    {-# LANGUAGE BangPatterns, CPP, MagicHash, UnboxedTuples #-}

    #if __GLASGOW_HASKELL__ >= 701
    {-# LANGUAGE Trustworthy #-}
    #endif

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Control.Concurrent.STM.TVar
    -- Copyright   :  (c) The University of Glasgow 2004
    -- License     :  BSD-style (see the file libraries/base/LICENSE)
    --
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  experimental
    -- Portability :  non-portable (requires STM)
    --
    -- TVar: Transactional variables
    --
    -----------------------------------------------------------------------------

    module Control.Concurrent.STM.TVar (
            -- * TVars
            TVar,
            newTVar,
            newTVarIO,
            readTVar,
            readTVarIO,
            writeTVar,
            modifyTVar,
            modifyTVar',
            stateTVar,
            swapTVar,
    #ifdef __GLASGOW_HASKELL__
            registerDelay,
    #endif
            mkWeakTVar
      ) where

    #ifdef __GLASGOW_HASKELL__
    import GHC.Base
    import GHC.Conc
    import GHC.Weak
    #else
    import Control.Sequential.STM
    #endif

    -- Like 'modifyIORef' but for 'TVar'.
    -- | Mutate the contents of a 'TVar'. /N.B./, this version is
    -- non-strict.
    --
    -- @since 2.3
    modifyTVar :: TVar a -> (a -> a) -> STM ()
    modifyTVar :: forall a. TVar a -> (a -> a) -> STM ()
    modifyTVar TVar a
    var a -> a
    f = do
        x <- TVar a -> STM a
    forall a. TVar a -> STM a
    readTVar TVar a
    var
        writeTVar var (f x)
    {-# INLINE modifyTVar #-}

    -- | Strict version of 'modifyTVar'.
    --
    -- @since 2.3
    modifyTVar' :: TVar a -> (a -> a) -> STM ()
    modifyTVar' :: forall a. TVar a -> (a -> a) -> STM ()
    modifyTVar' TVar a
    var a -> a
    f = do
        x <- TVar a -> STM a
    forall a. TVar a -> STM a
    readTVar TVar a
    var
        writeTVar var $! f x
    {-# INLINE modifyTVar' #-}

    -- | Like 'modifyTVar'' but the function is a simple state transition that can
    -- return a side value which is passed on as the result of the 'STM'.
    --
    -- @since 2.5.0
    stateTVar :: TVar s -> (s -> (a, s)) -> STM a
    stateTVar :: forall s a. TVar s -> (s -> (a, s)) -> STM a
    stateTVar TVar s
    var s -> (a, s)
    f = do
       s <- TVar s -> STM s
    forall a. TVar a -> STM a
    readTVar TVar s
    var
       let !(a, s') = f s
       writeTVar var s'
       return a
    {-# INLINE stateTVar #-}

    -- Like 'swapTMVar' but for 'TVar'.
    -- | Swap the contents of a 'TVar' for a new value.
    --
    -- @since 2.3
    swapTVar :: TVar a -> a -> STM a
    swapTVar :: forall a. TVar a -> a -> STM a
    swapTVar TVar a
    var a
    new = do
        old <- TVar a -> STM a
    forall a. TVar a -> STM a
    readTVar TVar a
    var
        writeTVar var new
        return old
    {-# INLINE swapTVar #-}

    -- | Make a 'Weak' pointer to a 'TVar', using the second argument as
    -- a finalizer to run when 'TVar' is garbage-collected
    --
    -- @since 2.4.3
    mkWeakTVar :: TVar a -> IO () -> IO (Weak (TVar a))
    mkWeakTVar :: forall a. TVar a -> IO () -> IO (Weak (TVar a))
    mkWeakTVar t :: TVar a
    t@(TVar TVar# RealWorld a
    t#) (IO State# RealWorld -> (# State# RealWorld, () #)
    finalizer) = (State# RealWorld -> (# State# RealWorld, Weak (TVar a) #))
    -> IO (Weak (TVar a))
    forall a. (State# RealWorld -> (# State# RealWorld, a #)) -> IO a
    IO ((State# RealWorld -> (# State# RealWorld, Weak (TVar a) #))
     -> IO (Weak (TVar a)))
    -> (State# RealWorld -> (# State# RealWorld, Weak (TVar a) #))
    -> IO (Weak (TVar a))
    forall a b. (a -> b) -> a -> b
    $ \State# RealWorld
    s ->
        case TVar# RealWorld a
    -> TVar a
    -> (State# RealWorld -> (# State# RealWorld, () #))
    -> State# RealWorld
    -> (# State# RealWorld, Weak# (TVar a) #)
    forall a b c.
    a
    -> b
    -> (State# RealWorld -> (# State# RealWorld, c #))
    -> State# RealWorld
    -> (# State# RealWorld, Weak# b #)
    mkWeak# TVar# RealWorld a
    t# TVar a
    t State# RealWorld -> (# State# RealWorld, () #)
    finalizer State# RealWorld
    s of (# State# RealWorld
    s1, Weak# (TVar a)
    w #) -> (# State# RealWorld
    s1, Weak# (TVar a) -> Weak (TVar a)
    forall v. Weak# v -> Weak v
    Weak Weak# (TVar a)
    w #)