Portability | non-portable (requires STM) |
---|---|
Stability | experimental |
Maintainer | libraries@haskell.org |
Safe Haskell | Trustworthy |
TChan: Transactional channels (GHC only)
- data TChan a
- newTChan :: STM (TChan a)
- newTChanIO :: IO (TChan a)
- newBroadcastTChan :: STM (TChan a)
- newBroadcastTChanIO :: IO (TChan a)
- dupTChan :: TChan a -> STM (TChan a)
- readTChan :: TChan a -> STM a
- tryReadTChan :: TChan a -> STM (Maybe a)
- peekTChan :: TChan a -> STM a
- tryPeekTChan :: TChan a -> STM (Maybe a)
- writeTChan :: TChan a -> a -> STM ()
- unGetTChan :: TChan a -> a -> STM ()
- isEmptyTChan :: TChan a -> STM Bool
- cloneTChan :: TChan a -> STM (TChan a)
TChans
TChan
is an abstract type representing an unbounded FIFO channel.
Construction
newTChanIO :: IO (TChan a)Source
IO
version of newTChan
. This is useful for creating top-level
TChan
s using unsafePerformIO
, because using
atomically
inside unsafePerformIO
isn't
possible.
newBroadcastTChan :: STM (TChan a)Source
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.
newBroadcastTChanIO :: IO (TChan a)Source
IO
version of newBroadcastTChan
.
dupTChan :: TChan a -> STM (TChan a)Source
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.
Reading and writing
tryReadTChan :: TChan a -> STM (Maybe a)Source
A version of readTChan
which does not retry. Instead it
returns Nothing
if no value is available.
peekTChan :: TChan a -> STM aSource
Get the next value from the TChan
without removing it,
retrying if the channel is empty.
tryPeekTChan :: TChan a -> STM (Maybe a)Source
A version of peekTChan
which does not retry. Instead it
returns Nothing
if no value is available.
unGetTChan :: TChan a -> a -> STM ()Source
Put a data item back onto a channel, where it will be the next item read.