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

module Playground4 where

import Data.Kind (Constraint)
import qualified Data.List as List

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

type EmptyConstraint a = ()

instance GFunctor EmptyConstraint [] where
  gfmap = List.map


-- error:
--    • Expected kind ‘* -> Constraint’,
--        but ‘EmptyConstraint’ has kind ‘* -> *’
--    • In the first argument of ‘GFunctor’, namely ‘EmptyConstraint’
--      In the instance declaration for ‘GFunctor EmptyConstraint []’