We’ve been using cowboy for some time at work for writing small REST-endpoints and lately Elm as been added to the mix to provide front-ends for those endpoints.

At the beginning the work flow was to compile the Elm assets, copy the result to the priv/ in our cowboy projects and let cowboy serve them from there. That work flow feelt a bit clunky. If you made changes to both the cowboy handlers and the Elm code you would have to run both elm make and mix compile.

To make that work flow a bit more smooth we have integrated the Elm compile cycle into mix compile. You can see the end result here: elm-compile.

If you’re just looking an easy way to integrate Elm into your mix projects just stop reading now and follow the link above. If on the other hand you’re interested in how to add external compilers to a mix project read on.

Two approaches

There are two approaches to adding Elm compilation to your Mix project: Override mix compile with an alias or add Elm compilation to the list of compilers used by mix compile.

An aside on writing your own Mix tasks

Both approaches will require us to write our own Mix tasks. Luckily thats easy.

A Mix task can be defined by simply using Mix.Task in a module starting with Mix.Tasks. and defining the run/1 function: – Mix.Task behaviour docs

It basically boils down to createing a module in the Mix.Task namespace, adding use Mix.Task and implementing run(args) as required by the Mix.Task behaviour. You can find an example here .

Using aliases

If you’ve already encountered Mix aliases if you’ve ever created a Phoenix. If you look at the mix.exs file in a Phoenix project you’ll see the following:

  def project do
    [app: .....
     version: .....
     elixir: .....
     elixirc_paths: .....
     aliases: aliases(),
     deps: deps()]

  defp aliases do
    ["ecto.setup": ["ecto.create", "ecto.migrate", "run priv/repo/seeds.exs"],
     "ecto.reset": ["ecto.drop", "ecto.setup"],
     "test": ["ecto.create --quiet", "ecto.migrate", "test"]]

Here they are using aliases to create convenience tasks like ecto.setup that creates, migrate and seeds the database.

But you can also use aliases to override existing Mix tasks. So given that we have created a Mix task in the ‘Mix.Tasks.Compile.Elm’ module you could create the following alias:

defp aliases do
   ["compile": ["compile", "compile.elm"]]

We are in essence telling mix to first run its normal compile and afterwards run the Elm compile task. Mix aliases are awesome and can be really helpful in smoothing out your work flow. But overriding the compile task is perhaps not the best use of aliases.

Adding a compiler

The other alternative is to place you task in the Mix.Tasks.Compile.Elm namespace and then adding it to the list of Mix compilers.

Again: Take a look at the mix.exs file from a Phoenix project. Under the project definition you’ll spot the following: compilers: [:phoenix, :gettext] ++ Mix.compilers.

They are adding :phoenix and :gettext to the compilers run by mix compile. Modify that to compilers: Mix.compilers ++ [:elm] and you’re good to go.