Documentation

Create a new MVar that holds the supplied argument.

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.

Create a new MVar that is initially empty.

Put a value into an MVar. If the MVar is currently full, putMVar will wait until it becomes empty.

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.

Multiple Wakeup: 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: On GHCs prior to 7.8, readMVar is a combination of takeMVar and putMVar. Consequently, its behavior differs in the following ways:

• It is single-wakeup instead of multiple-wakeup.
• It might not receive the value from the next call to putMVar if there is already a pending thread blocked on takeMVar.
• If another thread puts a value in the MVar in between the calls to takeMVar and putMVar, that value may be overridden.

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.

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.

Compatibility note: On GHCs prior to 7.8, tryReadMVar is a combination of tryTakeMVar and putMVar. Consequently, its behavior differs in the following ways:

• It is single-wakeup instead of multiple-wakeup.
• In the presence of other threads calling putMVar, tryReadMVar may block.
• If another thread puts a value in the MVar in between the calls to tryTakeMVar and putMVar, that value may be overridden.

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.