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

hasEvenLength :: List a -> Boolean
hasEvenLength l = do 
    even Nil        = True
    even (Cons _ t) = odd t
    odd  Nil        = False
    odd  (Cons _ t) = even t
    even l

main = hasEvenLength (Cons (1 :: Integer) (Cons (2 :: Integer) (Cons (3 :: Integer) (Cons (4 :: Integer) Nil))))
--
true