Re: Re[2]: state of the Lua nation on resource cleanup

[Mark Hamburg <mark@...>]

I think this really comes down to two cases:

1. You know the lifetime for the object and you can explicitly close  
it when done -- e.g., by calling f:close(). f:close() is much clearer  
that the goal is to close the resource. f = nil just means "I don't  
care about this particular reference to the resource anymore".

2. You don't know the lifetime. Then you are looking to the system to  
find all of the potentially live references and the only way to do  
that with absolute certainty is to complete a full GC. Reference  
counting won't get it for you because of cycles. Now, maybe the  
objects you are interested in -- e.g., file handles -- can't be  
directly involved in cycles, but the objects referencing them -- e.g.,  
tables -- could be.

One could patch the Lua VM so that assignments adjusted reference  
counts and this would result in some items getting collected faster,  
but it would not ensure that all objects would get finalized the  
moment they became unreachable because that's a problem that requires  
global rather than local information.

This is not unique to Lua. The languages that seem to give one  
immediate collection do so by restricting the data structures one can  
build or by expecting the programmer to restrict the choice of data  
structures and/or how they are used.

Mark