leiningen/TUTORIAL.md

# Tutorial

For those of you new to the JVM who have never touched Ant or Maven in
anger: don't panic. Leiningen is designed with you in mind. This
tutorial will help you get started and explain Leiningen's take on
project building and JVM-land dependency management.

## Creating a Project

We'll assume you've got Leiningen installed as per the
[README](http://github.com/technomancy/leiningen/blob/master/README.md). 
Generating a new project is easy:

    $ lein new myproject

    Created new project in: myproject

    $ cd myproject
    $ tree
    .
    |-- project.clj
    |-- README
    |-- src
    |   `-- myproject
    |       `-- core.clj
    `-- test
        `-- myproject
            `-- core_test.clj

Here we've got your project's README, a src/ directory containing
implementation code, a test/ directory, and a project.clj file which
describes your project to Leiningen. The src/myproject/core.clj file
corresponds to the myproject.core namespace.

Note that we use that instead of just myproject since single-segment
namespaces are discouraged in Clojure. Also the file
test/myproject/core_test.clj corresponds with the myproject.core-test
namespace--you need to remember to replace dashes in namespace names
with underscores in file names on disk since the JVM has trouble
loading files with dashes in the name.

## Packaging

You can package your project up now, even though at this stage it's
fairly useless:

    $ lein jar

    Created ~/src/myproject/myproject-1.0.0-SNAPSHOT.jar

Libraries for the JVM are packaged up as .jar files, which are
basically just .zip files with a little extra JVM-specific metadata.
They usually contain .class files (JVM bytecode) and .clj source
files, but they can also contain other things like config
files. Leiningen downloads them from remote Maven repositories for
you.

## project.clj

    $ cat project.clj

    (defproject myproject "1.0.0-SNAPSHOT"
      :description "FIXME: write"
      :dependencies [[org.clojure/clojure "1.1.0"]
                     [org.clojure/clojure-contrib "1.1.0"]])

Fill in the :description with a short paragraph so that your project
will show up in search results once you upload to Clojars (as
described below). At some point you'll need to flesh out the README
too, but for now let's skip ahead to setting :dependencies.  Note that
Clojure is just another dependency here. Unlike most languages, it's
easy to swap out any version of Clojure. If you're using Clojure
Contrib, make sure that version matches the Clojure version.

If you've got a simple pure-clojure project, you will be fine with the
default of depending only on Clojure and Contrib, but otherwise you'll
need to list other dependencies.

## Dependencies

[Clojars](http://clojars.org) is the Clojure community's centralized
jar repository, and it's where you'll find Clojure dependencies for your
project. Each dependency even lists out the snippet you'll need to put
in your project.clj to use it. Let's take a look at what it would take
to add a library named Robert Hooke:

It's [available on Clojars](http://clojars.org/robert/hooke) with the
Leiningen dependency notation shown as below:

    [robert/hooke "1.0.1"]

* "robert" is called the "group-id"
* "hooke" is called the "artifact-id"
* "1.0.1" is the version of the jar file you require

For projects on Clojars, often the group-id is the same as the
artifact-id, in which case you may leave it out of the Leiningen
dependency notation. For Java libraries often a domain name is used as
the group-id. The group and artifact names and version at the top of
the defproject form in project.clj follows the same rules.

Java libraries can be found by searching
[Jarvana](http://jarvana.com), though you'll need to translate the
Maven XML notation into
Leiningen's. [Lucene](http://jarvana.com/jarvana/archive-details/org/apache/lucene/lucene-core/3.0.2/lucene-core-3.0.2.jar)
is a typical example:

    <dependency>
       <groupId>org.apache.lucene</groupId>
       <artifactId>lucene-core</artifactId>
       <version>3.0.2</version>
    </dependency>

This becomes:

    [org.apache.lucene/lucene-core "3.0.2"]

Sometimes versions will end in "-SNAPSHOT". This means that it is not
an official release but a development build. Relying on snapshot
dependencies is discouraged but is sometimes necessary if you need bug
fixes etc. that have not made their way into a release yet. Adding a
snapshot dependency to your project will cause Leiningen to actively
go seek out the latest version of the dependency once a day when you
run <tt>lein deps</tt>, (whereas normal release versions are cached in
the local repository) so if you have a lot of snapshots it will slow
things down.

Speaking of the local repository, all the dependencies you pull in
using Leiningen or Maven get cached in $HOME/.m2/repository since
Leiningen uses the Maven API under the covers. You can install the
current project in the local repository with this command:

    $ lein install

    Wrote pom.xml
    [INFO] Installing myproject-1.0.0-SNAPSHOT.jar to ~/.m2/repository/myproject/myproject/1.0.0-SNAPSHOT/myproject-1.0.0-SNAPSHOT.jar

Generally Leiningen will fetch your dependencies when they're needed,
but if you have just added a new dependency and you want to force it
to fetch it, you can do that too:

    $ lein deps

    Copying 4 files to ~/src/myproject/lib
    Copied :dependencies into ~/src/myproject/lib.

Dependencies are downloaded from Clojars, the central Maven (Java)
repository, the [official Clojure build
server](http://build.clojure.org), and any other repositories that you
add to your project.clj file. See :repositories in
[sample.project.clj](http://github.com/technomancy/leiningen/blob/master/sample.project.clj).

If you've confirmed that your project will work with a number of
different versions of a given dependency, you can provide a range
instead of a single version:

    [org.clojure/clojure "[1.1,1.2]"] ; <= will match 1.1.0 through 1.2.0.

See [Maven's version range
specification](http://maven.apache.org/plugins/maven-enforcer-plugin/rules/versionRanges.html)
for details.

## Dev Dependencies

Sometimes you want to pull in dependencies that are really only for
your convenience while developing; they aren't strictly required for
the project to function. Leiningen calls these
:dev-dependencies. They're listed in project.clj alongside regular
dependencies and downloaded when you run <tt>lein deps</tt>, but they
are not brought along when another project depends on your
project. Using [swank-clojure](http://github.com/technomancy/swank-clojure)
for Emacs support would be a typical example; you may not want it
included at runtime, but it's useful while you're hacking on the project.

## Writing the Code

This is the part Leiningen can't really help you with; you're on your
own here. Well--not quite. Leiningen can help you with running your
tests.

    $ lein test

    Testing myproject.core-test
    FAIL in (replace-me) (core_test.clj:6)
    No tests have been written.
    expected: false
      actual: false
    Ran 1 tests containing 1 assertions.
    1 failures, 0 errors.

Of course, we haven't written any tests yet, so we've just got the
skeleton failing tests that Leiningen gave us with <tt>lein
new</tt>. But once we fill it in the test suite will become more
useful. Sometimes if you've got a large test suite you'll want to run
just one or two namespaces at a time:

    $ lein test myproject.parser-test

    Testing myproject.parser-test
    Ran 2 tests containing 10 assertions.
    0 failures, 0 errors.

Because it must start a new process, lein test is not a good solution
for a development cycle that involves running the tests often. For
that you would either need to look into better editor integration (see
[clojure-test-mode](http://github.com/technomancy/clojure-mode) for
Emacs) or keep a repl open and call <tt>run-tests</tt> from there as
you work.

## Compiling

If you're lucky you'll be able to get away without doing any AOT
(ahead-of-time) compilation. But there are some Java interop features
that require it, so if you need to use them you should add an :aot
option into your project.clj file. It should be a seq of namespaces
you want AOT-compiled. Again, the
[sample.project.clj](http://github.com/technomancy/leiningen/blob/master/sample.project.clj)
has example usage.

Like dependencies, this should happen for you automatically, but if
you need to force it you can:

    $ lein compile

    Compiling myproject.core

For your code to compile, it must be run. This means that you
shouldn't have any code with side-effects in the top-level. If you
have code that should run on startup, place it in a <tt>-main</tt>
function as explained below under "Uberjar".

## What to do with it

Generally speaking, there are three different goals that are typical
of Leiningen projects:

* An application you can distribute to end-users
* A library
* A server-side application

For the first, you will want to build an uberjar. For libraries, you
will want to have them published to a repository like Clojars. For
server-side applications it varies as described below.

### Uberjar

The <tt>uberjar</tt> task is used to create a standalone, executable
jar. For this to work you'll need to specify a namespace as your :main
in project.clj. By this point our project.clj file should look like this:

    (defproject myproject "1.0.0-SNAPSHOT"
      :description "This project is MINE."
      :dependencies [[org.clojure/clojure "1.1.0"]
                     [org.clojure/clojure-contrib "1.1.0"]
                     [org.apache.lucene/lucene-core "3.0.2"]
                     [robert/hooke "1.0.1"]]
      :main myproject.core)

The namespace you specify will need to contain a <tt>-main</tt>
function that will get called when your standalone jar is run. This
namespace should have a <tt>(:gen-class)</tt> declaration in the
<tt>ns</tt> form at the top. The <tt>-main</tt> function will get
passed the command-line arguments. Let's try something simple in
src/myproject/core.clj:

    (ns myproject.core
      (:gen-class))

    (defn -main [& args]
      (println "Welcome to my project! These are your args:" args))

Now we're ready to generate your uberjar:

    $ lein uberjar
    Cleaning up
    Copying 4 files to /home/phil/src/leiningen/myproject/lib
    Created ~/src/myproject/myproject-1.0.0.jar
    Including myproject-1.0.0-SNAPSHOT.jar
    Including clojure-contrib-1.1.0.jar
    Including hooke-1.0.1.jar
    Including clojure-1.1.0.jar
    Including lucene-core-3.0.2.jar
    Created myproject-1.0.0-standalone.jar

This creates a single jar file that contains the contents of all your
dependencies. Users can run it with a simple <tt>java</tt> invocation,
or on some systems just by double-clicking the jar file.

    $ java -jar myproject-1.0.0-standalone.jar Hello world.
    Welcome to my project! These are your args: (Hello world.)

You can run a regular (non-uber) jar with the <tt>java</tt>
command-line tool, but that requires constructing the classpath
yourself, so it's not a good solution for end-users.

### 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
repository or get it into Maven central, the easiest way is to publish
it at [Clojars](http://clojars.org). Once you have created an account
there, publishing is easy:

    $ lein jar && lein pom
    $ scp pom.xml myproject-1.0.0.jar clojars@clojars.org:

Once that succeeds it will be available as a package on which other
projects may depend. You will need to have permission to publish to
the project's group-id under Clojars, though if that group-id doesn't
exist yet then Clojars will automatically create it and give you
permissions.

Sometimes you'll need to publish libraries that you don't directly
maintain, either because the original maintainer hasn't published it
or because you need some bugfixes that haven't been applied upstream
yet. In this case you don't want to publish it under its original
group-id, since this will prevent the true maintainer from using that
group-id once they publish it. You should use "org.clojars.$USERNAME"
as the group-id instead.

### Server-side Projects

There are many ways to get your project deployed as a server-side
application. Simple programs can be packaged up as tarballs with
accompanied shell scripts using the [lein-release
plugin](http://github.com/technomancy/lein-release) and then deployed
using [chef](http://opscode.com/chef/),
[pallet](http://hugoduncan.github.com/pallet/), or other
mechanisms. Web applications may be deployed using the [lein-war
plugin](http://github.com/alienscience/leiningen-war). You can even
create [Hadoop
projects](http://github.com/ndimiduk/lein-hadoop). These kinds of
deployments are so varied that they are better-handled using plugins
rather than tasks that are built-in to Leiningen itself.

## That's It!

If you prefer a visual introduction, try the Full Disclojure
screencast on [project management](http://vimeo.com/8934942). Now go
start coding your next project!