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An Elixir build tool inspired by Clojure's Leiningen

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Mix is a build tool and development aid for the Elixir programming language. It is heavily inspired by the Leiningen build tool for Clojure and is written by one of its contributors.


In Mix, a task is simply an Elixir module named with a Mix.Tasks prefix. For example, the compile task is a module named Mix.Tasks.Compile.

Here is a simple example task:

defmodule Mix.Tasks.Hello do
  @behavior Mix.Task
  @shortdoc "This is short documentation, see."
  @moduledoc """
  A test task.
  def run(_) do
    IO.puts "Hello, World!"

This defines a task called hello. In order to make it a task, it defines a run function that takes a single argument that will be a list of binary strings which are the arguments that were passed to the task on the command line or from another task calling this one.

When you invoke mix hello, this task will run and print Hello, World!. Mix uses its first argument to lookup the task module and execute its run function.

You're probably wondering why we have a moduledoc and shortdoc. Mix has a help task for listing tasks and providing documentation of them. Let's try it out:

~/code/mix(master) $ mix help
Available tasks:

iex: Start iex with your project's settings.
compile: Compile Elixir source files.
test: Run a project's tests.
clean: Delete compile path and target.
help: Print help information for tasks.
codepath: Prints the current load path.
hello: This is short documentation, see.

This lists all of the tasks that Mix can currently see. See the short form documentation beside them? That is why we have shortdoc. It is a one line string of documentation to be printed alongside the task name in mix help.

Let's try looking at the long-form documentation of a task:

~/code/mix(master) $ mix help help
If given a task name, prints the documentation for that task.
If no task name is given, prints the short form documentation
for all tasks.

  task: Print the @doc documentation for this task.
  none: Print the short form documentation for all tasks.

Note that none of this is anything special. This is nothing more than the moduledoc of the Mix.Tasks.Help module. It is a good example of conventions to use when documenting Mix tasks.

Project Configuration

Like Leiningen, tasks are written and projects are configured using the same language they are being written for. You write your tasks in Elixir and you configure your projects in Elixir. Every Elixir project should have a mix.exs file that contains arbitrary Elixir code. This code can require other files, define tasks, and it should at the very least provide a basic project configuration. Here is a sample mix.exs file:

defmodule Mix.Project do
  def project do
    [name: "mix",
     version: "0.1.0",
     compile_options: [ignore_module_conflict: true, docs: true]]

Your project definition contains all of your project configuration. In this case, we're letting Mix know that the name of the project it is running in is mix (I know, how meta of me), is at version 0.1.0, that it should ignore module conflicts when compiling, and that it should compile with docs enabled (necessary for mix help to work).

There are a lot of configuration options you can set and any task is free to invent its own. Mix's configuration is simplistic and concise because Mix has sane defaults for options that most projects will use. Examples:

  • :compile_path = "ebin/"
  • :source_paths = ["lib/"]
  • :test_paths = ["test/"]

... and more. This file is the first thing that Mix loads, so you can do some pretty cool stuff in there.

What Mix Can Help You With

Mix does a lot of things out of the box.


Mix can handle your project's compilation for you. If you're a typical Elixir project that uses lib/ for source files and ebin/ for compiled beam files, you don't even have to provide any compilation-specific setup. Just throw a simple Mix.Project in your mix.exs (possibly with just name and version) and run mix compile. That should be all you need to do.

As a bonus, Mix will only compile when necessary. If the compile task gets invoked by you or by some other task that runs it, it will only compile if source files have actually changed. See mix help compile for more information.

You can, of course, customize this compilation however you want. If you use some other path for source files or you use multiple paths, Mix can help you.


Mix handles testing for you. When you run mix test, it will start up ExUnit and run all of the tests in test/ and its subdirectories. By default, it only runs files that end with _test.exs, so you can have test helpers and such in the same place as your tests. This is configurable along with the location of tests itself. You can even have multiple directories with tests. See mix help test for more information.


Have you gotten sick of typing iex -pa ebin/ yet? Yeah, so did I. You can run mix iex and you'll instantly have an iex session with your :compile_path on the code path. More importantly, since Mix runs your mix.exs file at startup, anything you do there is available to this session as well. Get creative.

Writing Tasks

Tasks are simple things in Mix. They are just modules named a certain way that have a run/1 function. They don't even have to have documentation, but I'll punch you in the face if you don't write some for your own tasks.

Tasks only need to be on the load path in order to be found. For example, tasks compiled to beam files will be found as long as they're on the code path. Furthermore, any tasks defined in mix.exs or any files that it requires/loads are also available for use. They'll even be picked up by mix help.

Look at existing tasks for examples. The hello task is extremely simple and a great place to start.

Namespaced Tasks

While tasks are simple, they can be used to accomplish complex things. Since they are just Elixir code, anything you can do in normal Elixir you can do in Mix tasks. This has been known to cause insanity in users of the build tool and make them do insane things that nobody would do in any other build tool. As such, they may be inclined to write a 'plugin'.

A Mix 'plugin' isn't anything special. As a matter of fact, it is just a collection of (or even just one really awesome) task modules. You can distribute tasks however you want just like normal libraries and thus they can be reused in many projects.

So, what do you do when you have a whole bunch of related tasks? If you name them all like foo, bar, baz, etc, eventually you'll end up with conflicts with other people's tasks. To prevent this, Mix allows you to namespace tasks.

Let's assume you have a bunch of tasks for working with MongoDB.

defmodule Mix.Tasks.Mongodb do
  defmodule Dostuff do

  defmodule Dootherstuff do

Now you'll have two different tasks under the modules Mix.Tasks.Mongodb.Dostuff and Mix.Tasks.Mongodb.Dootherstuff respectively. You can invoke these tasks like so: mix mongodb.dostuff and mix mongodb.dootherstuff. Pretty cool, huh?

You should use this feature when you have a bunch of related tasks that would be unwieldly if named completely independently of each other. If you have a few unrelated tasks, go ahead and name them however you like.

Lots To Do

Mix is still very much a work in progress. Refer to the issue tracker for a list of planned features/ideas. Please add issues for anything you'd like to see in Mix and feel free to contribute.

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