Control.Concurrent.MVar

Synchronising variables

MVars

data MVar a

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

Instances

Typeable1 MVar

Eq (MVar a)

newEmptyMVar :: IO (MVar a)

Create an MVar which is initially empty.

newMVar :: a -> IO (MVar a)

Create an MVar which contains the supplied value.

takeMVar :: MVar a -> IO a

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 MVars.

putMVar :: MVar a -> a -> IO ()

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 MVars.

readMVar :: MVar a -> IO a

This is a combination of takeMVar and putMVar; ie. it takes the value from the MVar, puts it back, and also returns it.

swapMVar :: MVar a -> a -> IO a

Take a value from an MVar, put a new value into the MVar and return the value taken. Note that there is a race condition whereby another process can put something in the MVar after the take happens but before the put does.

tryTakeMVar :: MVar a -> IO (Maybe a)

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.

tryPutMVar :: MVar a -> a -> IO Bool

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.

isEmptyMVar :: MVar a -> IO Bool

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.

withMVar :: MVar a -> (a -> IO b) -> IO b

withMVar is a safe wrapper for operating on the contents of an MVar. This operation is exception-safe: it will replace the original contents of the MVar if an exception is raised (see Control.Exception).

modifyMVar_ :: MVar a -> (a -> IO a) -> IO ()

A safe wrapper for modifying the contents of an MVar. Like withMVar, modifyMVar will replace the original contents of the MVar if an exception is raised during the operation.

modifyMVar :: MVar a -> (a -> IO (a, b)) -> IO b

A slight variation on modifyMVar_ that allows a value to be returned (b) in addition to the modified value of the MVar.

addMVarFinalizer :: MVar a -> IO () -> IO ()

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