4.26. Weak

The Weak library provides a "weak pointer" abstraction, giving the user some control over the garbage collection of specified objects, and allowing objects to be "finalized" with an arbitrary Haskell IO computation when they die.

Weak pointers partially replace the old foreign object interface, as we will explain later.

4.26.1. Module Signature

module Weak (
	Weak,	    		-- abstract
	-- instance Eq (Weak v)  

	mkWeak,      		-- :: k -> v -> Maybe (IO ()) -> IO (Weak v)
	deRefWeak, 		-- :: Weak v -> IO (Maybe v)
	finalize,		-- :: Weak v -> IO ()

	-- Not yet implemented
	-- replaceFinalizer	-- :: Weak v -> IO () -> IO ()

	mkWeakPtr, 		-- :: k -> Maybe (IO ()) -> IO (Weak k)
	mkWeakPair, 		-- :: k -> v -> Maybe (IO ()) -> IO (Weak (k,v))
	addFinalizer, 		-- :: k -> IO () -> IO ()
	addForeignFinalizer 	-- :: ForeignObj -> IO () -> IO ()
   ) where

4.26.2. Weak pointers

In general terms, a weak pointer is a reference to an object that is not followed by the garbage collector --- that is, the existence of a weak pointer to an object has no effect on the lifetime of that object. A weak pointer can be de-referenced to find out whether the object it refers to is still alive or not, and if so to return the object itself.

Weak pointers are particularly useful for caches and memo tables. To build a memo table, you build a data structure mapping from the function argument (the key) to its result (the value). When you apply the function to a new argument you first check whether the key/value pair is already in the memo table. The key point is that the memo table itself should not keep the key and value alive. So the table should contain a weak pointer to the key, not an ordinary pointer. The pointer to the value must not be weak, because the only reference to the value might indeed be from the memo table.

So it looks as if the memo table will keep all its values alive for ever. One way to solve this is to purge the table occasionally, by deleting entries whose keys have died.

The weak pointers in this library support another approach, called finalization. When the key referred to by a weak pointer dies, the storage manager arranges to run a programmer-specified finalizer. In the case of memo tables, for example, the finalizer could remove the key/value pair from the memo table.

Another difficulty with the memo table is that the value of a key/value pair might itself contain a pointer to the key. So the memo table keeps the value alive, which keeps the key alive, even though there may be no other references to the key so both should die. The weak pointers in this library provide a slight generalisation of the basic weak-pointer idea, in which each weak pointer actually contains both a key and a value. We describe this in more detail below.

4.26.3. The simple interface

mkWeakPtr    :: a -> Maybe (IO ()) -> IO (Weak a)
deRefWeak    :: Weak a -> IO (Maybe a)
addFinalizer :: a -> IO () -> IO ()

mkWeakPtr takes a value of any type a, and maybe a finalizer of type IO (), and returns a weak pointer object referring to the value, of type Weak a. It is in the IO monad because it has the side effect of arranging that the finalizer (if there is one) will be run when the object dies. In what follows, a ``weak pointer object'', or ``weak pointer'' for short, means precisely ``a Haskell value of type Weak t'' for some type t. A weak pointer (object) is a first-class Haskell value; it can be passed to functions, stored in data structures, and so on.

deRefWeak dereferences a weak pointer, returning Just v if the value is still alive. If the key has already died, then deRefWeak returns Nothing; that's why it's in the IO monad—the return value of deRefWeak depends on when the garbage collector runs.

addFinalizer is just another name for mkWeakPtr except that it throws the weak pointer itself away. (The runtime system will remember that the weak pointer and hence the finalizer exists even if the program has forgotten it.)

addFinalizer :: a -> IO () -> IO ()
addFinalizer v f = do { mkWeakPtr v f; return () }

The effect of addFinalizer is simply that the finalizer runs when the referenced object dies.

The following properties hold:

4.26.4. The general interface

The Weak library offers a slight generalisation of the simple weak pointers described so far:
mkWeak :: k -> v -> Maybe (IO ()) -> IO (Weak v)
mkWeak takes a key of any type k and a value of any type v, as well as a finalizer, and returns a weak pointer of type Weak v.

deRefWeak returns the value only, not the key, as its type (given above) implies:
deRefWeak :: Weak a -> IO (Maybe a)
However, deRefWeak returns Nothing if the key, not the value, has died. Furthermore, references from the value to the key do not keep the key alive, in the same way that the finalizer does not keep the key alive.

Simple weak pointers are readily defined in terms of these more general weak pointers:
mkWeakPtr :: a -> Maybe (IO ()) -> IO (Weak a)
mkWeakPtr v f = mkWeak v v f

These more general weak pointers are enough to implement memo tables properly.

A weak pointer can be finalized early, using the finalize operation:

finalize :: Weak v -> IO ()

4.26.5. A precise semantics

The above informal specification is fine for simple situations, but matters can get complicated. In particular, it needs to be clear exactly when a key dies, so that any weak pointers that refer to it can be finalized. Suppose, for example, the value of one weak pointer refers to the key of another...does that keep the key alive?

The behaviour is simply this:

This behaviour depends on what it means for a key to be reachable. Informally, something is reachable if it can be reached by following ordinary pointers from the root set, but not following weak pointers. We define reachability more precisely as follows A heap object is reachable if:

The root set consists of all runnable threads, and all stable pointers (see Section 4.24). NOTE: currently all top-level objects are considered to be reachable, although we hope to remove this restriction in the future. A Char or small Int will also be constantly reachable, since the garbage collector replaces heap-resident Chars and small Ints with pointers to static copies.

Notice that a pointer to the key from its associated value or finalizer does not make the key reachable. However, if the key is reachable some other way, then the value and the finalizer are reachable, and so, therefore, are any other keys they refer to directly or indirectly.

4.26.6. Finalization for foreign objects

A foreign object is some data that lives outside the Haskell heap, for example some malloced data in C land. It's useful to be able to know when the Haskell program no longer needs the malloced data, so it can be freed. We can use weak pointers and finalizers for this, but we have to be careful: the foreign data is usually referenced by an address, ie. an Addr (see Section 4.1), and we must retain the invariant that if the Haskell program still needs the foreign object, then it retains the Addr object in the heap. This invariant isn't guaranteed to hold if we use Addr, because an Addr consists of a box around a raw address Addr#. If the Haskell program can manipulate the Addr# object independently of the heap-resident Addr, then the foreign object could be inadvertently finalized early, because a weak pointer to the Addr would find no more references to its key and trigger the finalizer despite the fact that the program still holds the Addr# and intends to use it.

To avoid this somewhat subtle race condition, we use another type of foreign address, called ForeignObj (see Section 4.9). Historical note: ForeignObj is identical to the old ForeignObj except that it no longer supports finalization - that's provided by the weak pointer/finalization mechanism above.

A ForeignObj is basically an address, but the ForeignObj itself is a heap-resident object and can therefore be watched by weak pointers. A ForeignObj can be passed to C functions (in which case the C function gets a straightforward pointer), but it cannot be decomposed into an Addr#.