lib/mlton/queue/ephemeral.fun

   1 (* Copyright (C) 1999-2006 Henry Cejtin, Matthew Fluet, Suresh
   2  *    Jagannathan, and Stephen Weeks.
   3  *
   4  * MLton is released under a BSD-style license.
   5  * See the file MLton-LICENSE for details.
   6  *)
   7 (*-------------------------------------------------------------------*)
   8 (*                          Queue Ephemeral                          *)
   9 (*-------------------------------------------------------------------*)
  10
  11 functor QueueEphemeral(): BASIC_QUEUE =
  12 struct
  13
  14 val {error, ...} = Error.errors("queue", "ephemeral")
  15
  16 structure L = MutableList
  17
  18 datatype 'a t = T of {head: 'a L.t ref,
  19                       tail: 'a L.t ref}
  20
  21 fun destruct(q as T{head, tail}) =
  22    case L.destruct(!head) of
  23       NONE => NONE
  24     | SOME(x, _) =>
  25          (if L.eqTail(!head, !tail)
  26              then (head := L.empty() ; tail := L.empty())
  27           else head := L.tail(!head) ;
  28              SOME(x, q))
  29
  30 fun empty() = T{head = ref(L.empty()), tail = ref(L.empty())}
  31
  32 fun isEmpty(T{head, ...}) = L.isEmpty(!head)
  33
  34 fun enque(q as T{head, tail}, x) =
  35     (let val cell = L.cons(x, L.empty())
  36      in (if isEmpty q then head := cell
  37          else L.setTail(!tail, cell) ;
  38          tail := cell)
  39      end ;
  40      q)
  41
  42 end