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Guardians provide a way to be notified about objects that would otherwise be collected as garbage. Guarding them prevents the objects from being collected and cleanup actions can be performed on them, for example.
See R. Kent Dybvig, Carl Bruggeman, and David Eby (1993) "Guardians in a Generation-Based Garbage Collector". ACM SIGPLAN Conference on Programming Language Design and Implementation, June 1993.
Create a new guardian. A guardian protects a set of objects from garbage collection, allowing a program to apply cleanup or other actions.
make-guardianreturns a procedure representing the guardian. Calling the guardian procedure with an argument adds the argument to the guardian's set of protected objects. Calling the guardian procedure without an argument returns one of the protected objects which are ready for garbage collection, or#fif no such object is available. Objects which are returned in this way are removed from the guardian.You can put a single object into a guardian more than once and you can put a single object into more than one guardian. The object will then be returned multiple times by the guardian procedures.
An object is eligible to be returned from a guardian when it is no longer referenced from outside any guardian.
There is no guarantee about the order in which objects are returned from a guardian. If you want to impose an order on finalization actions, for example, you can do that by keeping objects alive in some global data structure until they are no longer needed for finalizing other objects.
Being an element in a weak vector, a key in a hash table with weak keys, or a value in a hash table with weak values does not prevent an object from being returned by a guardian. But as long as an object can be returned from a guardian it will not be removed from such a weak vector or hash table. In other words, a weak link does not prevent an object from being considered collectable, but being inside a guardian prevents a weak link from being broken.
A key in a weak key hash table can be thought of as having a strong reference to its associated value as long as the key is accessible. Consequently, when the key is only accessible from within a guardian, the reference from the key to the value is also considered to be coming from within a guardian. Thus, if there is no other reference to the value, it is eligible to be returned from a guardian.