Simple example of using a recursive datatype (ML)

	datatype exp = Constant of int
	| Negate of exp
	| Add of exp * exp
	| Multiply of exp * exp
	fun eval e =
	case e of
	Constant i => i
	| Negate e2 => ~ (eval e2)
	| Add(e1,e2) => (eval e1) + (eval e2)
	| Multiply(e1,e2) => (eval e1) * (eval e2)
	fun max_constant e =
	case e of
	Constant i => i
	| Negate e1 => max_constant e1
	| Add(e1,e2) => Int.max(max_constant e1,max_constant e2)
	| Multiply(e1,e2) => Int.max(max_constant e1,max_constant e2)
	val example_exp = Add (Constant (10+9), Negate (Constant 4))
	val result = eval example_exp
	val maxconstant = max_constant example_exp
	(* output of program
	- use "expression.sml";
	[opening expression.sml]
	datatype exp
	= Add of exp * exp | Constant of int | Multiply of exp * exp | Negate of exp
	val eval = fn : exp -> int
	val max_constant = fn : exp -> int
	val example_exp = Add (Constant 19,Negate (Constant 4)) : exp
	val result = 15 : int
	val maxconstant = 19 : int
	val it = () : unit
	*)

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