(* Carsten Schuermann *)
(* The visitor pattern in ML *)

let 
  datatype 'a List
    = Nil
    | Cons of 'a * 'a List  

  fun visitorList (Nil) c g = c
    | visitorList (Cons (H, T)) f g =
	g (H, visitorList T f g)

  val E = Cons (5, Cons (2, Nil))

  val N = visitorList E Nil (fn (x, y) => Cons (x*x, y))
  val S = String.implode (visitorList E 
			  [#"[", #"]"] 
			  (fn (x, [c1, c2]) => c1 :: String.explode (Int.toString x) @ [c2]
			    | (x, c :: s) => c :: String.explode (Int.toString x)
			                     @ #"," :: #" " :: s))
in
  (N, S)
end