DeriveFunctor works with RankNTypes in GHC

FunctorRankNTypes.hs

{-# LANGUAGE DeriveFunctor, RankNTypes, StandaloneDeriving #-}

-- The Glorious Glasgow Haskell Compilation System, version 7.4.1

data F x = C (([x] -> Bool) -> Maybe x) deriving Functor

data F1 x = C1 ((forall y . [(y,x)] -> y) -> Maybe x) deriving Functor

data F2 x = C2 (forall y . ([x] -> y) -> [(y,x)]) deriving Functor

data F3 x = C3 (forall y . ([x] -> y) -> Maybe y) deriving Functor

data F4 x = C4 (forall y . ([x] -> y) -> (y,x)) deriving Functor

deriving instance Functor ((,) y)

-- instance Functor F where
--   fmap f (C g) = C (\h -> fmap f (g (h . fmap f)))

-- instance Functor F1 where
--   fmap f (C1 g) = C1 (\h -> fmap f (g (h . fmap (fmap f))))

-- instance Functor F2 where
--   fmap f (C2 g) = C2 (\h -> fmap (fmap f) (g (h . fmap f)))

-- instance Functor F3 where
--   fmap f (C3 g) = C3 (\h -> (g (h . fmap f)))