typeclasses.hs

{-# LANGUAGE RankNTypes #-}

-- as a data type
data Functor1 f =
  Functor2 { fmap1 :: forall a b. (a -> b) -> f a -> f b }

-- as a type-class
class Functor2 f where
  fmap2 :: (a -> b) -> f a -> f b

flip1 :: Functor1 f -> f (a -> b) -> a -> f b
flip1 f x a = fmap1 f (\k -> k a) x

flip2 :: Functor2 f => f (a -> b) -> a -> f b
flip2 x a = fmap2 (\k -> k a) x