I know you can do stuff like:

    @type pair(t, u) :: {t, u}
    @type result(t, e) :: {:ok, t} | {:error, e}

I'm more talking about type parameters on functions, and also constraining by behaviors and protocols. For example the typespec for Elixir's Stream.map/2 (http://elixir-lang.org/docs/stable/elixir/Stream.html#map/2):

    @spec map(Enumerable.t, (element -> any)) :: Enumerable.t

Note all the other similarly unspecific type definitions. In a parametrically polmorphic lang, you could do something like:

    @spec map[e1: Enumerable, e2: Enumerable](e1.t, (e1.element -> e2.element)) :: e2.t

And perhaps even intersections of behaviours and protocols:

    @spec foo[a: Eq & Ord](a.t, a.t) :: bool

('scuse the Elixir syntax...)