wrappers.ex

defmodule Wrapper do
  defmacro apply(call, [do: block]) do
    {wrapper, _opts, args} = call
    func = makefun(do: block)
    {wrapper, [], [:wraps, func] ++ args}
  end

  def makefun(do: block), do:
    {:fn, [], [{:->, [], [[], block]}]}

end



defmodule Ex do
  require Wrapper

  # a wrapper that increments each of 2 values returned
  def incr(:wraps, f, x, y) do
    {a, b} = f.()
    {a+1, b+1}
  end

  def f(x, y) do
    incr(x, y) |> Wrapper.apply do
      {x, y}
    end
  end

  # a convenience macros for incr wrapper
  defmacro incr(x,y, do: block) do
    {wrapper, _opts, _args} = quote do Ex.incr end
    func = Wrapper.makefun(do: block)
    {wrapper, [], [:wraps, func, x, y]}
  end
  def g(x, y) do
    incr(x, y) do
      {x, y}
    end
  end


end