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 = Enum.map 0..arity, fn(i) ->
        { binary_to_atom(<< ?x, ?A + i >>), [], nil }
      end

      { name, [], tl(args) }
    end

    quote do
      defdelegate unquote(signatures), to: unquote(module)
      defoverridable unquote(functions)
    end
  end
end

defmodule MyList do
  import Extension

  extends List

  def flatten(_) do
    IO.puts "Overriden flatten"
  end
end

IO.inspect MyList.delete([1,2,3], 3)
MyList.flatten([1,2,3])

https://gist.github.com/josevalim/da654648b280f2707f2c