Remove Control.Parallel*, now in package parallel

[haskell-directory.git] / Data / IORef.hs

diff --git a/Data/IORef.hs b/Data/IORef.hs
index 284c2bf..7a6ec7d 100644 (file)
--- a/Data/IORef.hs
+++ b/Data/IORef.hs
@@ -20,20 +20,21 @@ module Data.IORef
         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__)
@@ -47,13 +48,10 @@ 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))
@@ -65,7 +63,31 @@ mkWeakIORef r@(IORef (STRef r#)) f = IO $ \s ->
 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