MVars

An MVar (pronounced "em-var") is a synchronising variable, used for communication between concurrent threads. It can be thought of as a box, which may be empty or full.

Constructors

Instances

Create an MVar which contains the supplied value.

Create an MVar which is initially empty.

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.

Atomically read the contents of an MVar . If the MVar is currently empty, readMVar will wait until it is full. readMVar is guaranteed to receive the next putMVar .

readMVar is multiple-wakeup, so when multiple readers are blocked on an MVar , all of them are woken up at the same time.

Compatibility note: Prior to base 4.7, readMVar was a combination of takeMVar and putMVar . This mean that in the presence of other threads attempting to putMVar , readMVar could block. Furthermore, readMVar would not receive the next putMVar if there was already a pending thread blocked on takeMVar . The old behavior can be recovered by implementing 'readMVar as follows:

 readMVar :: MVar a -> IO a
 readMVar m =
 mask_ $ do
 a <- takeMVar m
 putMVar m a
 return a

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.

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.

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.

A non-blocking version of readMVar . The tryReadMVar function returns immediately, with Nothing if the MVar was empty, or Just a if the MVar was full with contents a.

Since: base-4.7.0.0

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.

Add a finalizer to an MVar (GHC only). See Foreign.ForeignPtr and System.Mem.Weak for more about finalizers.