X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=GHC%2FWeak.lhs;h=92e1eb89b039f59a0f6232e9271091e35bcdb2d8;hb=7a97ec4b12e1fbec5505f82032cf4dc435b5a60c;hp=9b85f541135c2d95dc1f85aa34a8b68f5220639e;hpb=b706340c451952adf230b5b8daecad8a1f34d714;p=ghc-base.git

diff --git a/GHC/Weak.lhs b/GHC/Weak.lhs
index 9b85f54..92e1eb8 100644
--- a/GHC/Weak.lhs
+++ b/GHC/Weak.lhs
@@ -1,5 +1,14 @@
 \begin{code}
-{-# OPTIONS -fno-implicit-prelude #-}
+{-# LANGUAGE CPP
+           , NoImplicitPrelude
+           , BangPatterns
+           , MagicHash
+           , UnboxedTuples
+           , DeriveDataTypeable
+           , StandaloneDeriving
+  #-}
+{-# OPTIONS_HADDOCK hide #-}
+
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  GHC.Weak
@@ -14,31 +23,94 @@
 --
 -----------------------------------------------------------------------------
 
+-- #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
@@ -56,13 +128,13 @@ 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