[project @ 2001-06-28 14:15:04 by simonmar]
[packages/pretty.git] / Control / Concurrent / QSemN.hs
1 -----------------------------------------------------------------------------
2 --
3 -- Module : Control.Concurrent.QSemN
4 -- Copyright : (c) The University of Glasgow 2001
5 -- License : BSD-style (see the file libraries/core/LICENSE)
6 --
7 -- Maintainer : libraries@haskell.org
8 -- Stability : experimental
9 -- Portability : non-portable
10 --
11 -- $Id: QSemN.hs,v 1.1 2001/06/28 14:15:02 simonmar Exp $
12 --
13 -- Quantity semaphores
14 --
15 -----------------------------------------------------------------------------
16
17 module Control.Concurrent.QSemN
18 ( QSemN, -- abstract
19 newQSemN, -- :: Int -> IO QSemN
20 waitQSemN, -- :: QSemN -> Int -> IO ()
21 signalQSemN -- :: QSemN -> Int -> IO ()
22 ) where
23
24 import Prelude
25
26 import Control.Concurrent.MVar
27
28 newtype QSemN = QSemN (MVar (Int,[(Int,MVar ())]))
29
30 newQSemN :: Int -> IO QSemN
31 newQSemN init = do
32 sem <- newMVar (init,[])
33 return (QSemN sem)
34
35 waitQSemN :: QSemN -> Int -> IO ()
36 waitQSemN (QSemN sem) sz = do
37 (avail,blocked) <- takeMVar sem -- gain ex. access
38 if (avail - sz) >= 0 then
39 -- discharging 'sz' still leaves the semaphore
40 -- in an 'unblocked' state.
41 putMVar sem (avail-sz,[])
42 else do
43 block <- newEmptyMVar
44 putMVar sem (avail, blocked++[(sz,block)])
45 takeMVar block
46
47 signalQSemN :: QSemN -> Int -> IO ()
48 signalQSemN (QSemN sem) n = do
49 (avail,blocked) <- takeMVar sem
50 (avail',blocked') <- free (avail+n) blocked
51 putMVar sem (avail',blocked')
52 where
53 free avail [] = return (avail,[])
54 free avail ((req,block):blocked)
55 | avail >= req = do
56 putMVar block ()
57 free (avail-req) blocked
58 | otherwise = do
59 (avail',blocked') <- free avail blocked
60 return (avail',(req,block):blocked')