Koos Zevenhoven wrote:
Maybe you mean to propose that the concept of sum types should be introduced in typing/mypy.
I'm not deeply into category theory, but the proposal seems to be that Sum would be a special kind of type that's assumed to be the same type wherever it appears in the signature of a function.
That might work for the special case of a function of one parameter that returns the same type as its argument, but that's about all.
As has been pointed out, type parameters allow the same thing to be expressed, and a lot more besides, so a Sum type is not needed. We already have everything we neeed.