GHC/MVar.hs

    {-# LANGUAGE Unsafe #-}
    {-# LANGUAGE NoImplicitPrelude, MagicHash, UnboxedTuples #-}
    {-# OPTIONS_GHC -funbox-strict-fields #-}
    {-# OPTIONS_HADDOCK hide #-}

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  GHC.MVar
    -- Copyright   :  (c) The University of Glasgow 2008
    -- License     :  see libraries/base/LICENSE
    --
    -- Maintainer  :  cvs-ghc@haskell.org
    -- Stability   :  internal
    -- Portability :  non-portable (GHC Extensions)
    --
    -- The MVar type
    --
    -----------------------------------------------------------------------------

    module GHC.MVar (
            -- * MVars
              MVar(..)
            , newMVar
            , newEmptyMVar
            , takeMVar
            , readMVar
            , putMVar
            , tryTakeMVar
            , tryPutMVar
            , tryReadMVar
            , isEmptyMVar
            , addMVarFinalizer
        ) where

    import GHC.Base

    data MVar a = MVar (MVar# RealWorld a)
    {- ^
    An 'MVar' (pronounced \"em-var\") is a synchronising variable, used
    for communication between concurrent threads.  It can be thought of
    as a a box, which may be empty or full.
    -}

    -- pull in Eq (Mvar a) too, to avoid GHC.Conc being an orphan-instance module
    instance Eq (MVar a) where
            (MVar mvar1#) == (MVar mvar2#) = isTrue# (sameMVar# mvar1# mvar2#)

    {-
    M-Vars are rendezvous points for concurrent threads.  They begin
    empty, and any attempt to read an empty M-Var blocks.  When an M-Var
    is written, a single blocked thread may be freed.  Reading an M-Var
    toggles its state from full back to empty.  Therefore, any value
    written to an M-Var may only be read once.  Multiple reads and writes
    are allowed, but there must be at least one read between any two
    writes.
    -}

    --Defined in IOBase to avoid cycle: data MVar a = MVar (SynchVar# RealWorld a)

    -- |Create an 'MVar' which is initially empty.
    newEmptyMVar  :: IO (MVar a)
    newEmptyMVar = IO $ \ s# ->
        case newMVar# s# of
             (# s2#, svar# #) -> (# s2#, MVar svar# #)

    -- |Create an 'MVar' which contains the supplied value.
    newMVar :: a -> IO (MVar a)
    newMVar value =
        newEmptyMVar        >>= \ mvar ->
        putMVar mvar value  >>
        return mvar

    -- |Return the contents of the 'MVar'.  If the 'MVar' is currently
    -- empty, 'takeMVar' will wait until it is full.  After a 'takeMVar',
    -- the 'MVar' is left empty.
    --
    -- There are two further important properties of 'takeMVar':
    --
    --   * 'takeMVar' is single-wakeup.  That is, if there are multiple
    --     threads blocked in 'takeMVar', and the 'MVar' becomes full,
    --     only one thread will be woken up.  The runtime guarantees that
    --     the woken thread completes its 'takeMVar' operation.
    --
    --   * When multiple threads are blocked on an 'MVar', they are
    --     woken up in FIFO order.  This is useful for providing
    --     fairness properties of abstractions built using 'MVar's.
    --
    takeMVar :: MVar a -> IO a
    takeMVar (MVar mvar#) = IO $ \ s# -> takeMVar# mvar# s#

    -- |Atomically read the contents of an 'MVar'.  If the 'MVar' is
    -- currently empty, 'readMVar' will wait until its full.
    -- 'readMVar' is guaranteed to receive the next 'putMVar'.
    --
    -- 'readMVar' is multiple-wakeup, so when multiple readers are
    -- blocked on an 'MVar', all of them are woken up at the same time.
    --
    -- /Compatibility note:/ Prior to base 4.7, 'readMVar' was a combination
    -- of 'takeMVar' and 'putMVar'.  This mean that in the presence of
    -- other threads attempting to 'putMVar', 'readMVar' could block.
    -- Furthermore, 'readMVar' would not receive the next 'putMVar' if there
    -- was already a pending thread blocked on 'takeMVar'.  The old behavior
    -- can be recovered by implementing 'readMVar as follows:
    --
    -- @
    --  readMVar :: MVar a -> IO a
    --  readMVar m =
    --    mask_ $ do
    --      a <- takeMVar m
    --      putMVar m a
    --      return a
    -- @
    readMVar :: MVar a -> IO a
    readMVar (MVar mvar#) = IO $ \ s# -> readMVar# mvar# s#

    -- |Put a value into an 'MVar'.  If the 'MVar' is currently full,
    -- 'putMVar' will wait until it becomes empty.
    --
    -- There are two further important properties of 'putMVar':
    --
    --   * 'putMVar' is single-wakeup.  That is, if there are multiple
    --     threads blocked in 'putMVar', and the 'MVar' becomes empty,
    --     only one thread will be woken up.  The runtime guarantees that
    --     the woken thread completes its 'putMVar' operation.
    --
    --   * When multiple threads are blocked on an 'MVar', they are
    --     woken up in FIFO order.  This is useful for providing
    --     fairness properties of abstractions built using 'MVar's.
    --
    putMVar  :: MVar a -> a -> IO ()
    putMVar (MVar mvar#) x = IO $ \ s# ->
        case putMVar# mvar# x s# of
            s2# -> (# s2#, () #)

    -- |A non-blocking version of 'takeMVar'.  The 'tryTakeMVar' function
    -- returns immediately, with 'Nothing' if the 'MVar' was empty, or
    -- @'Just' a@ if the 'MVar' was full with contents @a@.  After 'tryTakeMVar',
    -- the 'MVar' is left empty.
    tryTakeMVar :: MVar a -> IO (Maybe a)
    tryTakeMVar (MVar m) = IO $ \ s ->
        case tryTakeMVar# m s of
            (# s', 0#, _ #) -> (# s', Nothing #)      -- MVar is empty
            (# s', _,  a #) -> (# s', Just a  #)      -- MVar is full

    -- |A non-blocking version of 'putMVar'.  The 'tryPutMVar' function
    -- attempts to put the value @a@ into the 'MVar', returning 'True' if
    -- it was successful, or 'False' otherwise.
    tryPutMVar  :: MVar a -> a -> IO Bool
    tryPutMVar (MVar mvar#) x = IO $ \ s# ->
        case tryPutMVar# mvar# x s# of
            (# s, 0# #) -> (# s, False #)
            (# s, _  #) -> (# s, True #)

    -- |A non-blocking version of 'readMVar'.  The 'tryReadMVar' function
    -- returns immediately, with 'Nothing' if the 'MVar' was empty, or
    -- @'Just' a@ if the 'MVar' was full with contents @a@.
    --
    -- @since 4.7.0.0
    tryReadMVar :: MVar a -> IO (Maybe a)
    tryReadMVar (MVar m) = IO $ \ s ->
        case tryReadMVar# m s of
            (# s', 0#, _ #) -> (# s', Nothing #)      -- MVar is empty
            (# s', _,  a #) -> (# s', Just a  #)      -- MVar is full

    -- |Check whether a given 'MVar' is empty.
    --
    -- Notice that the boolean value returned  is just a snapshot of
    -- the state of the MVar. By the time you get to react on its result,
    -- the MVar may have been filled (or emptied) - so be extremely
    -- careful when using this operation.   Use 'tryTakeMVar' instead if possible.
    isEmptyMVar :: MVar a -> IO Bool
    isEmptyMVar (MVar mv#) = IO $ \ s# ->
        case isEmptyMVar# mv# s# of
            (# s2#, flg #) -> (# s2#, isTrue# (flg /=# 0#) #)

    -- |Add a finalizer to an 'MVar' (GHC only).  See "Foreign.ForeignPtr" and
    -- "System.Mem.Weak" for more about finalizers.
    addMVarFinalizer :: MVar a -> IO () -> IO ()
    addMVarFinalizer (MVar m) (IO finalizer) =
        IO $ \s -> case mkWeak# m () finalizer s of { (# s1, _ #) -> (# s1, () #) }