Elixir module inheritance

enhanced_extension.ex

defmodule Extension do

  defmacro extends(module) do
    # As above...
  end

  defmacro implements(module, protocol: protocol) do
    quote do
      defimpl unquote(protocol), for: unquote(module) do
        import Extension
        extends unquote(module)
      end
    end
  end
end

enhanced_test.exs

Code.require_file "../test_helper.exs", __FILE__

import Extension

defprotocol Foo do
  def direct(this)
  def wrapper(this, arg)
end

defmodule Bar do
  @behaviour Foo
  def direct(_this) do
    :foo
  end
  def wrapper(this, arg) do
    [ Foo.direct(this), arg ]
  end
end

implements Bar, protocol: Foo

defmodule Baz do
  @behaviour Foo
  import Extension
  extends Bar
  def direct(_this) do
    :bar
  end
end

implements Baz, protocol: Foo

defmodule ExtensionTest do

  use ExUnit.Case, async: true

  test "foo" do
    assert Foo.direct({Bar}) == :foo
    assert Foo.wrapper({Bar}, 0) == [ :foo, 0 ]
  end

  test "bar" do
    assert Foo.direct({Baz}) == :bar
    assert Foo.wrapper({Baz}, 0) == [ :bar, 0 ]
  end

end

extension.ex

defmodule Extension do
  defmacro extends(module) do
    module = Macro.expand(module, __CALLER__)
    functions = module.__info__(:functions)
    signatures = Enum.map functions, fn { name, arity } ->
      args = if arity == 0 do
               []
             else
               Enum.map 1 .. arity, fn(i) ->
                 { binary_to_atom(<< ?x, ?A + i - 1 >>), [], nil }
               end
             end
      { name, [], args }
    end
    quote do
      defdelegate unquote(signatures), to: unquote(module)
      defoverridable unquote(functions)
    end
  end
end

extension_test.exs

Code.require_file "../test_helper.exs", __FILE__

defmodule Foo do

  def direct do
    :foo
  end

  def wrapper do
    direct
  end

end

defmodule Bar do

  import Extension

  extends Foo

  def direct do
    :bar
  end

end

defmodule Baz do

  import Extension

  extends Bar

  def direct do
    :baz
  end

  def wrapper do
    direct
  end

end

defmodule TodoxTest do

  use ExUnit.Case, async: true

  test "foo" do
    assert Foo.direct == :foo
    assert Foo.wrapper == :foo
  end

  test "bar" do
    assert Bar.direct == :bar
    assert Bar.wrapper == :bar # Actually returns :foo
  end

  test "baz" do
    assert Baz.direct == :baz
    assert Baz.wrapper == :baz
  end

end