINS.md for details.
See the complete list of known issues.
Copyright (C) 2009-2010 Phil Hagelberg, Alex Osborne, and Dan Larkin
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.