Re: Exceptions (was Re: Custom extensions to Lua)

[Lisa Parratt <lisa@...>]

David Given wrote:
> I'd just like to point out that the various Java people we have here will, if 
> you mention finalisers to them, start frothing at the mouth and using phrases 
> like 'utterly brain-damaged', 'my god, what were they thinking', 'spawn of 
> satan' and in some circumstances 'eeeeeeeaaaaaagh'.
>
> Just think about these:

*snip*
>         oo = o
*snip*

Tell me if this solution has any holes - I presume it must because it 
seems straightforward and simple - within a finaliser, make it illegal 
for the object undergoing finalisation to be on the right hand side of 
an assignment operation. Off the top of my head, this could be 
implemented by adding an extra field to lua_State that indicates which 
object, if any, is undergoing finalisation, and then modifying the load 
and move op codes so they won't allow the object under finalisation to 
be on the right hand side. This would admittedly slow down the VM 
slightly - does the parser have enough information to do thisat compile 
time, rather than run time? Care may be required to ensure that 
finalisers are called respecting hierarchy and that members for tables 
aren't marked for finalisation until the finaliser has completed, however.

> Lua gets away with __gc because it has very restricted functionality; 
> finalisers can only be written in C, the finalised objects can't be 
> resurrected, the finalisers can't block, etc. IMO, this is a Very Good Thing.

I can think of a few audit trail purposes where finalisers would be 
nice, and these could benefit from relaxing the C requirement. Currently 
one is limited into using work arounds and hacks to abuse metadata 
values to invoke finalisers. Providing the functionality, even if the 
documentation has a big "DO NOT USE" sign attached, would simplify and 
standardise such routines, making them more difficult to break for the 
uninitiated.

Sorry for rambling, and sorry for jumping in :)

--
Lisa
http://www.thecommune.org.uk/~lisa/