\begin{code}
-{-# OPTIONS -fno-implicit-prelude #-}
+{-# LANGUAGE CPP
+ , NoImplicitPrelude
+ , BangPatterns
+ , MagicHash
+ , UnboxedTuples
+ , DeriveDataTypeable
+ , StandaloneDeriving
+ #-}
+{-# OPTIONS_HADDOCK hide #-}
+
-----------------------------------------------------------------------------
-- |
-- Module : GHC.Weak
--
-----------------------------------------------------------------------------
+-- #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)
-mkWeak :: k -- key
- -> v -- value
- -> Maybe (IO ()) -- finalizer
- -> IO (Weak v) -- weak pointer
+#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 #) }
-mkWeakPtr :: k -> Maybe (IO ()) -> IO (Weak k)
-mkWeakPtr key finalizer = mkWeak key key finalizer
+{-|
+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 #)
-addFinalizer :: key -> IO () -> IO ()
-addFinalizer key finalizer = do
- mkWeakPtr key (Just finalizer) -- throw it away
- return ()
+-- | 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
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
- }}
+ 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
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