[antlr-interest] syntactic predicates and exceptions

[Prashant Deva]

cant u have something like this-
match(int tokenType, int guessing)
{
//do matching stuff
//on error, do this-
if (guessing>0)
return false;

else throw Excpetion;
}




On 10/18/05, Terence Parr <parrt at cs.usfca.edu> wrote:
>
> Howdy,
>
> Many people have complained about the fact that ANTLR uses exceptions
> for backtracking and it's slow in many languages and many grammars.
> For v3 I was thinking of changing to be return values and lots of IF
> checks everywhere...ugly but fast.
>
> Then i realized last night that it's not so easy. We use exceptions
> for throwing exceptions right? All the support code for match,
> matchAny, blah blah throws exceptions, which would trigger rewinding
> the backtracking predicate in v2. How can we use exceptions for
> error conditions and not for guessing? Do people propose that we
> duplicate the support library code or put IFs everywhere in the
> support code, perhaps slowing all cases down?
>
> Perhaps looking at javacc output (which avoids exceptions for
> backtracking I think) would be useful. I don't like a mixed model.
> Remember, normal case you want exceptions and backtracking case you
> don't. Pretty messy support code if you ask me. Easy to say "get
> rid of exceptions", but now when it comes to doing it I don't see a
> clean solution. I also hate the idea that even when not backtracking
> you'll see code like this everywhere (ugly and slower):
>
> if ( !match(ID) ) {code inserted to clean up; return false;} //
> indicate error if guessing
>
> instead of the current
>
> match(ID);
>
> Note: java can use a try/finally for the cleanup code but other
> languages will need perhaps a goto or to duplicate cleanup code at
> *all* exit sites...ick.
>
> Ideas for implementing backtracking without exceptions?
>
> Ter
>



--
Prashant Deva
Creator, ANTLR Studio, http://www.antlrstudio.com
Founder, Placid Systems
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