types.ex

defmodule M do
  @type mytype :: String.t
end

defmodule N do
  @spec hi(M.mytype) :: M.mytype
  def hi(x), do: x
end

defmodule Mac do
  defmacro mkfun(name, arg, type) do
    quote do
      @spec unquote(name)(unquote(type)) :: unquote(type)
      def unquote(name)(unquote(arg)), do: unquote(arg)
    end
  end
end

defmodule Foo do
  require Mac
  Mac.mkfun(:hello, x, M.mytype)
end

IO.puts Foo.hello "abc"