data Either a b = Left a | Right b

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

data Nat = Zero | Succ Nat

sum Zero a = a
sum a Zero = a
sum (Succ a) (Succ b) = Succ (Succ (sum a b))

sum_append xs ys
 = go Zero (Left xs)
 where
    go z (Left xs)
      = match xs with
        Nil -> go z (Right ys)
        Cons x xs' -> go (sum x z) (Left xs')
    go z (Right ys)
      = match ys with
        Nil -> z
        Cons y ys' -> go (sum y z) (Right ys')

main = "Hello world!"
--
Hello world!