Standard ML: Imperative vs. functional

gistfile1.sml

infixr 0 $
fun f $ x = f x
val concat = List.concat

fun good_cartesian_product (xs, ys) =
    concat (map (fn y => map (fn x => (x,y)) xs) ys)

fun evil_cartesian_product (xs', ys') =
    let
      val result = ref []
      val xs = ref xs'
      val ys = ref ys'
    in (
        while not (null !ys) do (
	  while not (null !xs) do (
	    result := !result @ [(hd (!xs), hd (!ys))];
	    xs := tl (!xs)
          );
	  xs := xs';
	  ys := tl (!ys)
        );
	!result
      )
    end

(* renaming some operations *)
val get = !
val set = op:=
val empty = null

fun evil_cartesian_product (xs', ys') =
    let
      val result = ref []
      val xs = ref xs'
      val ys = ref ys'
    in (
        while not(null(get(ys))) do (
	  while not(null(get(xs))) do (
	    set(result, get(result) @ [(hd(get(xs)), hd(get(ys)))]);
            set(xs, tl(get(xs)))
          );
	  set(xs, xs');
	  set(ys, tl(get(ys)))
        );
	get(result)
      )
    end