datatype expr =
	 Int of int
       | Neg of expr
       | Plus of expr * expr
       | Times of expr * expr

fun eval (Int i) = i
  | eval (Neg e) = ~(eval e)
  | eval (Plus (e1, e2)) = (eval e1) + (eval e2)
  | eval (Times (e1, e2)) = (eval e1) * (eval e2)

val result = eval (Plus (Int 2, Neg (Int 3)))