Use a Typespec function type

This is accomplished using behaviours:

defmodule A do
  @callback simple_function(integer()) :: non_neg_integer()
end

defmodule B do
  @behaviour A

  @impl A
  def simple_function(int), do: ...
end

I’ll add, however, that it’s not advisable to create behaviours if your goal is really only to re-use some typespecs. Two different functions sharing a common, duplicated typespec is not a code smell.

I thought you can do this. For example, I’ve done stuff like this before…

defmodule MyThing do

  @spec start_link(term) :: GenServer.server()

  def start_link(arg) do
    GenServer.start_link(__MODULE__, arg)
  end

end

This cover the return type I guess

You can write this spec, it just doesn’t mean what you want:

defmodule B do
  @spec my_fun :: A.simple_function()
  def my_fun do
    fn
      x when x >= 0 -> x + 1
      x -> -x + 1
    end
  end
end

Yes, you can reference remote types for any of the arguments or the return type, but you can’t do

@spec some_function :: some_generic_function_signature()

which is what the OP was asking.