Higher-Kinded TypeVars

aka type constructors, generic TypeVars

Has there already been discussion about those?
I do a lot of FP that results in impossible situations because of this. Consider an example:

A = TypeVar('A')
B = TypeVar('B')
F = TypeVar('F')

class M(Generic[F[X], A]):
    def x(fa: F[A], f: Callable[[A], B]) -> F[B]:
        return map(f, fa)

M().x([1], str)

I haven't found a way to make this work, does anyone know a trick or is it impossible?
If not, consider the syntax as a proposal.
Reference implementations would be Haskell, Scala.
optimally, the HK's type param would be indexable as well, allowing for F[X[X, X], X[X]]

Summary of current status (by @smheidrich, 2024-02-08):

• @JelleZijlstra has indicated interest in sponsoring a PEP, conditional on a prototype implementation in a major type checker and a well-specified draft PEP.
• Drafting the PEP takes place in @nekitdev's fork of the peps repo. The stub PEP draft so far contains a few examples of the proposed syntax.
• That same repo's GitHub Discussions forum forum has been designated as the place to discuss the PEP (and presumably the prototype implementation?). Some limited further discussions have taken place there.
  • If you want to be notified of new discussion threads, I think you have to set the whole repo as "watched" in GitHub?

Start with the stub PEP draft in the linked peps fork, then read the designated GitHub Discussions forum for the current design and prototype discussion. Done requires a well-specified PEP and a prototype implementation in a major type checker, as noted in the issue summary.