gistfile1.ex

defmodule Foo do
  
  defp make_guard(:any), do: true
  defp make_guard(token) do
    quote do
      head == token
    end
  end
  

  defmacro fast_forward(until, mode, body) do
    guard = make_guard(until)
    quote bind_quoted: [
      mode: mode,
      body: Macro.escape(body, unquote: true),
      guard: Macro.escape(guard, unquote: true)
    ] do
      def ff(<<head::binary-size(1), rest::binary>>, mode) when unquote(guard) do
        unquote(body[:do])
      end
    end
  end

end