data List a = Nil | Cons a (List a)

reverse :: List a -> List a
reverse l = do 
    reverseAux accum Nil        = accum
    reverseAux accum (Cons h t) = reverseAux (Cons h accum) t
    reverseAux Nil l
    
main = reverse (Cons (1 :: Integer) (Cons (2 :: Integer) (Cons (3 :: Integer) (Cons (4 :: Integer) Nil))))
--
(Cons 4 (Cons 3 (Cons 2 (Cons 1 Nil))))