Create union type from list

typespec_union.ex

defmodule MyTypespec do
  defmacro union_type({:"::", _, [name, types]}) when is_list(types) do
    union =
      types
      |> Enum.reverse
      |> Enum.reduce(fn x, acc ->
        {:|, [], [x, acc]}
      end)

    quote do
      @type unquote(name) :: unquote(union)
    end
  end
end

defmodule M1 do
  import MyTypespec

  @type a :: 1 | 2 | 3
  @type foo :: a | :a

  union_type foo2 :: [a, :a]
  union_type foo3 :: [foo2, :b, 1]

end

IO.inspect Kernel.Typespec.beam_types(M1)
# [type: {:a,
#   {:type, 19, :union, [{:integer, 0, 1}, {:integer, 0, 2}, {:integer, 0, 3}]},
#   []},
#  type: {:foo, {:type, 20, :union, [{:user_type, 20, :a, []}, {:atom, 0, :a}]},
#   []},
#  type: {:bar2, {:type, 22, :union, [{:user_type, 22, :a, []}, {:atom, 0, :a}]},
#   []},
#  type: {:bar3,
#   {:type, 23, :union,
#    [{:user_type, 23, :bar2, []}, {:atom, 0, :b}, {:integer, 0, 1}]}, []}]