Dynamically generate protocol impl w/no runtime overhead

enumerator.ex

defmodule Fstop.Model do

  defmacro __using__(_) do
    quote do
      use Ecto.Model
      @after_compile {Fstop.Enumerator, :generate}
    end
  end
end

defmodule Fstop.Enumerator do

  def generate(env, _bytecode) do
    quoted = define_enumerable(env.module)
    write_beam(quoted)
  end

  defp define_enumerable(module) do
    quote do
      defimpl Enumerable, for: unquote(module) do
        def count(v), do: Map.size(v) - 2
        def member?(v, :__struct__), do: {:ok, false}
        def member?(v, :__meta__), do: {:ok, false}
        def member?(v, key) do
          {:ok, Map.has_key?(v, key)}
        end
        # Too lazy to write this one :(
        def reduce(v, acc, fun) do
          raise RuntimeException, "Not implemented"
        end
      end
    end
  end

  defp write_beam(quoted) do
    [{module, bytecode}] = Code.compile_quoted(quoted)
    file_name = "#{Atom.to_string(module)}.beam"
    beam_path = Path.join(Mix.Project.compile_path, file_name)
    File.write!(beam_path, bytecode, [:write])
  end

end