---Defined in IOBase to avoid cycle: data MVar a = MVar (SynchVar# RealWorld a)
-
--- |Create an 'MVar' which is initially empty.
-newEmptyMVar :: IO (MVar a)
-newEmptyMVar = IO $ \ s# ->
- case newMVar# s# of
- (# s2#, svar# #) -> (# s2#, MVar svar# #)
-
--- |Create an 'MVar' which contains the supplied value.
-newMVar :: a -> IO (MVar a)
-newMVar value =
- newEmptyMVar >>= \ mvar ->
- putMVar mvar value >>
- return mvar
-
--- |Return the contents of the 'MVar'. If the 'MVar' is currently
--- empty, 'takeMVar' will wait until it is full. After a 'takeMVar',
--- the 'MVar' is left empty.
---
--- There are two further important properties of 'takeMVar':
---
--- * 'takeMVar' is single-wakeup. That is, if there are multiple
--- threads blocked in 'takeMVar', and the 'MVar' becomes full,
--- only one thread will be woken up. The runtime guarantees that
--- the woken thread completes its 'takeMVar' operation.
---
--- * When multiple threads are blocked on an 'MVar', they are
--- woken up in FIFO order. This is useful for providing
--- fairness properties of abstractions built using 'MVar's.
---
-takeMVar :: MVar a -> IO a
-takeMVar (MVar mvar#) = IO $ \ s# -> takeMVar# mvar# s#
-
--- |Put a value into an 'MVar'. If the 'MVar' is currently full,
--- 'putMVar' will wait until it becomes empty.
---
--- There are two further important properties of 'putMVar':
---
--- * 'putMVar' is single-wakeup. That is, if there are multiple
--- threads blocked in 'putMVar', and the 'MVar' becomes empty,
--- only one thread will be woken up. The runtime guarantees that
--- the woken thread completes its 'putMVar' operation.
---
--- * When multiple threads are blocked on an 'MVar', they are
--- woken up in FIFO order. This is useful for providing
--- fairness properties of abstractions built using 'MVar's.
---
-putMVar :: MVar a -> a -> IO ()
-putMVar (MVar mvar#) x = IO $ \ s# ->
- case putMVar# mvar# x s# of
- s2# -> (# s2#, () #)
-
--- |A non-blocking version of 'takeMVar'. The 'tryTakeMVar' function
--- returns immediately, with 'Nothing' if the 'MVar' was empty, or
--- @'Just' a@ if the 'MVar' was full with contents @a@. After 'tryTakeMVar',
--- the 'MVar' is left empty.
-tryTakeMVar :: MVar a -> IO (Maybe a)
-tryTakeMVar (MVar m) = IO $ \ s ->
- case tryTakeMVar# m s of
- (# s, 0#, _ #) -> (# s, Nothing #) -- MVar is empty
- (# s, _, a #) -> (# s, Just a #) -- MVar is full
-
--- |A non-blocking version of 'putMVar'. The 'tryPutMVar' function
--- attempts to put the value @a@ into the 'MVar', returning 'True' if
--- it was successful, or 'False' otherwise.
-tryPutMVar :: MVar a -> a -> IO Bool
-tryPutMVar (MVar mvar#) x = IO $ \ s# ->
- case tryPutMVar# mvar# x s# of
- (# s, 0# #) -> (# s, False #)
- (# s, _ #) -> (# s, True #)
-
--- |Check whether a given 'MVar' is empty.
---
--- 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. Use 'tryTakeMVar' instead if possible.
-isEmptyMVar :: MVar a -> IO Bool
-isEmptyMVar (MVar mv#) = IO $ \ s# ->
- case isEmptyMVar# mv# s# of
- (# s2#, flg #) -> (# s2#, not (flg ==# 0#) #)
-
--- |Add a finalizer to an 'MVar' (GHC only). See "Foreign.ForeignPtr" and
--- "System.Mem.Weak" for more about finalizers.
-addMVarFinalizer :: MVar a -> IO () -> IO ()
-addMVarFinalizer (MVar m) finalizer =
- IO $ \s -> case mkWeak# m () finalizer s of { (# s1, w #) -> (# s1, () #) }
-
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