Documentation

A synchronizing variable, used for communication between concurrent threads. It can be thought of as a box, which may be empty or full.

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.

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.

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.

• 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.

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.

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.

• 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.