Rebase.Control.Concurrent.STM

A monad supporting atomic memory transactions.

TArray is a transactional array, supporting the usual MArray interface for mutable arrays.

It is conceptually implemented as Array i (TVar e).

TBQueue is an abstract type representing a bounded FIFO channel.

TChan is an abstract type representing an unbounded FIFO channel.

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.

TQueue is an abstract type representing an unbounded FIFO channel.

Shared memory locations that support atomic memory transactions.

Compose two alternative STM actions (GHC only).

If the first action completes without retrying then it forms the result of the orElse. Otherwise, if the first action retries, then the second action is tried in its place. If both actions retry then the orElse as a whole retries.

The maximum number of elements the queue can hold.

Efficiently read the entire contents of a TBQueue into a list. This function never retries.

Returns True if the supplied TBQueue is empty.

Returns True if the supplied TBQueue is full.

Return the length of a TBQueue.

Builds and returns a new instance of TBQueue.

IO version of newTBQueue. This is useful for creating top-level TBQueues using unsafePerformIO, because using atomically inside unsafePerformIO isn't possible.

Get the next value from the TBQueue without removing it, retrying if the channel is empty.

Read the next value from the TBQueue.

A version of peekTBQueue which does not retry. Instead it returns Nothing if no value is available.

A version of readTBQueue which does not retry. Instead it returns Nothing if no value is available.

Put a data item back onto a channel, where it will be the next item read. Blocks if the queue is full.

Write a value to a TBQueue; blocks if the queue is full.

Clone a TChan: similar to dupTChan, but the cloned channel starts with the same content available as the original channel.

Duplicate a TChan: the duplicate channel begins empty, but data written to either channel from then on will be available from both. Hence this creates a kind of broadcast channel, where data written by anyone is seen by everyone else.

Returns True if the supplied TChan is empty.

Create a write-only TChan. More precisely, readTChan will retry even after items have been written to the channel. The only way to read a broadcast channel is to duplicate it with dupTChan.

Consider a server that broadcasts messages to clients:

serve :: TChan Message -> Client -> IO loop
serve broadcastChan client = do
    myChan <- dupTChan broadcastChan
    forever $ do
        message <- readTChan myChan
        send client message

The problem with using newTChan to create the broadcast channel is that if it is only written to and never read, items will pile up in memory. By using newBroadcastTChan to create the broadcast channel, items can be garbage collected after clients have seen them.

IO version of newBroadcastTChan.

Build and return a new instance of TChan

IO version of newTChan. This is useful for creating top-level TChans using unsafePerformIO, because using atomically inside unsafePerformIO isn't possible.

Get the next value from the TChan without removing it, retrying if the channel is empty.

Read the next value from the TChan.

A version of peekTChan which does not retry. Instead it returns Nothing if no value is available.

A version of readTChan which does not retry. Instead it returns Nothing if no value is available.

Put a data item back onto a channel, where it will be the next item read.

Write a value to a TChan.

Check whether a given TMVar is empty.

Make a Weak pointer to a TMVar, using the second argument as a finalizer to run when the TMVar is garbage-collected.

Create a TMVar which is initially empty.

IO version of newEmptyTMVar. This is useful for creating top-level TMVars using unsafePerformIO, because using atomically inside unsafePerformIO isn't possible.

Create a TMVar which contains the supplied value.

IO version of newTMVar. This is useful for creating top-level TMVars using unsafePerformIO, because using atomically inside unsafePerformIO isn't possible.

Put a value into a TMVar. If the TMVar is currently full, putTMVar will retry.

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.

Return the contents of the TMVar. If the TMVar is currently empty, the transaction will retry. After a takeTMVar, the TMVar is left empty.

A version of putTMVar that does not retry. The tryPutTMVar function attempts to put the value a into the TMVar, returning True if it was successful, or False otherwise.

A version of readTMVar which does not retry. Instead it returns Nothing if no value is available.

A version of takeTMVar that does not retry. The tryTakeTMVar function returns Nothing if the TMVar was empty, or Just a if the TMVar was full with contents a. After tryTakeTMVar, the TMVar is left empty.

Non-blocking write of a new value to a TMVar Puts if empty. Replaces if populated.

Efficiently read the entire contents of a TQueue into a list. This function never retries.

Returns True if the supplied TQueue is empty.

Build and returns a new instance of TQueue

IO version of newTQueue. This is useful for creating top-level TQueues using unsafePerformIO, because using atomically inside unsafePerformIO isn't possible.

Get the next value from the TQueue without removing it, retrying if the channel is empty.

Read the next value from the TQueue.

A version of peekTQueue which does not retry. Instead it returns Nothing if no value is available.

A version of readTQueue which does not retry. Instead it returns Nothing if no value is available.

Put a data item back onto a channel, where it will be the next item read.

Write a value to a TQueue.

Make a Weak pointer to a TVar, using the second argument as a finalizer to run when TVar is garbage-collected

Mutate the contents of a TVar. N.B., this version is non-strict.

Strict version of modifyTVar.

Like modifyTVar' but the function is a simple state transition that can return a side value which is passed on as the result of the STM.

Swap the contents of a TVar for a new value.

Check that the boolean condition is true and, if not, retry.

In other words, check b = unless b retry.

Switch the value of returned TVar from initial value False to True after a given number of microseconds. The caveats associated with threadDelay also apply.

Be careful not to exceed maxBound :: Int, which on 32-bit machines is only 2147483647 μs, less than 36 minutes.

Perform a series of STM actions atomically.

Using atomically inside an unsafePerformIO or unsafeInterleaveIO subverts some of guarantees that STM provides. It makes it possible to run a transaction inside of another transaction, depending on when the thunk is evaluated. If a nested transaction is attempted, an exception is thrown by the runtime. It is possible to safely use atomically inside unsafePerformIO or unsafeInterleaveIO, but the typechecker does not rule out programs that may attempt nested transactions, meaning that the programmer must take special care to prevent these.

However, there are functions for creating transactional variables that can always be safely called in unsafePerformIO. See: newTVarIO, newTChanIO, newBroadcastTChanIO, newTQueueIO, newTBQueueIO, and newTMVarIO.

Using unsafePerformIO inside of atomically is also dangerous but for different reasons. See unsafeIOToSTM for more on this.

Exception handling within STM actions.

catchSTM m f catches any exception thrown by m using throwSTM, using the function f to handle the exception. If an exception is thrown, any changes made by m are rolled back, but changes prior to m persist.

Create a new TVar holding a value supplied

IO version of newTVar. This is useful for creating top-level TVars using unsafePerformIO, because using atomically inside unsafePerformIO isn't possible.

Return the current value stored in a TVar.

Return the current value stored in a TVar. This is equivalent to

 readTVarIO = atomically . readTVar

but works much faster, because it doesn't perform a complete transaction, it just reads the current value of the TVar.

Retry execution of the current memory transaction because it has seen values in TVars which mean that it should not continue (e.g. the TVars represent a shared buffer that is now empty). The implementation may block the thread until one of the TVars that it has read from has been updated. (GHC only)

A variant of throw that can only be used within the STM monad.

Throwing an exception in STM aborts the transaction and propagates the exception. If the exception is caught via catchSTM, only the changes enclosed by the catch are rolled back; changes made outside of catchSTM persist.

If the exception is not caught inside of the STM, it is re-thrown by atomically, and the entire STM is rolled back.

Although throwSTM has a type that is an instance of the type of throw, the two functions are subtly different:

throw e    `seq` x  ===> throw e
throwSTM e `seq` x  ===> x

The first example will cause the exception e to be raised, whereas the second one won't. In fact, throwSTM will only cause an exception to be raised when it is used within the STM monad. The throwSTM variant should be used in preference to throw to raise an exception within the STM monad because it guarantees ordering with respect to other STM operations, whereas throw does not.

Write the supplied value into a TVar.