2 % (c) The GRASP/AQUA Project, Glasgow University, 1995-97
4 \section[Channel]{Unbounded Channels}
6 Standard, unbounded channel abstraction.
11 {- abstract type defined -}
15 newChan, -- :: IO (Chan a)
18 writeChan, -- :: Chan a -> a -> IO ()
19 readChan, -- :: Chan a -> IO a
20 dupChan, -- :: Chan a -> IO (Chan a)
21 unGetChan, -- :: Chan a -> a -> IO ()
23 {- stream interface -}
24 getChanContents, -- :: Chan a -> IO [a]
25 writeList2Chan -- :: Chan a -> [a] -> IO ()
32 import PrelUnsafe ( unsafeInterleaveIO )
35 A channel is represented by two @MVar@s keeping track of the two ends
36 of the channel contents,i.e., the read- and write ends. Empty @MVar@s
37 are used to handle consumers trying to read from an empty channel.
41 = Chan (MVar (Stream a))
44 type Stream a = MVar (ChItem a)
46 data ChItem a = ChItem a (Stream a)
49 See the Concurrent Haskell paper for a diagram explaining the
50 how the different channel operations proceed.
52 @newChan@ sets up the read and write end of a channel by initialising
53 these two @MVar@s with an empty @MVar@.
56 newChan :: IO (Chan a)
61 return (Chan read write)
64 To put an element on a channel, a new hole at the write end is created.
65 What was previously the empty @MVar@ at the back of the channel is then
66 filled in with a new stream element holding the entered value and the
70 writeChan :: Chan a -> a -> IO ()
71 writeChan (Chan read write) val = do
72 new_hole <- newEmptyMVar
73 old_hole <- takeMVar write
74 putMVar write new_hole
75 putMVar old_hole (ChItem val new_hole)
77 readChan :: Chan a -> IO a
78 readChan (Chan read write) = do
79 read_end <- takeMVar read
80 (ChItem val new_read_end) <- takeMVar read_end
81 putMVar read new_read_end
85 dupChan :: Chan a -> IO (Chan a)
86 dupChan (Chan read write) = do
87 new_read <- newEmptyMVar
88 hole <- readMVar write
90 return (Chan new_read write)
92 unGetChan :: Chan a -> a -> IO ()
93 unGetChan (Chan read write) val = do
94 new_read_end <- newEmptyMVar
95 read_end <- takeMVar read
96 putMVar new_read_end (ChItem val read_end)
97 putMVar read new_read_end
101 Operators for interfacing with functional streams.
104 getChanContents :: Chan a -> IO [a]
106 = unsafeInterleaveIO (do
108 xs <- getChanContents ch
113 writeList2Chan :: Chan a -> [a] -> IO ()
114 writeList2Chan ch ls = sequence (map (writeChan ch) ls)