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|
-----------------------------------------------------------------------------
-- |
-- Module : Data.IORef
-- Copyright : (c) The University of Glasgow 2001
-- License : BSD-style (see the file libraries/base/LICENSE)
--
-- Maintainer : libraries@haskell.org
-- Stability : experimental
-- Portability : portable
--
-- Mutable references in the IO monad.
--
-----------------------------------------------------------------------------
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 ()
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 -- Explicit dependency helps 'make depend' do the right thing
#ifdef __HUGS__
import Hugs.IORef
#endif
#ifdef __GLASGOW_HASKELL__
import GHC.Base ( mkWeak#, atomicModifyMutVar# )
import GHC.STRef
import GHC.IOBase
#if !defined(__PARALLEL_HASKELL__)
import GHC.Weak
#endif
#endif /* __GLASGOW_HASKELL__ */
#ifdef __NHC__
import NHC.IOExtras
( IORef
, newIORef
, readIORef
, writeIORef
, excludeFinalisers
)
#endif
#if defined(__GLASGOW_HASKELL__) && !defined(__PARALLEL_HASKELL__)
-- |Make a 'Weak' pointer to an 'IORef'
mkWeakIORef :: IORef a -> IO () -> IO (Weak (IORef a))
mkWeakIORef r@(IORef (STRef r#)) f = IO $ \s ->
case mkWeak# r# r f s of (# s1, w #) -> (# s1, Weak w #)
#endif
-- |Mutate the contents of an 'IORef'
modifyIORef :: IORef a -> (a -> a) -> IO ()
modifyIORef ref f = writeIORef ref . f =<< readIORef ref
-- |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 '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
|
