id-2.hs

instance Functor [] where
  fmap _ [] = []
  fmap f (x::xs) = f x :: fmap f xs

instance Functor Maybe where
  fmap _ Nothing = Nothing
  fmap f (Just x) = Just (f x)
  
instance Functor (Either a) where
  fmap _ x@(Left _) = x
  fmap f (Right x) = Right (f x)
  
instance Functor (-> a) where
  fmap = (.)