pexam.sml

(*val x = 50;
val y = 3;
val z = 10;
val f = fn z => z;
val a = 
	let 
		val x = 3*x;
		val z = y*z;
	in
		 x*z
	end
fun f x z = x + y + z
*)

(*
fun maybeEven x = 
	if x = 0 
	then true
	else
	if x = 50
	then false
	else maybeOdd (x-1)
and
maybeOdd y =
	if y = 0
	then false
	else 
	if y = 99
	then true
	else maybeEven (y-1)

*)
	
signature DIGIT = 
sig
type digit
val increment : digit -> digit
val decrement : digit -> digit
val down_and_up : digit -> digit
val test : digit -> unit
end


structure Digit :> DIGIT =
struct
type digit = int
exception BadDigit
exception FailTest
fun make_digit i = if i < 0 orelse i > 9 then raise BadDigit else i
fun increment d = if d=9 then 0 else d+1
fun decrement d = if d=0 then 9 else d-1
val down_and_up = increment o decrement (* recall o is function composition *)
fun test d = if down_and_up d = d then () else raise FailTest
end