# Leiningen
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
Leiningen bootstraps itself using the 'lein' shell script you
download, there is no separate 'install script'. It installs its
dependencies in $HOME/.m2/repository.
1. [Download the script](http://github.com/technomancy/leiningen/raw/stable/bin/lein).
2. Place it on your path and chmod it to be executable.
3. Run: <tt>lein self-install</tt>
This only works with stable versions of Leiningen; for development
versions see "Hacking" below.
On Windows you can download
[lein.bat](http://github.com/technomancy/leiningen/raw/stable/bin/lein.bat),
instead, though support on that platform is still experimental.
## Usage
## Configuration
Place a project.clj file in the project root that looks something like this:
Other keys accepted:
* :aot - a list of namespaces on which to perform AOT-compilation.
* :main - specify a namespace to use as main for an executable jar.
* :repositories - additional maven repositories to search for dependencies.
Specify this as a map of repo IDs to URLs.
* :source-path, :compile-path, :library-path, :test-path, :resources-path -
alternate paths for src/, classes/, lib/, resources/, and test/ directories.
* :jvm-opts - a list of arguments to pass to the project-level JVM
TODO: spin this list off into its own file and explain each option better
## FAQ
**Q:** How do you pronounce Leiningen?
**A:** It's LINE-ing-en. I think.
**Q:** What does this offer over [Lancet](http://github.com/stuarthalloway/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](http://www.defmacro.org/ramblings/lisp.html). Ant is
an interpreter for a [procedural language with a regrettable
syntax](http://blogs.tedneward.com/2005/08/22/When+Do+You+Use+XML+Again.aspx).
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](http://github.com/technomancy/leiningen/blob/master/INTRO.md)
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 groupId 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 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.<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. 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:** It looks like the classpath isn't honoring project.clj.
**A:** Leiningen runs many things in a subclassloader so it can
control the classpath and other things. Because of this, the
standard (System/getProperty "java.class.path") call will return the
classpath that Leiningen runs in, not the one that your project is
run in. Your project's classpath is stored in the user/*classpath* var.
**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, as demonstrated here:
**Q:** How should I pick my version numbers?
**A:** Use [semantic versioning](http://semver.org).
**Q:** What happened to [Corkscrew](http://github.com/technomancy/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. The maven-ant-tasks API is much more manageable.
## 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](http://clojars.org), which is a Clojure-specific repository
for open-source code. Once you have created an account there,
publishing is easy:
Once that succeeds it will be available for other projects to depend
on. Leiningen adds Clojars and [the Clojure nightly build
snapshots](http://build.clojure.org) to the default repositories.
## Hacking
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](http://groups.google.com/group/leiningen) 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](http://clojure.org/patches). Please
use standard indentation with no tabs, trailing whitespace, or lines
longer than 80 columns. If you've got some time on your hands, reading
this [style guide](http://mumble.net/~campbell/scheme/style.txt)
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 [complete list of known
issues](http://github.com/technomancy/leiningen/issues).
## License
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.