Data/IORef.hs

    {-# LANGUAGE Trustworthy #-}
    {-# LANGUAGE NoImplicitPrelude, MagicHash, UnboxedTuples #-}
    {-# LANGUAGE BangPatterns #-}

    -----------------------------------------------------------------------------
    -- |
    -- 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,
            readIORef,
            writeIORef,
            modifyIORef,
            modifyIORef',
            atomicModifyIORef,
            atomicModifyIORef',
            atomicWriteIORef,
            mkWeakIORef,
            -- ** Memory Model

            -- $memmodel

            ) where

    import GHC.Base
    import GHC.STRef
    import GHC.IORef
    import GHC.Weak

    -- |Make a 'Weak' pointer to an 'IORef', using the second argument as a finalizer
    -- to run when 'IORef' is garbage-collected
    mkWeakIORef :: IORef a -> IO () -> IO (Weak (IORef a))
    mkWeakIORef r@(IORef (STRef r#)) (IO finalizer) = IO $ \s ->
        case mkWeak# r# r finalizer s of (# s1, w #) -> (# s1, Weak w #)

    -- |Mutate the contents of an 'IORef'.
    --
    -- Be warned that 'modifyIORef' does not apply the function strictly.  This
    -- means if the program calls 'modifyIORef' many times, but seldom uses the
    -- value, thunks will pile up in memory resulting in a space leak.  This is a
    -- common mistake made when using an IORef as a counter.  For example, the
    -- following will likely produce a stack overflow:
    --
    -- >ref <- newIORef 0
    -- >replicateM_ 1000000 $ modifyIORef ref (+1)
    -- >readIORef ref >>= print
    --
    -- To avoid this problem, use 'modifyIORef'' instead.
    modifyIORef :: IORef a -> (a -> a) -> IO ()
    modifyIORef ref f = readIORef ref >>= writeIORef ref . f

    -- |Strict version of 'modifyIORef'
    --
    -- @since 4.6.0.0
    modifyIORef' :: IORef a -> (a -> a) -> IO ()
    modifyIORef' ref f = do
        x <- readIORef ref
        let x' = f x
        x' `seq` writeIORef ref x'

    -- |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' does not apply the function strictly.  This is important
    -- to know even if all you are doing is replacing the value.  For example, this
    -- will leak memory:
    --
    -- >ref <- newIORef '1'
    -- >forever $ atomicModifyIORef ref (\_ -> ('2', ()))
    --
    -- Use 'atomicModifyIORef'' or 'atomicWriteIORef' to avoid this problem.
    --
    atomicModifyIORef :: IORef a -> (a -> (a,b)) -> IO b
    atomicModifyIORef ref f = do
      (_old, ~(_new, res)) <- atomicModifyIORef2 ref f
      pure res

    -- | Variant of 'writeIORef' with the \"barrier to reordering\" property that
    -- 'atomicModifyIORef' has.
    --
    -- @since 4.6.0.0
    atomicWriteIORef :: IORef a -> a -> IO ()
    atomicWriteIORef ref a = do
      _ <- atomicSwapIORef ref a
      pure ()

    {- $memmodel

      In a concurrent program, 'IORef' operations may appear out-of-order
      to another thread, depending on the memory model of the underlying
      processor architecture.  For example, on x86, loads can move ahead
      of stores, so in the following example:

      > import Data.IORef
      > import Control.Monad (unless)
      > import Control.Concurrent (forkIO, threadDelay)
      >
      > maybePrint :: IORef Bool -> IORef Bool -> IO ()
      > maybePrint myRef yourRef = do
      >   writeIORef myRef True
      >   yourVal <- readIORef yourRef
      >   unless yourVal $ putStrLn "critical section"
      >
      > main :: IO ()
      > main = do
      >   r1 <- newIORef False
      >   r2 <- newIORef False
      >   forkIO $ maybePrint r1 r2
      >   forkIO $ maybePrint r2 r1
      >   threadDelay 1000000

      it is possible that the string @"critical section"@ is printed
      twice, even though there is no interleaving of the operations of the
      two threads that allows that outcome.  The memory model of x86
      allows 'readIORef' to happen before the earlier 'writeIORef'.

      The implementation is required to ensure that reordering of memory
      operations cannot cause type-correct code to go wrong.  In
      particular, when inspecting the value read from an 'IORef', the
      memory writes that created that value must have occurred from the
      point of view of the current thread.

      'atomicModifyIORef' acts as a barrier to reordering.  Multiple
      'atomicModifyIORef' operations occur in strict program order.  An
      'atomicModifyIORef' is never observed to take place ahead of any
      earlier (in program order) 'IORef' operations, or after any later
      'IORef' operations.

    -}