MVar package:concurrency

An MVar t is mutable location that is either empty or contains a value of type t. It has two fundamental operations: putMVar which fills an MVar if it is empty and blocks otherwise, and takeMVar which empties an MVar if it is full and blocks otherwise. They can be used in multiple different ways:

1. As synchronized mutable variables,
2. As channels, with takeMVar and putMVar as receive and send, and
3. As a binary semaphore MVar (), with takeMVar and putMVar as wait and signal.

Deviations: There is no Eq instance for MonadConc the MVar type. Furthermore, the mkWeakMVar and addMVarFinalizer functions are not provided. Finally, normal MVars have a fairness guarantee, which dejafu does not currently make use of when generating schedules to test, so your program may be tested with unfair schedules.

Check if a MVar is empty. The boolean value returned is just a snapshot of the state of the MVar, it may have been emptied (or filled) by the time you actually access it. Generally prefer tryPutMVar, tryTakeMVar, and tryReadMVar.

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

Like modifyMVar, but the IO action in the second argument is executed with asynchronous exceptions masked.

Like modifyMVar_, but the IO action in the second argument is executed with asynchronous exceptions masked.

An exception-safe wrapper for modifying the contents of a 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.

Create a new empty MVar.

newEmptyMVar = newEmptyMVarN ""

Create a new empty MVar, but it is given a name which may be used to present more useful debugging information.

newEmptyMVarN _ = newEmptyMVar

Create a new MVar containing a value.

Create a new MVar containing a value, but it is given a name which may be used to present more useful debugging information.

Put a value into a MVar. If there is already a value there, this will block until that value has been taken, at which point the value will be stored.

Block until a value is present in the MVar, and then return it. This does not "remove" the value, multiple reads are possible.

Swap the contents of a MVar, and return the value taken. This function is atomic only if there are no other producers fro this MVar.

Take a value from a MVar. This "empties" the MVar, allowing a new value to be put in. This will block if there is no value in the MVar already, until one has been put.

Attempt to put a value in a MVar non-blockingly, returning True (and filling the MVar) if there was nothing there, otherwise returning False.

Attempt to take a value from a MVar non-blockingly, returning a Just (and emptying the MVar) if there was something there, otherwise returning Nothing.

Operate on the contents of a MVar, replacing the contents after finishing. This operation is exception-safe: it will replace the original contents of the MVar if an exception is raised. However, it is only atomic if there are no other producers for this MVar.

Like withMVar, but the IO action in the second argument is executed with asynchronous exceptions masked.

Transactional MVars, for use with MonadSTM. Deviations: TMVar as defined here does not have an Eq instance, this is because the MonadSTM TVar type does not have an Eq constraint. Furthermore, the newTMVarIO, newEmptyTMVarIO, and mkWeakTMVar functions are not provided.

A TMVar is like an MVar or a mVar, but using transactional memory. As transactions are atomic, this makes dealing with multiple TMVars easier than wrangling multiple mVars.

Check if a TMVar is empty or not.

Create a new empty TMVar.

Create a new empty TMVar with the given name. Name conflicts are handled as usual for TVars. The name is prefixed with "ctmvar-".

Create a TMVar containing the given value.

Create a TMVar containing the given value, with the given name. Name conflicts are handled as usual for TVars. The name is prefixed with "ctmvar-".