, putMVar -- :: MVar a -> a -> IO ()
, readMVar -- :: MVar a -> IO a
, swapMVar -- :: MVar a -> a -> IO a
+ , takeMaybeMVar -- :: MVar a -> IO (Maybe a)
, isEmptyMVar -- :: MVar a -> IO Bool
) where
import PrelBase
+import PrelMaybe
import PrelErr ( parError, seqError )
import PrelST ( liftST )
import PrelIOBase ( IO(..), MVar(..), unsafePerformIO )
--Defined in IOBase to avoid cycle: data MVar a = MVar (SynchVar# RealWorld a)
newEmptyMVar :: IO (MVar a)
-
newEmptyMVar = IO $ \ s# ->
case newMVar# s# of
(# s2#, svar# #) -> (# s2#, MVar svar# #)
takeMVar :: MVar a -> IO a
-
takeMVar (MVar mvar#) = IO $ \ s# -> takeMVar# mvar# s#
putMVar :: MVar a -> a -> IO ()
-
putMVar (MVar mvar#) x = IO $ \ s# ->
case putMVar# mvar# x s# of
s2# -> (# s2#, () #)
newMVar :: a -> IO (MVar a)
-
newMVar value =
newEmptyMVar >>= \ mvar ->
putMVar mvar value >>
return mvar
readMVar :: MVar a -> IO a
-
readMVar mvar =
takeMVar mvar >>= \ value ->
putMVar mvar value >>
return value
swapMVar :: MVar a -> a -> IO a
-
swapMVar mvar new =
takeMVar mvar >>= \ old ->
putMVar mvar new >>
return old
+-- takeMaybeMVar is a non-blocking takeMVar
+takeMaybeMVar :: MVar a -> IO (Maybe a)
+takeMaybeMVar (MVar m) = IO $ \ s ->
+ case takeMaybeMVar# m s of
+ (# s, 0#, _ #) -> (# s, Nothing #) -- MVar is empty
+ (# s, _, a #) -> (# s, Just a #) -- MVar is full
+
{-
Low-level op. for checking whether an MVar is filled-in or not.
Notice that the boolean value returned is just a snapshot of
the state of the MVar. By the time you get to react on its result,
the MVar may have been filled (or emptied) - so be extremely
- careful when using this operation.
+ careful when using this operation.
+
+ Use takeMaybeMVar instead if possible.
If you can re-work your abstractions to avoid having to
depend on isEmptyMVar, then you're encouraged to do so,