    \begin{code}
    {-# OPTIONS_GHC -XNoImplicitPrelude #-}
    {-# OPTIONS_HADDOCK hide #-}
    -----------------------------------------------------------------------------
    -- |
    -- Module      :  GHC.Weak
    -- Copyright   :  (c) The University of Glasgow, 1998-2002
    -- License     :  see libraries/base/LICENSE
    --
    -- Maintainer  :  cvs-ghc@haskell.org
    -- Stability   :  internal
    -- Portability :  non-portable (GHC Extensions)
    --
    -- Weak pointers.
    --
    -----------------------------------------------------------------------------

    -- #hide
    module GHC.Weak where

    import GHC.Base
    import Data.Maybe
    import GHC.IOBase       ( IO(..), unIO )
    import Data.Typeable

    {-|
    A weak pointer object with a key and a value.  The value has type @v@.

    A weak pointer expresses a relationship between two objects, the
    /key/ and the /value/:  if the key is considered to be alive by the
    garbage collector, then the value is also alive.  A reference from
    the value to the key does /not/ keep the key alive.

    A weak pointer may also have a finalizer of type @IO ()@; if it does,
    then the finalizer will be run at most once, at a time after the key
    has become unreachable by the program (\"dead\").  The storage manager
    attempts to run the finalizer(s) for an object soon after the object
    dies, but promptness is not guaranteed.

    It is not guaranteed that a finalizer will eventually run, and no
    attempt is made to run outstanding finalizers when the program exits.
    Therefore finalizers should not be relied on to clean up resources -
    other methods (eg. exception handlers) should be employed, possibly in
    addition to finalisers.

    References from the finalizer to the key are treated in the same way
    as references from the value to the key: they do not keep the key
    alive.  A finalizer may therefore ressurrect the key, perhaps by
    storing it in the same data structure.

    The finalizer, and the relationship between the key and the value,
    exist regardless of whether the program keeps a reference to the
    'Weak' object or not.

    There may be multiple weak pointers with the same key.  In this
    case, the finalizers for each of these weak pointers will all be
    run in some arbitrary order, or perhaps concurrently, when the key
    dies.  If the programmer specifies a finalizer that assumes it has
    the only reference to an object (for example, a file that it wishes
    to close), then the programmer must ensure that there is only one
    such finalizer.

    If there are no other threads to run, the runtime system will check
    for runnable finalizers before declaring the system to be deadlocked.
    -}
    data Weak v = Weak (Weak# v)

    #include "Typeable.h"
    INSTANCE_TYPEABLE1(Weak,weakTc,"Weak")

    -- | Establishes a weak pointer to @k@, with value @v@ and a finalizer.
    --
    -- This is the most general interface for building a weak pointer.
    --
    mkWeak  :: k                            -- ^ key
            -> v                            -- ^ value
            -> Maybe (IO ())                -- ^ finalizer
            -> IO (Weak v)                  -- ^ returns: a weak pointer object

    mkWeak key val (Just finalizer) = IO $ \s ->
       case mkWeak# key val finalizer s of { (# s1, w #) -> (# s1, Weak w #) }
    mkWeak key val Nothing = IO $ \s ->
       case mkWeak# key val (unsafeCoerce# 0#) s of { (# s1, w #) -> (# s1, Weak w #) }

    {-|
    Dereferences a weak pointer.  If the key is still alive, then
    @'Just' v@ is returned (where @v@ is the /value/ in the weak pointer), otherwise
    'Nothing' is returned.

    The return value of 'deRefWeak' depends on when the garbage collector
    runs, hence it is in the 'IO' monad.
    -}
    deRefWeak :: Weak v -> IO (Maybe v)
    deRefWeak (Weak w) = IO $ \s ->
       case deRefWeak# w s of
            (# s1, flag, p #) -> case flag of
                                    0# -> (# s1, Nothing #)
                                    _  -> (# s1, Just p #)

    -- | Causes a the finalizer associated with a weak pointer to be run
    -- immediately.
    finalize :: Weak v -> IO ()
    finalize (Weak w) = IO $ \s ->
       case finalizeWeak# w s of
            (# s1, 0#, _ #) -> (# s1, () #) -- already dead, or no finaliser
            (# s1, _,  f #) -> f s1

    {-
    Instance Eq (Weak v) where
      (Weak w1) == (Weak w2) = w1 `sameWeak#` w2
    -}

    -- run a batch of finalizers from the garbage collector.  We're given
    -- an array of finalizers and the length of the array, and we just
    -- call each one in turn.
    --
    -- the IO primitives are inlined by hand here to get the optimal
    -- code (sigh) --SDM.

    runFinalizerBatch :: Int -> Array# (IO ()) -> IO ()
    runFinalizerBatch (I# n) arr =
       let  go m  = IO $ \s ->
                      case m of
                      0# -> (# s, () #)
                      _  -> let m' = m -# 1# in
                            case indexArray# arr m' of { (# io #) ->
                            case unIO io s of          { (# s', _ #) ->
                            unIO (go m') s'
                            }}
       in
            go n

    \end{code}