Koos Zevenhoven writes:
On Mon, May 16, 2016 at 2:37 AM, Greg Ewing email@example.com wrote:
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.
Indeed, that's TypeVar.
True as far as the statement itself makes sense (categorically, a type is what it is, so of course it is the same type wherever it appears!), but a Sum would not be a TypeVar. It's a type, it's far more limited in analyzing Python programs, and I would like to withdraw the term itself, as well as any proposal to add it to typing, from this discussion.
All concerned have my apologies for bringing it up at all.
Footnotes:  And useful in category theory for that very reason -- it's easier to reason about in categorical contexts. But it makes things harder in Mypy.