[antlr-interest] syntactic predicates and exceptions

[Pete Gonzalez]

Terence Parr wrote:
> The *LEXER* is backtracking?  Oh gawd, it will kill you.  See what  the 
> java grammar does for matching numbers...I think it avoids them.   An 
> exception per char is deadly.

Thanks!  I tried the lexer from your Java grammar and this eliminated the 
exceptions.  The improvement is significant:  0.406 secs outside of 
debugger (i.e. 48% of original), 0.456 secs in the debugger (i.e. 0.8%!).  :-)

FYI I noticed that Todd King recently posted an updated C# grammar on the 
Antlr web site saying:

"I designed this grammar for speed so I avoided using arbitrary lookaheads 
where I could. I got it down to just 7 lookaheads."

However, he is using the exact same lexer (from Robin Debreuil) with 
backtracking.  I didn't see an e-mail address, but someone should notify 
him about the technique used in your Java lexer.

> I'll be doing an auto-converter to 3.0.  Current version I just  
> released is pretty usable if you ask me, but I'm going to try it out  on 
> a real app soon to see if it does anything useful.  It crashes on  bad 
> input at the moment etc...  I wouldn't bet my job on it yet ;)   3.0 eta 
> will be next summer probably.

I took a look at the 3.0 examples and this new design really does "kick 
ass" to quote a PowerPoint slide.  I think you are doing some genuinely 
innovative work, and I think your approach to the problem is right on.  I'm 
really looking forward to this release, and will give the "early access" 
build a try as soon as the C# codegen is in place.  Anyway, thanks for your 
help!

Cheers,
-Pete