X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=Data%2FIORef.hs;h=f8857510e45109cbd42efe99189bef553a90562b;hb=b5c54282dc54cc861277ae532224775076a4818e;hp=284c2bf123b879ee93ff6ad85a636f5be862ec57;hpb=d12c4f3de454ea031f2258627d388d3d994a3cfd;p=ghc-base.git

diff --git a/Data/IORef.hs b/Data/IORef.hs
index 284c2bf..f885751 100644
--- a/Data/IORef.hs
+++ b/Data/IORef.hs
@@ -1,3 +1,4 @@
+{-# OPTIONS_GHC -XNoImplicitPrelude #-}
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Data.IORef
@@ -14,28 +15,29 @@
 
 module Data.IORef
   ( 
-	-- * IORefs
-	IORef,		      -- abstract, instance of: Eq, Typeable
-	newIORef,	      -- :: a -> IO (IORef a)
-        readIORef,	      -- :: IORef a -> IO a
-        writeIORef,	      -- :: IORef a -> a -> IO ()
-	modifyIORef,	      -- :: IORef a -> (a -> a) -> IO ()
+        -- * IORefs
+        IORef,                -- abstract, instance of: Eq, Typeable
+        newIORef,             -- :: a -> IO (IORef a)
+        readIORef,            -- :: IORef a -> IO a
+        writeIORef,           -- :: IORef a -> a -> IO ()
+        modifyIORef,          -- :: IORef a -> (a -> a) -> IO ()
+        atomicModifyIORef,    -- :: IORef a -> (a -> (a,b)) -> IO b
 
 #if !defined(__PARALLEL_HASKELL__) && defined(__GLASGOW_HASKELL__)
-	mkWeakIORef,          -- :: IORef a -> IO () -> IO (Weak (IORef a))
+        mkWeakIORef,          -- :: IORef a -> IO () -> IO (Weak (IORef a))
 #endif
-	) where
-
-import Prelude
+        ) where
 
 #ifdef __HUGS__
 import Hugs.IORef
 #endif
 
 #ifdef __GLASGOW_HASKELL__
-import GHC.Base		( mkWeak# )
+import GHC.Base
 import GHC.STRef
-import GHC.IOBase
+-- import GHC.IO
+import GHC.IORef hiding (atomicModifyIORef)
+import qualified GHC.IORef
 #if !defined(__PARALLEL_HASKELL__)
 import GHC.Weak
 #endif
@@ -47,13 +49,10 @@ import NHC.IOExtras
     , newIORef
     , readIORef
     , writeIORef
+    , excludeFinalisers
     )
 #endif
 
-#ifndef __NHC__
-import Data.Dynamic
-#endif
-
 #if defined(__GLASGOW_HASKELL__) && !defined(__PARALLEL_HASKELL__)
 -- |Make a 'Weak' pointer to an 'IORef'
 mkWeakIORef :: IORef a -> IO () -> IO (Weak (IORef a))
@@ -63,9 +62,33 @@ mkWeakIORef r@(IORef (STRef r#)) f = IO $ \s ->
 
 -- |Mutate the contents of an 'IORef'
 modifyIORef :: IORef a -> (a -> a) -> IO ()
-modifyIORef ref f = writeIORef ref . f =<< readIORef ref
+modifyIORef ref f = readIORef ref >>= writeIORef ref . f
+
+
+-- |Atomically modifies the contents of an 'IORef'.
+--
+-- This function is useful for using 'IORef' in a safe way in a multithreaded
+-- program.  If you only have one 'IORef', then using 'atomicModifyIORef' to
+-- access and modify it will prevent race conditions.
+--
+-- Extending the atomicity to multiple 'IORef's is problematic, so it
+-- is recommended that if you need to do anything more complicated
+-- then using 'Control.Concurrent.MVar.MVar' instead is a good idea.
+--
+atomicModifyIORef :: IORef a -> (a -> (a,b)) -> IO b
+#if defined(__GLASGOW_HASKELL__)
+atomicModifyIORef = GHC.IORef.atomicModifyIORef
 
-#ifndef __NHC__
-#include "Dynamic.h"
-INSTANCE_TYPEABLE1(IORef,ioRefTc,"IORef")
+#elif defined(__HUGS__)
+atomicModifyIORef = plainModifyIORef    -- Hugs has no preemption
+  where plainModifyIORef r f = do
+                a <- readIORef r
+                case f a of (a',b) -> writeIORef r a' >> return b
+#elif defined(__NHC__)
+atomicModifyIORef r f =
+  excludeFinalisers $ do
+    a <- readIORef r
+    let (a',b) = f a
+    writeIORef r a'
+    return b
 #endif