Idea for map-type generics

[tmke8]

(I hope this is the right place to suggest something like this.)

The syntax for "map-type" generics is not very nice: Callable[[int, str], bool], Dict[str, int], Mapping[str, int], DefaultDict[str, int]. And there was a PEP to improve the Callable syntax but it was rejected. But maybe we can solve all of these at once by introducing a special map-type generic (maybe GenericMap) which has its own syntax that visually makes it clearer that it's a map. The steering committee didn't seem to like the use of -> so maybe it could be => instead: Callable[[int, str] => bool] (or, if you alias Callable as Fn, then Fn[[int, str] => bool]), Dict[str => int], Mapping[str => int], DefaultDict[str => int].

Fn = Callable
def map(d: Dict[str => int], f: Fn[[int] => float]) -> Dict[str => float]: ...

The selling point is that it's not only useful for Callables but for all the dictionary variants as well.

[JelleZijlstra]

This is a good place to start a discussion for something like this!

Personally I don't feel that changing a comma into an arrow is useful enough to justify totally new syntax, though. I also haven't really felt the need to change the syntax for mapping types; dict[str, int] seems fine to me.