For simplicity written with a type synonym, but it cannot be partially applied. So I would use constraint synonym encoding (https://gist.github.com/Icelandjack/

For simplicity written with a type synonym, but it cannot be partially applied. So I would use constraint synonym encoding (https://gist.github.com/Icelandjack/5afdaa32f41adf3204ef9025d9da2a70#constraint-synonym-encoding-or-class-synonym https://gist.github.com/Icelandjack/5afdaa32f41adf3204ef9025...) type FunctorOf :: forall (s :: Type) (t :: Type). Cat s -> Cat t -> Constraint class (Functor f, Src f ~ src, Tgt f ~ tgt) => FunctorOf src tgt f instance (Functor f, Src f ~ src, Tgt f ~ tgt) => FunctorOf src tgt f The other definitions can be eta reduced type EndofunctorOf cat = FunctorOf cat cat type OldFunctor = EndofunctorOf (->)