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|
{-# LANGUAGE CPP #-}
-----------------------------------------------------------------------------
-- |
-- Module : Control.Concurrent.Chan
-- Copyright : (c) The University of Glasgow 2001
-- License : BSD-style (see the file libraries/base/LICENSE)
--
-- Maintainer : libraries@haskell.org
-- Stability : experimental
-- Portability : non-portable (concurrency)
--
-- Unbounded channels.
--
-----------------------------------------------------------------------------
module Control.Concurrent.Chan
(
-- * The 'Chan' type
Chan, -- abstract
-- * Operations
newChan, -- :: IO (Chan a)
writeChan, -- :: Chan a -> a -> IO ()
readChan, -- :: Chan a -> IO a
dupChan, -- :: Chan a -> IO (Chan a)
unGetChan, -- :: Chan a -> a -> IO ()
isEmptyChan, -- :: Chan a -> IO Bool
-- * Stream interface
getChanContents, -- :: Chan a -> IO [a]
writeList2Chan, -- :: Chan a -> [a] -> IO ()
) where
import Prelude
import System.IO.Unsafe ( unsafeInterleaveIO )
import Control.Concurrent.MVar
import Data.Typeable
#include "Typeable.h"
-- A channel is represented by two @MVar@s keeping track of the two ends
-- of the channel contents,i.e., the read- and write ends. Empty @MVar@s
-- are used to handle consumers trying to read from an empty channel.
-- |'Chan' is an abstract type representing an unbounded FIFO channel.
data Chan a
= Chan (MVar (Stream a))
(MVar (Stream a))
deriving Eq
INSTANCE_TYPEABLE1(Chan,chanTc,"Chan")
type Stream a = MVar (ChItem a)
data ChItem a = ChItem a (Stream a)
-- See the Concurrent Haskell paper for a diagram explaining the
-- how the different channel operations proceed.
-- @newChan@ sets up the read and write end of a channel by initialising
-- these two @MVar@s with an empty @MVar@.
-- |Build and returns a new instance of 'Chan'.
newChan :: IO (Chan a)
newChan = do
hole <- newEmptyMVar
readVar <- newMVar hole
writeVar <- newMVar hole
return (Chan readVar writeVar)
-- To put an element on a channel, a new hole at the write end is created.
-- What was previously the empty @MVar@ at the back of the channel is then
-- filled in with a new stream element holding the entered value and the
-- new hole.
-- |Write a value to a 'Chan'.
writeChan :: Chan a -> a -> IO ()
writeChan (Chan _ writeVar) val = do
new_hole <- newEmptyMVar
modifyMVar_ writeVar $ \old_hole -> do
putMVar old_hole (ChItem val new_hole)
return new_hole
-- |Read the next value from the 'Chan'.
readChan :: Chan a -> IO a
readChan (Chan readVar _) = do
modifyMVar readVar $ \read_end -> do
(ChItem val new_read_end) <- readMVar read_end
-- Use readMVar here, not takeMVar,
-- else dupChan doesn't work
return (new_read_end, val)
-- |Duplicate a 'Chan': 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.
--
-- (Note that a duplicated channel is not equal to its original.
-- So: @fmap (c /=) $ dupChan c@ returns @True@ for all @c@.)
dupChan :: Chan a -> IO (Chan a)
dupChan (Chan _ writeVar) = do
hole <- readMVar writeVar
newReadVar <- newMVar hole
return (Chan newReadVar writeVar)
-- |Put a data item back onto a channel, where it will be the next item read.
unGetChan :: Chan a -> a -> IO ()
unGetChan (Chan readVar _) val = do
new_read_end <- newEmptyMVar
modifyMVar_ readVar $ \read_end -> do
putMVar new_read_end (ChItem val read_end)
return new_read_end
{-# DEPRECATED unGetChan "if you need this operation, use Control.Concurrent.STM.TChan instead. See http://hackage.haskell.org/trac/ghc/ticket/4154 for details" #-}
-- |Returns 'True' if the supplied 'Chan' is empty.
isEmptyChan :: Chan a -> IO Bool
isEmptyChan (Chan readVar writeVar) = do
withMVar readVar $ \r -> do
w <- readMVar writeVar
let eq = r == w
eq `seq` return eq
{-# DEPRECATED isEmptyChan "if you need this operation, use Control.Concurrent.STM.TChan instead. See http://hackage.haskell.org/trac/ghc/ticket/4154 for details" #-}
-- Operators for interfacing with functional streams.
-- |Return a lazy list representing the contents of the supplied
-- 'Chan', much like 'System.IO.hGetContents'.
getChanContents :: Chan a -> IO [a]
getChanContents ch
= unsafeInterleaveIO (do
x <- readChan ch
xs <- getChanContents ch
return (x:xs)
)
-- |Write an entire list of items to a 'Chan'.
writeList2Chan :: Chan a -> [a] -> IO ()
writeList2Chan ch ls = sequence_ (map (writeChan ch) ls)
|
