Control.Concurrent.STM.TBMChan

stm-chans-3.0.0.4: Additional types of channels for STM.

Copyright | Copyright (c) 2011--2015 wren gayle romano
License | BSD
Maintainer | wren@community.haskell.org
Stability | provisional
Portability | non-portable (GHC STM, DeriveDataTypeable)
Safe Haskell | Trustworthy
Language | Haskell98

Control.Concurrent.STM.TBMChan

Contents

- The TBMChan type
  - Creating TBMChans
  - Reading from TBMChans
  - Writing to TBMChans
  - Closing TBMChans
  - Predicates
  - Other functionality

Description

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

Synopsis

- data TBMChan a
- newTBMChan :: Int -> STM (TBMChan a)
- newTBMChanIO :: Int -> IO (TBMChan a)
- readTBMChan :: TBMChan a -> STM (Maybe a)
- tryReadTBMChan :: TBMChan a -> STM (Maybe (Maybe a))
- peekTBMChan :: TBMChan a -> STM (Maybe a)
- tryPeekTBMChan :: TBMChan a -> STM (Maybe (Maybe a))
- writeTBMChan :: TBMChan a -> a -> STM ()
- tryWriteTBMChan :: TBMChan a -> a -> STM (Maybe Bool)
- unGetTBMChan :: TBMChan a -> a -> STM ()
- closeTBMChan :: TBMChan a -> STM ()
- isClosedTBMChan :: TBMChan a -> STM Bool
- isEmptyTBMChan :: TBMChan a -> STM Bool
- isFullTBMChan :: TBMChan a -> STM Bool
- estimateFreeSlotsTBMChan :: TBMChan a -> STM Int
- freeSlotsTBMChan :: TBMChan a -> STM Int

# The TBMChan type

data TBMChan a

`TBMChan` is an abstract type representing a bounded closeable FIFO channel.

## Creating TBMChans

newTBMChan :: Int -> STM (TBMChan a)

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

newTBMChanIO :: Int -> IO (TBMChan a)

`IO` version of `newTBMChan`. This is useful for creating top-level `TBMChan`s using `unsafePerformIO`, because using `atomically` inside `unsafePerformIO` isn't possible.

## Reading from TBMChans

readTBMChan :: TBMChan a -> STM (Maybe a)

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

tryReadTBMChan :: TBMChan a -> STM (Maybe (Maybe a))

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

peekTBMChan :: TBMChan a -> STM (Maybe a)

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

tryPeekTBMChan :: TBMChan a -> STM (Maybe (Maybe a))

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

## Writing to TBMChans

writeTBMChan :: TBMChan a -> a -> STM ()

Write a value to a `TBMChan`, retrying if the channel is full. If the channel is closed then the value is silently discarded. Use `isClosedTBMChan` to determine if the channel is closed before writing, as needed.

tryWriteTBMChan :: TBMChan a -> a -> STM (Maybe Bool)

A version of `writeTBMChan` which does not retry. Returns `Just True` if the value was successfully written, `Just False` if it could not be written (but the channel was open), and `Nothing` if it was discarded (i.e., the channel was closed).

unGetTBMChan :: TBMChan a -> a -> STM ()

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

## Closing TBMChans

closeTBMChan :: TBMChan a -> STM ()

Closes the `TBMChan`, preventing any further writes.

## Predicates

isClosedTBMChan :: TBMChan a -> STM Bool

Returns `True` if the supplied `TBMChan` has been closed.

isEmptyTBMChan :: TBMChan a -> STM Bool

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

isFullTBMChan :: TBMChan a -> STM Bool

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

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

## Other functionality

estimateFreeSlotsTBMChan :: TBMChan 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 `unGetTBMChan` 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 `freeSlotsTBMChan`.

freeSlotsTBMChan :: TBMChan a -> STM Int

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

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