1 -----------------------------------------------------------------------------
3 -- Module : Control.Concurrent.MVar
4 -- Copyright : (c) The University of Glasgow 2001
5 -- License : BSD-style (see the file libraries/base/LICENSE)
7 -- Maintainer : libraries@haskell.org
8 -- Stability : experimental
9 -- Portability : non-portable (concurrency)
11 -- Synchronising variables
13 -----------------------------------------------------------------------------
15 module Control.Concurrent.MVar
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 , addMVarFinalizer -- :: MVar a -> IO () -> IO ()
35 import ConcBase ( MVar, newEmptyMVar, newMVar, takeMVar, putMVar,
36 tryTakeMVar, tryPutMVar, isEmptyMVar,
41 #ifdef __GLASGOW_HASKELL__
42 import GHC.Conc ( MVar, newEmptyMVar, newMVar, takeMVar, putMVar,
43 tryTakeMVar, tryPutMVar, isEmptyMVar, addMVarFinalizer
48 import Control.Exception as Exception
51 -- This is as close as Hugs gets to providing throw
52 throw :: Exception -> IO a
56 #ifdef __GLASGOW_HASKELL__
58 This is a combination of 'takeMVar' and 'putMVar'; ie. it takes the value
59 from the 'MVar', puts it back, and also returns it.
61 readMVar :: MVar a -> IO a
68 -- |Swap the contents of an 'MVar' for a new value.
69 swapMVar :: MVar a -> a -> IO a
70 swapMVar mvar new = modifyMVar mvar (\old -> return (new,old))
74 'withMVar' is a safe wrapper for operating on the contents of an
75 'MVar'. This operation is exception-safe: it will replace the
76 original contents of the 'MVar' if an exception is raised (see
79 withMVar :: MVar a -> (a -> IO b) -> IO b
83 b <- Exception.catch (unblock (io a))
84 (\e -> do putMVar m a; throw e)
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.
93 modifyMVar_ :: MVar a -> (a -> IO a) -> IO ()
97 a' <- Exception.catch (unblock (io a))
98 (\e -> do putMVar m a; throw e)
102 A slight variation on 'modifyMVar_' that allows a value to be
103 returned (@b@) in addition to the modified value of the 'MVar'.
105 modifyMVar :: MVar a -> (a -> IO (a,b)) -> IO b
109 (a',b) <- Exception.catch (unblock (io a))
110 (\e -> do putMVar m a; throw e)