MVars

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

Create an MVar which is initially empty.

Create an MVar which contains the supplied value.

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.

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.

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

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.

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.

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 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).

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.

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

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