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On Sep 12, 2013, at 7:00 AM, Marc Lepage <> wrote:

> Suppose I have tables referring to tables (by their field values), or functions referring to upvalues (by closure).
> Given a table, I can know what it refers to by iterating over its contents. And given a closure, I can use the debug facilities to find its upvalues. I can do both recursively to obtain all objects or upvalues referenced.
> Is it possible to go in the other direction? To find all references to a given value (say a table) in other tables or closures (as upvalues)?
> Wouldn't the garbage collector have this information? It's not exposed in any debug facilities, is it?
> I'm just pondering about the idea of one object being able to notify other objects that reference it, without having to keep a list of listeners that explicitly subscribed for notifications.

You really don't want to use the GC for this (not that you can), and I would advise against the debug library for any production code (it makes very few claims regarding performance). There are lots of design patterns that could help you out with this problem, without having to resort to "tricks" .. your best friend here is a weak table, as what you are really looking for is a variant on pub/sub, and a weak table is the ideal way of tracking subscriptions.