[antlr-interest] Re: updated "antlr 2 bashing list"

[lgcraymer]

Ter--

I hate to say it, but I think that that's a cop out.  Building
exception objects and stack frames costs, and there is no point when
in guessing mode.  They are also obnoxious when debugging (especially
C++) and hamper code generation for languages which do not have
exceptions.  It would be better to have a "synpredFailed flag as part
of the generated classes-the flag will always be in cache, so is not a
performance issue--or to have a return flag from rules and wrap rule
invocations (foo) in
if (foo() == false) return;

and add
if (guessing)
    return false;
else {
    <throw exception>
}

around the <throw exception>s in ANTLR.  Come to think of it,
implementing this with the current ANTLR runtime would be only about
10 lines of code or so (maybe a little more--I think that each
exception would have to be fixed separately).

--Loring


--- In antlr-interest at yahoogroups.com, Terence Parr <parrt at c...> wrote:
> 
> On Mar 9, 2004, at 12:03 PM, Robin Debreuil wrote:
> 
> > Well, I like it a lot already - have to say that first : ).
> >
> > One thing I find strange is using exceptions for flow control in the
> > generated code. I prefer exceptions to just be used for things you 
> > aren't
> > expecting in code, probably that is just a style thing though. Just 
> > this way
> > they seem to be much like gotos with global variables for state...
> 
> "Exceptions are for exceptional conditions and not for control-flow" is 
> a great rule....unless you are automatically generating that code.  The 
> syn pred mechanism is meant to be used sparingly because it has 
> exponential complexity when they start to nest.  The extra catch 
> overhead (which is only invoked upon failure) is not a huge part of 
> that I'd wager.  Further, I find the try/catch more readable than say 
> wrapping all method calls in
> 
> boolean failed = rule();
> if ( failed ) { ... }
> 
> also makes it kind of hard to return values efficiently.   i agree that 
> it might be interesting to look at the two different approaches.
> 
> I'm adding the rest to the list... thanks...
> 
> Ter
> 
> > For the C# version, it would be really nice to use Enums for all the 
> > tokens,
> > or better yet, categories of them. It makes debugging much easier,
and 
> > the
> > whole thing becomes a bit more 'solid'.
> >
> > Above all though, it would be great to have more error information
> > available, both in the grammars and when running the generated code. 
> > The
> > program itself is designed to facilitate building that kind of thing 
> > into
> > languages, so it seems kind of like the cobblers kids going without 
> > shoes.
> > Maybe restrict what is valid syntax in a grammar and catch more common
> > errors. For the generated code, maybe a debug version - where it can 
> > tell
> > you things like the statement that it couldn't get past etc. Maybe
even
> > things like setting breakpoints on input files.. Probably that
would be
> > hard, but with all the guessing levels, gotos, exceptions, etc, it
can 
> > get
> > pretty hard to trace. I may be overlooking a few exisiting techniques 
> > here
> > though, I'm pretty new to it.
> >
> > Regardless, it is a great tool - the above would only make it
easier (I
> > think anyway) to do what is already pretty easy, considering...
> >
> > Cheers,
> > Robin
> >
> >
> >
> >
> >> Folks,
> >>
> >> Keep that hate mail coming ;)  Thanks for all the words of
> >> encouragement also :)  I have updated the page to include your
> >> comments:
> >>
> >> http://www.antlr.org/blog/antlr3/antlr2.bashing.tml
> >>
> >> Terence
> >> --
> >> Professor Comp. Sci., University of San Francisco
> >> Creator, ANTLR Parser Generator, http://www.antlr.org
> >> Cofounder, http://www.jguru.com
> >> Cofounder, http://www.knowspam.net enjoy email again!
> >> Cofounder, http://www.peerscope.com pure link sharing
> >>
> >>
> >>
> >>
> >>
> >>
> >> Yahoo! Groups Links
> >>
> >>
> >>
> >>
> >>
> >>
> >
> >
> >
> >
> > Yahoo! Groups Links
> >
> >
> >
> >
> >
> >
> --
> Professor Comp. Sci., University of San Francisco
> Creator, ANTLR Parser Generator, http://www.antlr.org
> Cofounder, http://www.jguru.com
> Cofounder, http://www.knowspam.net enjoy email again!
> Cofounder, http://www.peerscope.com pure link sharing



 
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