System/IO/Unsafe.hs

{-# LANGUAGE Unsafe #-}
{-# LANGUAGE NoImplicitPrelude #-}

-----------------------------------------------------------------------------
-- |
-- Module : System.IO.Unsafe
-- Copyright : (c) The University of Glasgow 2001
-- License : BSD-style (see the file libraries/base/LICENSE)
-- 
-- Maintainer : libraries@haskell.org
-- Stability : provisional
-- Portability : portable
--
-- \"Unsafe\" IO operations.
--
-----------------------------------------------------------------------------

module System.IO.Unsafe (
 -- * Unsafe 'System.IO.IO' operations
 unsafePerformIO,
 unsafeDupablePerformIO,
 unsafeInterleaveIO,
 unsafeFixIO,
 ) where

import GHC.Base
import GHC.IO
import GHC.IORef
import GHC.Exception
import Control.Exception

-- | A slightly faster version of `System.IO.fixIO` that may not be
-- safe to use with multiple threads. The unsafety arises when used
-- like this:
--
-- > unsafeFixIO $ \r -> do
-- > forkIO (print r)
-- > return (...)
--
-- In this case, the child thread will receive a @NonTermination@
-- exception instead of waiting for the value of @r@ to be computed.
--
-- /Since: 4.5.0.0/
unsafeFixIO :: (a -> IO a) -> IO a
unsafeFixIO k = do
 ref <- newIORef (throw NonTermination)
 ans <- unsafeDupableInterleaveIO (readIORef ref)
 result <- k ans
 writeIORef ref result
 return result