Control.Concurrent.STM.TMQueue

A version of Control.Concurrent.STM.TQueue where the queue is closeable. This is similar to a TQueue (Maybe a) with a monotonicity guarantee that once there's a Nothing there will always be Nothing.

Since: 2.0.0

The TMQueue type

data TMQueue a

TMQueue is an abstract type representing a closeable FIFO queue.

Instances

Typeable1 TMQueue

Creating TMQueues

newTMQueue :: STM (TMQueue a)

Build and returns a new instance of TMQueue.

newTMQueueIO :: IO (TMQueue a)

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

Reading from TMQueues

readTMQueue :: TMQueue a -> STM (Maybe a)

Read the next value from the TMQueue, retrying if the queue is empty (and not closed). We return Nothing immediately if the queue is closed and empty.

tryReadTMQueue :: TMQueue a -> STM (Maybe (Maybe a))

A version of readTMQueue which does not retry. Instead it returns Just Nothing if the queue is open but no value is available; it still returns Nothing if the queue is closed and empty.

peekTMQueue :: TMQueue a -> STM (Maybe a)

Get the next value from the TMQueue without removing it, retrying if the queue is empty.

tryPeekTMQueue :: TMQueue a -> STM (Maybe (Maybe a))

A version of peekTMQueue which does not retry. Instead it returns Just Nothing if the queue is open but no value is available; it still returns Nothing if the queue is closed and empty.

Writing to TMQueues

writeTMQueue :: TMQueue a -> a -> STM ()

Write a value to a TMQueue. If the queue is closed then the value is silently discarded. Use isClosedTMQueue to determine if the queue is closed before writing, as needed.

unGetTMQueue :: TMQueue a -> a -> STM ()

Put a data item back onto a queue, where it will be the next item read. If the queue is closed then the value is silently discarded; you can use peekTMQueue to circumvent this in certain circumstances.

Closing TMQueues

closeTMQueue :: TMQueue a -> STM ()

Closes the TMQueue, preventing any further writes.

Predicates

isClosedTMQueue :: TMQueue a -> STM Bool

Returns True if the supplied TMQueue has been closed.

isEmptyTMQueue :: TMQueue a -> STM Bool

Returns True if the supplied TMQueue is empty.