Control.Concurrent.STM.TBChan

A version of Control.Concurrent.STM.TChan where the queue is bounded in length. This variant incorporates ideas from Thomas M. DuBuisson's bounded-tchan package in order to reduce contention between readers and writers.

The TBChan type

data TBChan a

TBChan is an abstract type representing a bounded FIFO channel.

Instances

Typeable (* -> *) TBChan

Creating TBChans

newTBChan :: Int -> STM (TBChan a)

Build and returns a new instance of TBChan with the given capacity. N.B., we do not verify the capacity is positive, but if it is non-positive then writeTBChan will always retry and isFullTBChan will always be true.

newTBChanIO :: Int -> IO (TBChan a)

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

Reading from TBChans

readTBChan :: TBChan a -> STM a

Read the next value from the TBChan, retrying if the channel is empty.

tryReadTBChan :: TBChan a -> STM (Maybe a)

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

peekTBChan :: TBChan a -> STM a

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

tryPeekTBChan :: TBChan a -> STM (Maybe a)

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

Writing to TBChans

writeTBChan :: TBChan a -> a -> STM ()

Write a value to a TBChan, retrying if the channel is full.

tryWriteTBChan :: TBChan a -> a -> STM Bool

A version of writeTBChan which does not retry. Returns True if the value was successfully written, and False otherwise.

unGetTBChan :: TBChan a -> a -> STM ()

Put a data item back onto a channel, where it will be the next item read. N.B., this could allow the channel to temporarily become longer than the specified limit, which is necessary to ensure that the item is indeed the next one read.

Predicates

isEmptyTBChan :: TBChan a -> STM Bool

Returns True if the supplied TBChan is empty (i.e., has no elements). N.B., a TBChan can be both ``empty'' and ``full'' at the same time, if the initial limit was non-positive.

isFullTBChan :: TBChan a -> STM Bool

Returns True if the supplied TBChan is full (i.e., is over its limit). N.B., a TBChan can be both ``empty'' and ``full'' at the same time, if the initial limit was non-positive. N.B., a TBChan may still be full after reading, if unGetTBChan was used to go over the initial limit.

This is equivalent to: liftM (<= 0) estimateFreeSlotsTBMChan

Other functionality

estimateFreeSlotsTBChan :: TBChan a -> STM Int

Estimate the number of free slots. If the result is positive, then it's a minimum bound; if it's non-positive then it's exact. It will only be negative if the initial limit was negative or if unGetTBChan was used to go over the initial limit.

This function always contends with writers, but only contends with readers when it has to; compare against freeSlotsTBChan.

freeSlotsTBChan :: TBChan a -> STM Int

Return the exact number of free slots. The result can be negative if the initial limit was negative or if unGetTBChan was used to go over the initial limit.

This function always contends with both readers and writers; compare against estimateFreeSlotsTBChan.