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))
#endif
) where
-import Prelude
+import Prelude -- Explicit dependency helps 'make depend' do the right thing
#ifdef __HUGS__
import Hugs.IORef
#endif
#ifdef __GLASGOW_HASKELL__
-import GHC.Base ( mkWeak# )
+import GHC.Base ( mkWeak#, atomicModifyMutVar# )
import GHC.STRef
import GHC.IOBase
#if !defined(__PARALLEL_HASKELL__)
, 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))
modifyIORef :: IORef a -> (a -> a) -> IO ()
modifyIORef ref f = writeIORef ref . f =<< readIORef ref
-#ifndef __NHC__
-#include "Dynamic.h"
-INSTANCE_TYPEABLE1(IORef,ioRefTc,"IORef")
+
+-- |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 (IORef (STRef r#)) f = IO $ \s -> atomicModifyMutVar# r# f s
+
+#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