How about add optional dependencies?

defmodule MyApp.MixFile do
  use Mix.Project

  def project do
    [
      app: :my_app,
      version: "0.1.0",
      elixir: "~> 1.4",
      elixirc_paths: elixirc_paths(Mix.env),
      build_embedded: Mix.env == :prod,
      start_permanent: Mix.env == :prod,
      aliases: aliases(),
      deps: deps(),
      optional_deps: optional_deps(),
    ]
  end

  # ...
end

and example code:

defmodule MyApp.Things.Special do
  @behavior MyApp.Thing
  # don't compile this module if we don't have an optional library
  @module_depends_on: :optional_library

  def example(), do: OptionalLibrary.example()

  # or:

  # don't compile this method if we don't have an another optional library
  @depends_on: :another_optional_library
  def something(), do: AnotherOptionalLibrary.something()
end

mix deps.get should ask if we want an optional dependency in our setup.

Here I think like Gentoo user. In this source based distribution we can choose USE flags so we can compile package for example: with sound support and without bluetooth support. Of course it’s only simple example that probably should be more explained and discussed.

What do you think about my idea? Maybe instead of choosing packages we should choose something like USE flags, so we could provide easy pre-setup for deployment administrators?