Basic stack implementation in haskell

Stack.hs

data Stack a
  = Nil
  | Stack a (Stack a)

instance Functor Stack where
  fmap f s =
    case s of
      Stack e next ->
        Stack (f e) $ fmap f next
      Nil -> Nil
    
instance Foldable Stack where
  foldr f current rest =
    case rest of
      Stack item next ->
        foldr f (f item current) next
      Nil -> current
        
instance Show a => Show (Stack a) where
  show st = "Stack: " <> show (foldr (:) [] st)

stack :: a -> Stack a
stack a = Stack a Nil

push :: Stack a -> a -> Stack a
push Nil item = Stack item Nil
push next item = Stack item next

pop :: Stack a -> (Maybe a, Stack a)
pop (Stack item next) = (Just item, next)
pop Nil = (Nothing, Nil)

main :: IO ()
main = print $ foldl push (stack 2) [1..10]