Generic functor, abstracting over a constraint

GenericFunctor.hs

{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE ExplicitForAll #-}
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE PolyKinds #-}

{-# LANGUAGE TypeSynonymInstances #-}
{-# LANGUAGE FlexibleInstances #-}

module Playground where

import Data.Kind
import qualified Data.Set as Set

-- typeclass
class GFunctor (c :: * -> Constraint) (f :: * -> *) | f -> c where
  gfmap :: forall a b. c b => (a -> b) -> (f a -> f b)

-- instance for sets
instance GFunctor Ord Set.Set where
  gfmap = Set.map

-- instance for lists
type Const (a :: Constraint) (b :: k) = a

instance GFunctor (Const ()) [] where
-- error:
--     • The type synonym ‘Const’ should have 2 arguments, but has been given 1
--     • In the instance declaration for ‘GFunctor (Const ()) []’
--    |
-- 35 | instance GFunctor (Const ()) [] where
--    |          ^^        ^^