defmodule Dec do

  alias Dec.Pipeline, as: Pipeline

  defmacro __using__(_) do
    quote do
      import Dec, only: [dec: 2]
    end
  end

  defmacro dec(head, do: block) do
    {:__block__, _, [dec | statements]} = block
    dec_fun = make_dec_fn(dec)
    base_fun = {:fn, [], [{:->, [], [[], {:__block__, [], statements}]}]}
    expr = quote do
      unquote(dec_fun).(unquote(base_fun)).()
    end

    quote do
      Kernel.def(unquote(head), do: unquote(expr))
    end
  end

  def make_dec_fn({:|>, _, _} = ppln) do
    {dec, items} = ppln |> Pipeline.split()
    {:dec, arg1, arg2} = dec
    arg_name = :base_function
    f = {arg_name, arg1, arg2}
    ppln = Pipeline.combine(items, f)
    {:fn, [], [{:->, [], [[f], ppln]}]}
  end

  def make_dec_fn({:@, _, [{:dec, _, [{:fn, _, _} = dec_fun]}]}) do
    dec_fun
  end

  defmacro make_dec_fun(do: ppln) do
    # for interactive testing only
    make_dec_fn(ppln)
  end
end