[project @ 2002-05-27 15:43:44 by simonmar]
[ghc.git] / libraries / base / System / Mem / Weak.hs
1 -----------------------------------------------------------------------------
2 -- |
3 -- Module : System.Mem.Weak
4 -- Copyright : (c) The University of Glasgow 2001
5 -- License : BSD-style (see the file libraries/base/LICENSE)
6 --
7 -- Maintainer : libraries@haskell.org
8 -- Stability : experimental
9 -- Portability : non-portable
10 --
11 -- In general terms, a weak pointer is a reference to an object that is
12 -- not followed by the garbage collector - that is, the existence of a
13 -- weak pointer to an object has no effect on the lifetime of that
14 -- object. A weak pointer can be de-referenced to find out
15 -- whether the object it refers to is still alive or not, and if so
16 -- to return the object itself.
17 --
18 -- Weak pointers are particularly useful for caches and memo tables.
19 -- To build a memo table, you build a data structure
20 -- mapping from the function argument (the key) to its result (the
21 -- value). When you apply the function to a new argument you first
22 -- check whether the key\/value pair is already in the memo table.
23 -- The key point is that the memo table itself should not keep the
24 -- key and value alive. So the table should contain a weak pointer
25 -- to the key, not an ordinary pointer. The pointer to the value must
26 -- not be weak, because the only reference to the value might indeed be
27 -- from the memo table.
28 --
29 -- So it looks as if the memo table will keep all its values
30 -- alive for ever. One way to solve this is to purge the table
31 -- occasionally, by deleting entries whose keys have died.
32 --
33 -- The weak pointers in this library
34 -- support another approach, called /finalization/.
35 -- When the key referred to by a weak pointer dies, the storage manager
36 -- arranges to run a programmer-specified finalizer. In the case of memo
37 -- tables, for example, the finalizer could remove the key\/value pair
38 -- from the memo table.
39 --
40 -- Another difficulty with the memo table is that the value of a
41 -- key\/value pair might itself contain a pointer to the key.
42 -- So the memo table keeps the value alive, which keeps the key alive,
43 -- even though there may be no other references to the key so both should
44 -- die. The weak pointers in this library provide a slight
45 -- generalisation of the basic weak-pointer idea, in which each
46 -- weak pointer actually contains both a key and a value.
47 --
48 -----------------------------------------------------------------------------
49
50 module System.Mem.Weak (
51 -- * The @Weak@ type
52 Weak, -- abstract
53
54 -- * The general interface
55 mkWeak, -- :: k -> v -> Maybe (IO ()) -> IO (Weak v)
56 deRefWeak, -- :: Weak v -> IO (Maybe v)
57 finalize, -- :: Weak v -> IO ()
58
59 -- * Specialised versions
60 mkWeakPtr, -- :: k -> Maybe (IO ()) -> IO (Weak k)
61 addFinalizer, -- :: key -> IO () -> IO ()
62 mkWeakPair, -- :: k -> v -> Maybe (IO ()) -> IO (Weak (k,v))
63 -- replaceFinaliser -- :: Weak v -> IO () -> IO ()
64
65 -- * A precise semantics
66
67 -- $precise
68 ) where
69
70 import Prelude
71
72 import Data.Dynamic
73
74 #ifdef __GLASGOW_HASKELL__
75 import GHC.Base
76 import GHC.IOBase
77 import GHC.Weak
78
79 #include "Dynamic.h"
80 INSTANCE_TYPEABLE1(Weak,weakTc,"Weak")
81
82 {-|
83 Dereferences a weak pointer. If the key is still alive, then
84 @'Just' v@ is returned (where @v@ is the /value/ in the weak pointer), otherwise
85 'Nothing' is returned.
86
87 The return value of 'deRefWeak' depends on when the garbage collector
88 runs, hence it is in the 'IO' monad.
89 -}
90 deRefWeak :: Weak v -> IO (Maybe v)
91 deRefWeak (Weak w) = IO $ \s ->
92 case deRefWeak# w s of
93 (# s1, flag, p #) -> case flag of
94 0# -> (# s1, Nothing #)
95 _ -> (# s1, Just p #)
96
97 -- | A specialised version of 'mkWeak' where the value is actually a pair
98 -- of the key and value passed to 'mkWeakPair':
99 --
100 -- > mkWeakPair key val finalizer = mkWeak key (key,val) finalizer
101 --
102 -- The advantage of this is that the key can be retrieved by 'deRefWeak'
103 -- in addition to the value.
104 mkWeakPair :: k -> v -> Maybe (IO ()) -> IO (Weak (k,v))
105 mkWeakPair key val finalizer = mkWeak key (key,val) finalizer
106
107 -- | Causes a the finalizer associated with a weak pointer to be run
108 -- immediately.
109 finalize :: Weak v -> IO ()
110 finalize (Weak w) = IO $ \s ->
111 case finalizeWeak# w s of
112 (# s1, 0#, _ #) -> (# s1, () #) -- already dead, or no finaliser
113 (# s1, _, f #) -> f s1
114 #endif
115
116
117 {- $precise
118
119 The above informal specification is fine for simple situations, but
120 matters can get complicated. In particular, it needs to be clear
121 exactly when a key dies, so that any weak pointers that refer to it
122 can be finalized. Suppose, for example, the value of one weak pointer
123 refers to the key of another...does that keep the key alive?
124
125 The behaviour is simply this:
126
127 * If a weak pointer (object) refers to an /unreachable/
128 key, it may be finalized.
129
130 * Finalization means (a) arrange that subsequent calls
131 to 'deRefWeak' return 'Nothing'; and (b) run the finalizer.
132
133 This behaviour depends on what it means for a key to be reachable.
134 Informally, something is reachable if it can be reached by following
135 ordinary pointers from the root set, but not following weak pointers.
136 We define reachability more precisely as follows A heap object is
137 reachable if:
138
139 * It is a member of the /root set/.
140
141 * It is directly pointed to by a reachable object, other than
142 a weak pointer object.
143
144 * It is a weak pointer object whose key is reachable.
145
146 * It is the value or finalizer of an object whose key is reachable.
147 -}