Remove Control.Parallel*, now in package parallel
[packages/random.git] / Control / Concurrent / MVar.hs
1 -----------------------------------------------------------------------------
2 -- |
3 -- Module : Control.Concurrent.MVar
4 -- Copyright : (c) The University of Glasgow 2001
5 -- License : BSD-style (see the file libraries/base/LICENSE)
6 --
7 -- Maintainer : libraries@haskell.org
8 -- Stability : experimental
9 -- Portability : non-portable (concurrency)
10 --
11 -- Synchronising variables
12 --
13 -----------------------------------------------------------------------------
14
15 module Control.Concurrent.MVar
16 (
17 -- * @MVar@s
18 MVar -- abstract
19 , newEmptyMVar -- :: IO (MVar a)
20 , newMVar -- :: a -> IO (MVar a)
21 , takeMVar -- :: MVar a -> IO a
22 , putMVar -- :: MVar a -> a -> IO ()
23 , readMVar -- :: MVar a -> IO a
24 , swapMVar -- :: MVar a -> a -> IO a
25 , tryTakeMVar -- :: MVar a -> IO (Maybe a)
26 , tryPutMVar -- :: MVar a -> a -> IO Bool
27 , isEmptyMVar -- :: MVar a -> IO Bool
28 , withMVar -- :: MVar a -> (a -> IO b) -> IO b
29 , modifyMVar_ -- :: MVar a -> (a -> IO a) -> IO ()
30 , modifyMVar -- :: MVar a -> (a -> IO (a,b)) -> IO b
31 #ifndef __HUGS__
32 , addMVarFinalizer -- :: MVar a -> IO () -> IO ()
33 #endif
34 ) where
35
36 #ifdef __HUGS__
37 import Hugs.ConcBase ( MVar, newEmptyMVar, newMVar, takeMVar, putMVar,
38 tryTakeMVar, tryPutMVar, isEmptyMVar,
39 )
40 #endif
41
42 #ifdef __GLASGOW_HASKELL__
43 import GHC.Conc ( MVar, newEmptyMVar, newMVar, takeMVar, putMVar,
44 tryTakeMVar, tryPutMVar, isEmptyMVar, addMVarFinalizer
45 )
46 #endif
47
48 import Prelude
49 import Control.Exception as Exception
50
51 {-|
52 This is a combination of 'takeMVar' and 'putMVar'; ie. it takes the value
53 from the 'MVar', puts it back, and also returns it.
54 -}
55 readMVar :: MVar a -> IO a
56 readMVar m =
57 block $ do
58 a <- takeMVar m
59 putMVar m a
60 return a
61
62 -- |Swap the contents of an 'MVar' for a new value.
63 swapMVar :: MVar a -> a -> IO a
64 swapMVar mvar new =
65 block $ do
66 old <- takeMVar mvar
67 putMVar mvar new
68 return old
69
70 {-|
71 'withMVar' is a safe wrapper for operating on the contents of an
72 'MVar'. This operation is exception-safe: it will replace the
73 original contents of the 'MVar' if an exception is raised (see
74 "Control.Exception").
75 -}
76 {-# INLINE withMVar #-}
77 -- inlining has been reported to have dramatic effects; see
78 -- http://www.haskell.org//pipermail/haskell/2006-May/017907.html
79 withMVar :: MVar a -> (a -> IO b) -> IO b
80 withMVar m io =
81 block $ do
82 a <- takeMVar m
83 b <- Exception.catch (unblock (io a))
84 (\e -> do putMVar m a; throw e)
85 putMVar m a
86 return b
87
88 {-|
89 A safe wrapper for modifying the contents of an 'MVar'. Like 'withMVar',
90 'modifyMVar' will replace the original contents of the 'MVar' if an
91 exception is raised during the operation.
92 -}
93 {-# INLINE modifyMVar_ #-}
94 modifyMVar_ :: MVar a -> (a -> IO a) -> IO ()
95 modifyMVar_ m io =
96 block $ do
97 a <- takeMVar m
98 a' <- Exception.catch (unblock (io a))
99 (\e -> do putMVar m a; throw e)
100 putMVar m a'
101
102 {-|
103 A slight variation on 'modifyMVar_' that allows a value to be
104 returned (@b@) in addition to the modified value of the 'MVar'.
105 -}
106 {-# INLINE modifyMVar #-}
107 modifyMVar :: MVar a -> (a -> IO (a,b)) -> IO b
108 modifyMVar m io =
109 block $ do
110 a <- takeMVar m
111 (a',b) <- Exception.catch (unblock (io a))
112 (\e -> do putMVar m a; throw e)
113 putMVar m a'
114 return b