type 'a enumerator = unit -> 'a option
type 'a observer = 'a option -> unit

fun list_enumerator xs =
    let val cell = ref xs in
	(fn () =>
	    if !cell = []
	    then NONE
	    else let val v = !cell in
		     cell := tl v;
		     SOME (hd v)
		 end)
    end

fun dump_enumerator e =
    let fun loop NONE = ()
	  | loop (SOME a) = (print (Int.toString a);
			     print "\n";
			     loop (e ()))
    in loop (e ()) end

; dump_enumerator (list_enumerator [1, 2, 3])