1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
|
{-# LANGUAGE Trustworthy #-}
{-# 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.
--
-- The channels are implemented with @MVar@s and therefore inherit all the
-- caveats that apply to @MVar@s (possibility of races, deadlocks etc). The
-- stm (software transactional memory) library has a more robust implementation
-- of channels called @TChan@s.
--
-----------------------------------------------------------------------------
module Control.Concurrent.Chan
(
-- * The 'Chan' type
Chan, -- abstract
-- * Operations
newChan,
writeChan,
readChan,
dupChan,
-- * Stream interface
getChanContents,
writeList2Chan,
) where
import System.IO.Unsafe ( unsafeInterleaveIO )
import Control.Concurrent.MVar
import Control.Exception (mask_)
#define _UPK_(x) {-# UNPACK #-} !(x)
-- 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 _UPK_(MVar (Stream a))
_UPK_(MVar (Stream a)) -- Invariant: the Stream a is always an empty MVar
deriving Eq -- ^ @since 4.4.0.0
type Stream a = MVar (ChItem a)
data ChItem a = ChItem a _UPK_(Stream a)
-- benchmarks show that unboxing the MVar here is worthwhile, because
-- although it leads to higher allocation, the channel data takes up
-- less space and is therefore quicker to GC.
-- 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
mask_ $ do
old_hole <- takeMVar writeVar
putMVar old_hole (ChItem val new_hole)
putMVar writeVar new_hole
-- The reason we don't simply do this:
--
-- modifyMVar_ writeVar $ \old_hole -> do
-- putMVar old_hole (ChItem val new_hole)
-- return new_hole
--
-- is because if an asynchronous exception is received after the 'putMVar'
-- completes and before modifyMVar_ installs the new value, it will set the
-- Chan's write end to a filled hole.
-- |Read the next value from the 'Chan'. Blocks when the channel is empty. Since
-- the read end of a channel is an 'MVar', this operation inherits fairness
-- guarantees of 'MVar's (e.g. threads blocked in this operation are woken up in
-- FIFO order).
--
-- Throws 'Control.Exception.BlockedIndefinitelyOnMVar' when the channel is
-- empty and no other thread holds a reference to the channel.
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)
-- 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)
|
