What are uses of polymorphic kinds?

Question

Polymorphic kinds are an extension to Haskell\'s type system, supported by UHC, allowing

data A x y = A (y x)

to be typed (kinded?) as a

Answer 1

One possible usage example can be using conal's TypeCompose for composing monad transformers in point-free style.

type MyT = StateT Foo :. MaybeT :. ContT Bar

(just as an example, I have no idea what one's going to do with those foos and bars..)

Instead of:

type MyT m = StateT Foo (MaybeT (ContT Bar m))

(this would have the same result apart from newtype-wrappers)

Currently you'll need to duplicate the combinators code for different kinds, and this extension abolishes the repetition and allows using one piece of code to rule them all.