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(*
Title: Weak references
Author: David Matthews
Copyright David Matthews 2008
This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
*)
(* A weak reference or array contains option values. The SOME variant of
the option must contain a reference. This restriction is imposed because
they require pointer equality. A weak reference or array behaves just like
a normal ref or array with one difference. The garbage collector may set
a weak ref or the field of weak array to NONE if it currently contains
SOME r but r is not reachable other than through weak references. The
one proviso is that if r is contained in the executable it is always
reachable.
*)
signature WEAK =
sig
val weak: 'a ref option -> 'a ref option ref
val weakArray: int * 'a ref option -> 'a ref option array
val weakLock: Thread.Mutex.mutex
and weakSignal: Thread.ConditionVar.conditionVar
end;
structure Weak: WEAK =
struct
fun weak (v: 'a ref option): 'a ref option ref =
let
val System_alloc =
RunCall.run_call3 RuntimeCalls.POLY_SYS_alloc_store
in
System_alloc(0w1, 0wx60, v)
end
fun weakArray(n: int, v: 'a ref option): 'a ref option array =
let
(* An array consists of n+1 words with the first containing
the length. *)
val System_alloc =
RunCall.run_call3 RuntimeCalls.POLY_SYS_alloc_store
val System_setw = RunCall.run_call3 RuntimeCalls.POLY_SYS_assign_word;
val arr = System_alloc(n+1, 0wx60, v)
in
System_setw(arr, 0, n);
arr
end
val weakLock = Thread.Mutex.mutex()
and weakSignal = Thread.ConditionVar.conditionVar()
end;
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