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(* Title: Pure/Concurrent/single_assignment.ML
Author: Makarius
Single-assignment variables with locking/signalling.
*)
signature SINGLE_ASSIGNMENT =
sig
type 'a var
val var: string -> 'a var
val peek: 'a var -> 'a option
val await: 'a var -> 'a
val assign: 'a var -> 'a -> unit
end;
structure Single_Assignment: SINGLE_ASSIGNMENT =
struct
datatype 'a state =
Set of 'a
| Unset of {lock: Mutex.mutex, cond: ConditionVar.conditionVar};
fun init_state () =
Unset {lock = Mutex.mutex (), cond = ConditionVar.conditionVar ()};
abstype 'a var = Var of {name: string, state: 'a state Unsynchronized.ref}
with
fun var name = Var {name = name, state = Unsynchronized.ref (init_state ())};
fun peek (Var {name, state}) =
(case ! state of
Set x => SOME x
| Unset {lock, ...} =>
Multithreading.synchronized name lock (fn () =>
(case ! state of
Set x => SOME x
| Unset _ => NONE)));
fun await (Var {name, state}) =
(case ! state of
Set x => x
| Unset {lock, cond} =>
Multithreading.synchronized name lock (fn () =>
let
fun wait () =
(case ! state of
Unset _ =>
(case Multithreading.sync_wait NONE cond lock of
Exn.Res _ => wait ()
| Exn.Exn exn => Exn.reraise exn)
| Set x => x);
in wait () end));
fun assign_fail name = raise Fail ("Duplicate assignment to " ^ name);
fun assign (Var {name, state}) x =
(case ! state of
Set _ => assign_fail name
| Unset {lock, cond} =>
Multithreading.synchronized name lock (fn () =>
(case ! state of
Set _ => assign_fail name
| Unset _ =>
Thread_Attributes.uninterruptible (fn _ => fn () =>
(state := Set x; RunCall.clearMutableBit state; ConditionVar.broadcast cond)) ())));
end;
end;
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(vorverarbeitet)
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