2. Given below is SML code that implements a simple way to define expressions and

operations on them:

datatype exp = Constant of int

Negate of exp

| Add of exp * exp

fun eval e = case e of

Constant (i) => i

Negate el => ~ (eval el)

| Add (el, e2) => (eval el) + (eval e2)

eval (Add (Constant 19, Negate (Constant 4))); (* result is 15

*)