defmodule Binar do

  defmacro define_extractor(args) do
    signature_patterns =
      args
      |> Enum.map(fn item -> IO.inspect item end)
      |> Enum.map(fn {variable, sized} ->
        {:::, [], [{variable, [], __MODULE__}, {:size, [], [sized]}]}
      end)
      |> Enum.reduce([], fn (pat, acc) -> [ pat | acc ] end)
      |> Enum.reverse
    return_patterns =
        args
        |> Enum.map(fn {variable, _} -> {variable, {variable, [], __MODULE__}} end)
        |> Enum.reverse # not that it matters

    signature = {:<<>>, [], signature_patterns}
    returns = {:%{}, [], return_patterns}
    quote do
      def extract(unquote(signature)) do
        unquote(returns)
      end
    end
  end

end

defmodule Extractors do
  require Binar
  Binar.define_extractor(x: 8, y: 8)
  Binar.define_extractor(a: 24, b: 16, c: 8)
end

IO.inspect Extractors.extract("hi")
IO.inspect Extractors.extract("hellow")
IO.inspect Extractors.extract("no match here. crash.")