/usr/lib/hugs/packages/base/Data/IORef.hs is in libhugs-base-bundled 98.200609.21-5.3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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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
) where
import Prelude -- Explicit dependency helps 'make depend' do the right thing
import Hugs.IORef
-- |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 'Control.Concurrent.MVar.MVar' instead is a good idea.
--
atomicModifyIORef :: IORef a -> (a -> (a,b)) -> IO b
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
|