38 Exempel datatype expr = Const of int | Var of string | Plus of expr * expr | Times of expr * expr fun eval env (Const n) = n | eval env (Var i) = env i | eval env (Plus (e1,e2)) = eval env e1 + eval env e2 | eval env (Times (e1,e2)) = eval env e1 * eval env e2 fun diff id (Const n) = Const 0 | diff id (Var i) = if id = i then Const 1 else Const 0 | diff id (Plus (e1,e2)) = Plus (diff id e1, diff id e2) | diff id (Times (e1,e2)) = Plus (Times (e1,diff id e2),Times (diff id e1,e2))