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.

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.

A specialised version of mkWeak, where the key and the value are the same object:

 mkWeakPtr key finalizer = mkWeak key key finalizer

A specialised version of mkWeak where the value is actually a pair of the key and value passed to mkWeakPair:

 mkWeakPair key val finalizer = mkWeak key (key,val) finalizer

The advantage of this is that the key can be retrieved by deRefWeak in addition to the value.

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:

• If a weak pointer (object) refers to an unreachable key, it may be finalized.

• Finalization means (a) arrange that subsequent calls to deRefWeak return Nothing; and (b) run the finalizer.

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:

• It is a member of the root set.

• It is directly pointed to by a reachable object, other than a weak pointer object.

• It is a weak pointer object whose key is reachable.

• It is the value or finalizer of an object whose key is reachable.