Weak keyed tables with primitive keys

[Gary Bringhurst <garyb@...>]

My google fu is failing me on this one.

Where can I find a definitive discussion of weak keyed tables and the  
behavior of primitive data types as those weak keys?  My tests say  
that unique string keys are never collected, and while I can see a  
justification for that I'd really like to know the relationship of  
primitive types such as strings to the garbage collector. Do we have  
to use tables or user data as keys in weak keyed tables in order for  
them to be collected?

Many thanks in advance,
Gary Bringhurst