MVar
Synchronising variables
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.
The thread is blocked on an MVar, but there are no other references to the MVar so it can't ever continue.
The current thread was executing a call to Control.Concurrent.MVar.takeMVar that could never return, because there are no other references to this MVar.
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.
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.
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.
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.
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.
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).
TMVar: Transactional MVars, for use in the STM monad (GHC only)
expiring-mvar provides the type ExpiringMVar. ExpiringMVar is a container for a single value. When creating an ExpiringMVar, a thread is spawned which deletes the value held in the ExpiringMVar after a given period of time. The timer can be reset, cancelled, or restarted with a new time amount.
Version 0.1
Check whether a given TMVar is empty.
Create a TMVar which is initially empty.
Create a TMVar which contains the supplied value.
This is a combination of takeTMVar and putTMVar; ie. it takes the value from the TMVar, puts it back, and also returns it.
Swap the contents of a TMVar for a new value.
A TMVar is a synchronising variable, used for communication between concurrent threads. It can be thought of as a box, which may be empty or full.
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