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jimblandy/weak-references.md

Last active May 15, 2019 00:29
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Take a stab at specifying the conditions under which weak references can be cut
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weak-references.md

From the spec's point of view, all objects live forever. Garbage collection is just an optimization, permitted only because it is undetectable. Even though weak references are generally seen as making GC visible, we can actually specify weak references in a way that preserves the traditional approach: the spec will still assume all objects live forever, and will merely describe when it is permitted to remove certain edges to them.

An object is "relevant" if replacing it with any arbitrary other object could have a visible effect on the future execution of the program.

If there is a set of weak references such that none of their referent objects are relevant unless some weak reference within the set is followed, then it is permitted to cut all references in the set.

Suppose an object A is pointed to by a weak reference B, and by a variable. Consider the set {B}: A is certainly relevant even if {B} is never followed, since A can be used via the variable.

However, if the implementation can prove that the variable will never be used, then A can only be relevant if B is dereferenced, and it is permitted to cut all the references in {B} (in other words, just B itself).

Suppose two weak references R and S point to two objects O and P. Suppose further that the two objects point to each other, but are not otherwise reachable.

  • Consider the set {R}. Certainly O is relevant even if R is not followed; it is reachable via S, and then P. It is not permitted to clear R alone.

  • Consider the set {R, S}. O and P are irrelevant unless some weak reference in {R, S} is followed, so it is permitted to clear R and S simultaneously.

Suppose, in addition to O, P, R, and S, there is another object Q reachable only via a weak reference T; Q itself refers to nothing else. Consider the set {R, S, T}: none of their referent objects are relevant unless some weak reference in that set is followed, so it is permitted to cut all three together.

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