Defunctionalization of type families

[langston-barrett]

The technique of defunctionalizing type families to support partial application (and especially higher-order type families) has been applied in at least two parameterized-utils-adjacent spots: #103, and the CtxFuns module mentioned in #84. Seems like it's probably worth creating a small module in parameterized-utils that at least provides a canonical home for the Apply family:

data TyFun a b
type a ~> b = TyFun a b -> Type
type family Apply (f :: a ~> b) (x :: a) :: b

[RyanGlScott]

Alternatively, we could just depend on the singletons library, which provides this in Data.Singletons. singletons-3.0 and later have no external dependencies, so it's a pretty lightweight dependency.

[travitch]

Yeah, there is also a version in semmc somewhere, I think. It would be nice to have just one. I'm open to singletons core now that it has slimmed down.

[masaeedu]

FWIW I would find it very useful to have defunctionalization boilerplate that isn't entangled with the rest of singletons.