Re: block-scope finalization

[Coda Highland <chighland@...>]

On Tue, Nov 24, 2015 at 12:42 PM, Dibyendu Majumdar
<mobile@majumdar.org.uk> wrote:
> On 24 November 2015 at 16:52, Roberto Ierusalimschy
> <roberto@inf.puc-rio.br> wrote:
>> What is wrong with the following proposal?
>>
>>   local <some mark to be invented> name = exp
>>
>> Unlike a regular 'local' declaration, this one must define only one
>> variable and it must be initialized. (Both restrictions could be easily
>> removed; they are more about programming style.)
>>
>> When the local 'name' goes out of scope, then:
>>
>> 1 - if its value is a function, that function is called (no parameters)
>> 2 - if its value is a table/userdata, its __close (or some other new
>> name) metamethod, if present, is called.
>>
>> Otherwise, 'name' is like any other local variable.
>>
>
> I haven't followed the whole conversation so apologies if I have got
> this wrong. If the aim is to provide a 'finally' type feature then an
> important aspect is that the finally code must execute even when
> exceptions occur. In the above proposal how will Lua ensure that the
> function will always be called when the block exits even if there was
> an exception which jumped down the stack?
>
> Regards
> Dibyendu
>

Because "going out of scope" is really just another name for "is
popped off the stack." If an error unwinds the stack, then it would
have to check for the presence of these marked slots on the way down
instead of just changing the stack top index.

/s/ Adam