Enabled PolyKinds for Cayley, Procompose, Rift, ProfunctorFunctor, Ran, …

[Icelandjack]

Enabled PolyKinds for Cayley, Procompose, Rift, ProfunctorFunctor, Ran, Codensity, Prep, Coprep, Star, Costar, WrappedArrow, Forget are now polykinded.

I also added some kind signatures in comments until we can use StandaloneKindSignatures. Let me know and I'll remove it. The reason for this is a polymorphic Cayley, I wanted to define Data.Monoid.Ap in terms of it.

[Icelandjack]

@RyanGlScott I mentioned my motivation in the description, I want to Ap behaviour via Cayley, in the end I had to use Representational1 so I couldn't

deriving (Semigroup, Monoid)
via f `Cayley` OneObject a `Endo` '() 

but it works standalone (basically I create a category from a monoid, I lift it with Applicative, and apply it to dummy arguments with a generalized Endo)

deriving
  via Endo (Cayley f (OneObject a)) '()
  instance (Representational1 f, Applicative f, Monoid a) => Semigroup (MyAp f a)
deriving
  via Endo (Cayley f (OneObject a)) '()
  instance (Representational1 f, Applicative f, Monoid a) => Monoid (MyAp f a)

where OneObject and newtype Endo cat a = Endo (cat a a)

[Icelandjack]

This forces Semigroup to have a Monoid constraint. If we were able to subtract methods from a type class we could write something like

type Semigroupoid :: Cat ob -> Constraint
type Semigroupoid cat = Category cat - id

instance Semigroup a => Semigroupoid (OneObject a) where (.) = .. (<>) ..
instance Monoid    a => Category     (OneObject a) where id  = .. mempty ..

instance Semigroupoid cat => Semigroup (Endo cat a) where (<>)   = .. (.) ..
instance Category     cat => Monoid    (Endo cat a) where mempty = .. id .. 

I hope we can avoid the ..s soon, writing instances via

instance Semigroupoid cat => Semigroup (Endo cat a) where
  via cat a a

  (<>) :: cat a a -> cat a a -> cat a a
  (<>)   = (.)