Where's the functor in the natural transformation?

Question

I've had this question on the very back of my mind ever since I saw the definition of natural transformations in the Edward Kmett's old category-extras package:

-- | A natural transformation between functors f and g.
type f :~> g = forall a. f a -> g a

But now reading Stephen Diehl's blog post on adjunctions, I find this:

A natural transformation in our context will be a polymorphic function associated with two Haskell functor instances f and g with type signature (Functor f, Functor g) => forall a. f a -> g a. Which could be written with the following type synonym.

type Nat f g = forall a. f a -> g a

Which was a slap in the face of my "I'll continue ignoring this" attitude. So for the question: Why is okay to suddenly drop the functor constraints?

Answer 1

It is not allowed in Haskell to put a constraint on a type synonym. And even for datatypes, it has been deprecated in Haskell 2010. Instead, the constraint should be put on the functions that operate on values of this type.