[m-rev.] for review: add two mvar operations

[Peter Wang]

Branches: main, 11.07

library/thread.mvar.m:
	Add two operations: mvar.read, mvar.try_put.

NEWS:
	Announce the additions.

diff --git a/NEWS b/NEWS
index e41662d..4714e1f 100644
--- a/NEWS
+++ b/NEWS
@@ -111,6 +111,8 @@ Changes to the Mercury standard library:
 	hash_table.init/3
 	hash_table.init_default/1
 	mvar.init/0
+	mvar.read/4
+	mvar.try_put/5
 	store.init/1
 	semaphore.init/1
 	semaphore.init/3
diff --git a/library/thread.mvar.m b/library/thread.mvar.m
index 8a0ac4e..19df3c3 100644
--- a/library/thread.mvar.m
+++ b/library/thread.mvar.m
@@ -23,6 +23,7 @@
 :- module thread.mvar.
 :- interface.
 
+:- import_module bool.
 :- import_module io.
 :- import_module maybe.
 
@@ -54,6 +55,17 @@
     %
 :- pred mvar.put(mvar(T)::in, T::in, io::di, io::uo) is det.
 
+    % Place the value of type T into an empty mvar, returning yes on success.
+    % If the mvar is full, return no immediately without blocking.
+    %
+:- pred mvar.try_put(mvar(T)::in, T::in, bool::out, io::di, io::uo) is det.
+
+    % Read the contents of mvar, without taking it out.
+    % If the mvar is empty, block until it is full.
+    % This is equivalent to mvar.take followed by mvar.put.
+    %
+:- pred mvar.read(mvar(T)::in, T::out, io::di, io::uo) is det.
+
 %-----------------------------------------------------------------------------%
 %-----------------------------------------------------------------------------%
 
@@ -109,5 +121,24 @@ mvar.put(mvar(Full, Empty, Ref), Data, !IO) :-
     impure set_mutvar(Ref, Data),
     semaphore.signal(Full, !IO).
 
+:- pragma promise_pure(mvar.try_put/5).
+
+mvar.try_put(mvar(Full, Empty, Ref), Data, Success, !IO) :-
+    semaphore.try_wait(Empty, Success, !IO),
+    (
+        Success = yes,
+        impure set_mutvar(Ref, Data),
+        semaphore.signal(Full, !IO)
+    ;
+        Success = no
+    ).
+
+:- pragma promise_pure(mvar.read/4).
+
+mvar.read(mvar(Full, _Empty, Ref), Data, !IO) :-
+    semaphore.wait(Full, !IO),
+    impure get_mutvar(Ref, Data),
+    semaphore.signal(Full, !IO).
+
 %---------------------------------------------------------------------------%
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