X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=Control%2FConcurrent%2FMVar.hs;h=9a95b8ff0817236d34d952bd8e0673da8c6b2f97;hb=41e8fba828acbae1751628af50849f5352b27873;hp=7832c2eca38172dd683743d6716cd8646cd381a4;hpb=7f1f4e7a695c402ddd3a1dc2cc7114e649a78ebc;p=ghc-base.git

diff --git a/Control/Concurrent/MVar.hs b/Control/Concurrent/MVar.hs
index 7832c2e..9a95b8f 100644
--- a/Control/Concurrent/MVar.hs
+++ b/Control/Concurrent/MVar.hs
@@ -1,95 +1,123 @@
+{-# LANGUAGE CPP, NoImplicitPrelude #-}
+
 -----------------------------------------------------------------------------
--- 
+-- |
 -- Module      :  Control.Concurrent.MVar
 -- Copyright   :  (c) The University of Glasgow 2001
--- License     :  BSD-style (see the file libraries/core/LICENSE)
+-- License     :  BSD-style (see the file libraries/base/LICENSE)
 -- 
 -- Maintainer  :  libraries@haskell.org
 -- Stability   :  experimental
--- Portability :  non-portable
---
--- $Id: MVar.hs,v 1.1 2001/06/28 14:15:02 simonmar Exp $
+-- Portability :  non-portable (concurrency)
 --
--- MVars: Synchronising variables
+-- Synchronising variables
 --
 -----------------------------------------------------------------------------
 
 module Control.Concurrent.MVar
-	( MVar		-- abstract
-	, newEmptyMVar  -- :: IO (MVar a)
-	, newMVar 	-- :: a -> IO (MVar a)
-	, takeMVar 	-- :: MVar a -> IO a
-	, putMVar  	-- :: MVar a -> a -> IO ()
-	, readMVar 	-- :: MVar a -> IO a
-	, swapMVar 	-- :: MVar a -> a -> IO a
-	, tryTakeMVar   -- :: MVar a -> IO (Maybe a)
-	, tryPutMVar    -- :: MVar a -> a -> IO Bool
-	, isEmptyMVar	-- :: MVar a -> IO Bool
-	, withMVar	-- :: MVar a -> (a -> IO b) -> IO b
-	, modifyMVar_ 	-- :: MVar a -> (a -> IO a) -> IO ()
-	, modifyMVar 	-- :: MVar a -> (a -> IO (a,b)) -> IO b
-	, addMVarFinalizer -- :: MVar a -> IO () -> IO ()
+        (
+          -- * @MVar@s
+          MVar          -- abstract
+        , newEmptyMVar  -- :: IO (MVar a)
+        , newMVar       -- :: a -> IO (MVar a)
+        , takeMVar      -- :: MVar a -> IO a
+        , putMVar       -- :: MVar a -> a -> IO ()
+        , readMVar      -- :: MVar a -> IO a
+        , swapMVar      -- :: MVar a -> a -> IO a
+        , tryTakeMVar   -- :: MVar a -> IO (Maybe a)
+        , tryPutMVar    -- :: MVar a -> a -> IO Bool
+        , isEmptyMVar   -- :: MVar a -> IO Bool
+        , withMVar      -- :: MVar a -> (a -> IO b) -> IO b
+        , modifyMVar_   -- :: MVar a -> (a -> IO a) -> IO ()
+        , modifyMVar    -- :: MVar a -> (a -> IO (a,b)) -> IO b
+#ifndef __HUGS__
+        , addMVarFinalizer -- :: MVar a -> IO () -> IO ()
+#endif
     ) where
 
 #ifdef __HUGS__
-import ConcBase	( MVar, newEmptyMVar, newMVar, takeMVar, putMVar,
-		  tryTakeMVar, tryPutMVar, isEmptyMVar,
-                  readMVar, swapMVar,
-		)
-import Prelude hiding( catch )
+import Hugs.ConcBase ( MVar, newEmptyMVar, newMVar, takeMVar, putMVar,
+                  tryTakeMVar, tryPutMVar, isEmptyMVar,
+                )
 #endif
 
 #ifdef __GLASGOW_HASKELL__
-import GHC.Conc	( MVar, newEmptyMVar, newMVar, takeMVar, putMVar,
-		  tryTakeMVar, tryPutMVar, isEmptyMVar, addMVarFinalizer
-		)
+import GHC.MVar ( MVar, newEmptyMVar, newMVar, takeMVar, putMVar,
+                  tryTakeMVar, tryPutMVar, isEmptyMVar, addMVarFinalizer
+                )
 #endif
 
-import Control.Exception as Exception
-
-#ifdef __HUGS__
--- This is as close as Hugs gets to providing throw
-throw :: Exception -> IO a
-throw = throwIO
+#ifdef __GLASGOW_HASKELL__
+import GHC.Base
+#else
+import Prelude
 #endif
 
-#ifdef __GLASGOW_HASKELL__
+import Control.Exception.Base
+
+{-|
+  This is a combination of 'takeMVar' and 'putMVar'; ie. it takes the value
+  from the 'MVar', puts it back, and also returns it.
+-}
 readMVar :: MVar a -> IO a
 readMVar m =
-  block $ do
+  mask_ $ do
     a <- takeMVar m
     putMVar m a
     return a
 
+{-|
+  Take a value from an 'MVar', put a new value into the 'MVar' and
+  return the value taken. Note that there is a race condition whereby
+  another process can put something in the 'MVar' after the take
+  happens but before the put does.
+-}
 swapMVar :: MVar a -> a -> IO a
-swapMVar mvar new = modifyMVar mvar (\old -> return (new,old))
-#endif
+swapMVar mvar new =
+  mask_ $ do
+    old <- takeMVar mvar
+    putMVar mvar new
+    return old
 
--- put back the same value, return something
+{-|
+  'withMVar' is a safe wrapper for operating on the contents of an
+  'MVar'.  This operation is exception-safe: it will replace the
+  original contents of the 'MVar' if an exception is raised (see
+  "Control.Exception").
+-}
+{-# INLINE withMVar #-}
+-- inlining has been reported to have dramatic effects; see
+-- http://www.haskell.org//pipermail/haskell/2006-May/017907.html
 withMVar :: MVar a -> (a -> IO b) -> IO b
-withMVar m io = 
-  block $ do
+withMVar m io =
+  mask $ \restore -> do
     a <- takeMVar m
-    b <- Exception.catch (unblock (io a))
-      	    (\e -> do putMVar m a; throw e)
+    b <- restore (io a) `onException` putMVar m a
     putMVar m a
     return b
 
--- put back a new value, return ()
+{-|
+  A safe wrapper for modifying the contents of an 'MVar'.  Like 'withMVar', 
+  'modifyMVar' will replace the original contents of the 'MVar' if an
+  exception is raised during the operation.
+-}
+{-# INLINE modifyMVar_ #-}
 modifyMVar_ :: MVar a -> (a -> IO a) -> IO ()
-modifyMVar_ m io = 
-  block $ do
+modifyMVar_ m io =
+  mask $ \restore -> do
     a  <- takeMVar m
-    a' <- Exception.catch (unblock (io a))
-      	    (\e -> do putMVar m a; throw e)
+    a' <- restore (io a) `onException` putMVar m a
     putMVar m a'
 
--- put back a new value, return something
+{-|
+  A slight variation on 'modifyMVar_' that allows a value to be
+  returned (@b@) in addition to the modified value of the 'MVar'.
+-}
+{-# INLINE modifyMVar #-}
 modifyMVar :: MVar a -> (a -> IO (a,b)) -> IO b
-modifyMVar m io = 
-  block $ do
+modifyMVar m io =
+  mask $ \restore -> do
     a      <- takeMVar m
-    (a',b) <- Exception.catch (unblock (io a))
-      	        (\e -> do putMVar m a; throw e)
+    (a',b) <- restore (io a) `onException` putMVar m a
     putMVar m a'
     return b