(* Copyright (C) 1999-2005 Henry Cejtin, Matthew Fluet, Suresh
 *    Jagannathan, and Stephen Weeks.
 *
 * MLton is released under a BSD-style license.
 * See the file MLton-LICENSE for details.
 *)

functor ExnHetContainer():> HET_CONTAINER =
   struct
      type t = exn

      fun 'a new() =
         let exception E of 'a
         in {make = E,
             pred = fn E _ => true | _ => false,
             peek = fn E x => SOME x | _ => NONE}
         end
   end

functor RefHetContainer():> HET_CONTAINER =
   struct
      type t = unit ref * (unit -> unit)

      fun 'a new() =
         let
            val id = ref()
            val r: 'a option ref = ref NONE
            fun make v = (id, fn () => r := SOME v)
            fun peek ((id', f): t) =
               if id = id' then (f(); !r before r := NONE)
               else NONE
            fun pred(id', _) = id = id'
         in {make = make, pred = pred, peek = peek}
         end
   end