(define main
  (display (concat (Cons "Hello" (Cons ", " (Cons "World!" Nil))))))

(define concat
  (foldr ++ ""))

(define (foldr f init)
  (fun-match
    [Nil init]
    [(Cons x xs) (Cons (f x) (foldr f init xs))]))

(define ++
  (fun-match
    [Nil snd]
    [(Cons x xs) (. (Cons x) (++ xs))]))

(define (const c x) c)
(define (snd a b) b)