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

let 
  datatype Exp 
    = Int of int
    | Add of Exp * Exp 

  fun visitorExp (Int e) f g = 
        f (Int e)
    | visitorExp (Add (e1, e2)) f g =
	g (visitorExp e1 f g, visitorExp e2 f g)

  val E = Add (Int 5, Add (Add (Int 7, Int 9), Int 16))
  val N = visitorExp E (fn Int x => x) (fn (x, y) => x + y)
  val S = visitorExp E (fn Int x => Int.toString x) (fn (x, y) => x ^ "+" ^ y)
in
  (N, S)
end