On Mon, May 16, 2016 at 2:37 AM, Greg Ewing firstname.lastname@example.org 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.
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.
I was actually trying to interpret the (vague) proposal as going even a step further, closer to what may have helped slightly in the fspath type hinting problem. Perhaps something like this:
M = TagMatcher(3) # 3 is the number of "summed" types # (and yes, I just made up the term TagMatcher)
def onehalf(value: M.sum[int, float, complex]) -> M.sum[float, float, complex]: return 0.5 * value
That would then match the "tag" between the argument and return types (within the sum types which are tagged unions):
int -> float float -> float complex -> complex
As you can see, "onehalf(...)" will turn int, float and complex into float, float and complex, respectively. I suppose one or more "functors" could be seen in there to make this happen in theory XD. This is not solved by a TypeVar.
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.
So, despite what I write above, you still seem to agree with me, assuming you did not literally mean *everything* ;-).
The reason why we agree is that we can already do:
@overload def onehalf(value: int) -> float: ... @overload def onehalf(value: float) -> float: ... @overload def onehalf(value: complex) -> complex: ...
So, the sum type, even with my "TagMatcher" described above, does not provide anything new. After all, the whole point of sum types is to give *statically typed* languages something that resembles dynamic typing.
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