MVar package:ihaskell

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.

A shared mutable variable (not the same as a MutVar#!). (Note: in a non-concurrent implementation, (MVar# a) can be represented by (MutVar# (Maybe a)).)

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.

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

An exception-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. This function is only atomic if there are no other producers for this MVar. In other words, it cannot guarantee that, by the time modifyMVar_ gets the chance to write to the MVar, the value of the MVar has not been altered by a write operation from another thread.

Create an MVar which is initially empty.

Create an MVar which contains the supplied value.

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. 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

Compare the underlying pointers of two MVar#s.

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.