[m-rev.] for review: concurrency additions

[Peter Wang]

Branches: main

Add predicates from MC's versions of the concurrency modules.

library/thread.channel.m:
	Add channel.try_take/4, a non-blocking version of channel.take/4.

library/thread.mvar.m:
	Add mvar.try_take/4, a non-blocking version of mvar.take/4.

NEWS:
	Announce the additions.


Index: NEWS
===================================================================
RCS file: /home/mercury/mercury1/repository/mercury/NEWS,v
retrieving revision 1.452
diff -u -r1.452 NEWS
--- NEWS	18 Mar 2007 23:34:44 -0000	1.452
+++ NEWS	12 Apr 2007 03:00:26 -0000
@@ -37,7 +37,8 @@
 * We have moved some of the concurrency primitives out of the extras
   distribution and into a new standard library module called `thread',
   and its submodules `thread.channel', `thread.mvar', and `thread.semaphore'.
-  The predicate `thread.can_spawn' has also been added.
+  The predicates `thread.can_spawn', `thread.channel.try_take'
+  and `thread.mvar.try_take' have also been added.
 
 * The following predicates have been added to the list module:
 	list.map_corresponding/4
Index: library/thread.channel.m
===================================================================
RCS file: /home/mercury/mercury1/repository/mercury/library/thread.channel.m,v
retrieving revision 1.1
diff -u -r1.1 thread.channel.m
--- library/thread.channel.m	1 Feb 2007 08:07:59 -0000	1.1
+++ library/thread.channel.m	12 Apr 2007 03:00:26 -0000
@@ -24,6 +24,7 @@
 :- interface.
 
 :- import_module io.
+:- import_module maybe.
 
 %-----------------------------------------------------------------------------%
 
@@ -42,6 +43,11 @@
     %
 :- pred channel.take(channel(T)::in, T::out, io::di, io::uo) is det.
 
+    % Take an item from the start of the channel.
+    % Returns immediately with no if the channel was empty.
+    %
+:- pred channel.try_take(channel(T)::in, maybe(T)::out, io::di, io::uo) is det.
+
     % Duplicate a channel.  The new channel sees all (and only) the
     % data written to the channel after the channel.duplicate call.
     % 
@@ -93,6 +99,19 @@
     mvar.take(Head, item(Val, NewHead), !IO),
     mvar.put(Read, NewHead, !IO).
 
+channel.try_take(channel(Read, _Write), MaybeVal, !IO) :-
+    mvar.take(Read, Head, !IO),
+    mvar.try_take(Head, MaybeItem, !IO),
+    (
+        MaybeItem = yes(item(Val, NewHead)),
+        MaybeVal = yes(Val)
+    ;
+        MaybeItem = no,
+        MaybeVal = no,
+        NewHead = Head
+    ),
+    mvar.put(Read, NewHead, !IO).
+
 channel.duplicate(channel(_Read, Write), channel(NewRead, Write), !IO) :-
     mvar.init(NewRead, !IO),
     mvar.take(Write, Hole, !IO),
Index: library/thread.mvar.m
===================================================================
RCS file: /home/mercury/mercury1/repository/mercury/library/thread.mvar.m,v
retrieving revision 1.1
diff -u -r1.1 thread.mvar.m
--- library/thread.mvar.m	1 Feb 2007 08:07:59 -0000	1.1
+++ library/thread.mvar.m	12 Apr 2007 03:00:26 -0000
@@ -24,6 +24,7 @@
 :- interface.
 
 :- import_module io.
+:- import_module maybe.
 
 %-----------------------------------------------------------------------------%
 
@@ -38,6 +39,12 @@
     %
 :- pred mvar.take(mvar(T)::in, T::out, io::di, io::uo) is det.
 
+    % Take the contents of the mvar out leaving the mvar empty.
+    % Returns immediately with no if the mvar was empty, or yes(X) if
+    % the mvar contained X.
+    %
+:- pred mvar.try_take(mvar(T)::in, maybe(T)::out, io::di, io::uo) is det.
+
     % Place the value of type T into an empty mvar.
     % If the mvar is full block until it becomes empty.
     %
@@ -48,6 +55,7 @@
 
 :- implementation.
 
+:- import_module bool.
 :- import_module mutvar.
 :- import_module thread.semaphore.
 
@@ -73,6 +81,19 @@
     impure get_mutvar(Ref, Data),
     semaphore.signal(Empty, !IO).
 
+:- pragma promise_pure(mvar.try_take/4).
+mvar.try_take(mvar(Full, Empty, Ref), MaybeData, !IO) :-
+    semaphore.try_wait(Full, Success, !IO),
+    (
+        Success = yes,
+        impure get_mutvar(Ref, Data),
+        semaphore.signal(Empty, !IO),
+        MaybeData = yes(Data)
+    ;
+        Success = no,
+        MaybeData = no
+    ).
+
 :- pragma promise_pure(mvar.put/4).
 mvar.put(mvar(Full, Empty, Ref), Data, !IO) :-
     semaphore.wait(Empty, !IO),
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