regression/weak.3.sml

   1 fun find cache x =
   2    case (List.find (fn (y,_) => x = y) (!cache)) of
   3       NONE => NONE
   4     | SOME (_,r) => SOME r
   5 fun remove cache x =
   6    cache := (List.filter (fn (y,_) => not (x = y)) (!cache))
   7 fun insert cache (x,r) =
   8    cache := (x,r)::(!cache)
   9
  10 val cache = ref []
  11
  12 fun lookup (x : int) =
  13    case find cache x of
  14       SOME r  => (case MLton.Weak.get r of
  15                      SOME r' => r'
  16                    | NONE => (remove cache x; lookup x))
  17     | NONE    => let val res = x + 1
  18                      val wres = MLton.Weak.new res
  19                  in insert cache (x, wres);
  20                     res
  21                  end
  22
  23 val _ = List.app (fn x => print (concat [Int.toString (lookup x), "\n"])) [5,4,3,2,1]