def_guard.ex

defmodule DefGuard do

  defp defguardp(defwith, name, meta = [{:when, _}, {:args, args} | _]) do
    meta = meta |> Macro.escape
    quote do
      @expects {unquote(name), unquote(meta)}
      unquote(defwith)(unquote(:"#{name}?")(unquote_splicing(args))) do
        values = unquote(args |> Enum.map(fn value = {name, _, nil} -> {name, value} end))
        unquote(meta[:when]) |> Macro.prewalk(fn
          {name, _, nil} -> Keyword.get(values, name)
          exp -> exp
        end)
      end
    end
  end

  defmacro defguard({:when, _, [{{:unquote, _, [name]}, _, args}, guard]}, options) when is_atom(name) do
    defexpectp(:defmacro, name, [{:when, guard}, {:args, args} | options])
  end
  
  defmacro defguard({:when, _, [{name, _, args}, guard]}, options) do
    defexpectp(:defmacro, name, [{:when, guard}, {:args, args} | options])
  end

end