defmodule A do
  @type size :: pos_integer()
  defguard is_size(x) when is_integer(x) and x > 0
  def default_size(), do: 1
end

defmodule B do
  @type t :: %__MODULE__{size: size()}
  defstruct [size: default_size()]
  def put_size(opts, size) when is_list(opts) and is_size(size), do: Keyword.put(opts, :size, size)
end

@type foo :: Foo.foo

defmodule MyApp.Types do

    defmacro __using__(_opts) do 
          quote do
             @type greek_t :: :alpha | :beta
             @type mystruct_t :: MyApp.MyStruct.t # Beware of circulars here!!
          end
    end
...