filter : (a -> Bool) -> List a -> List a
filter p xs = case xs of
    Nil => Nil
    Cons x xs => case p x of
        True => Cons x (filter p xs)
        False => filter p xs

quicksort : List Int -> List Int
quicksort xs = case xs of
    Nil => Nil
    Cons a as => let smaller = quicksort (filter (\y. y < a) xs) in
                 let bigger = quicksort (filter (\y. a < y) xs) in
                 smaller ++ (Cons a bigger)

-- [5, 2, 8, 9, 6, 0, 1]
main = let list = Cons 5 (Cons 2 (Cons 8 (Cons 9 (Cons 6 (Cons 0 (Cons 1 Nil)))))) in 
       printList (quicksort list)