regression/ref-flatten.4.sml

   1 structure CList =
   2    struct
   3       datatype 'a clist' = Cons of 'a * 'a clist ref
   4       withtype 'a clist = 'a clist' option
   5
   6       fun cnil () = NONE
   7       fun ccons (h, t) = SOME (Cons (h, ref t))
   8
   9       fun match cl nilCase consCase =
  10          case cl of
  11             NONE => nilCase ()
  12           | SOME (Cons (h, t)) => consCase (h, !t)
  13
  14       fun fromList l =
  15          case l of
  16             [] => cnil ()
  17           | h::t => ccons (h, fromList t)
  18
  19       fun repeat x =
  20          let
  21             val r = ref NONE
  22             val cl = SOME (Cons (x, r))
  23             val () = r := cl
  24          in
  25             cl
  26          end
  27
  28       local
  29          val max = 1000
  30          fun length' (cl, n) =
  31             if n >= max
  32                then NONE
  33                else match cl
  34                           (fn () => SOME n)
  35                           (fn (_,t) => length' (t, n + 1))
  36       in
  37          fun length cl = length' (cl, 0)
  38       end
  39    end
  40
  41 val cl = CList.repeat #"x"
  42 val n = CList.length cl
  43 val () =
  44    case n of
  45       NONE => print "NONE\n"
  46     | SOME n => print (concat ["SOME ", Int.toString n, "\n"])
  47
  48 val cl = CList.fromList [1,2,3,4,5,6,7,8,9]
  49 val n = CList.length cl
  50 val () =
  51    case n of
  52       NONE => print "NONE\n"
  53     | SOME n => print (concat ["SOME ", Int.toString n, "\n"])