[antlr-interest] Re: Antlr grammar to parse Java classfile?

lgcraymer lgc at mail1.jpl.nasa.gov
Thu Dec 6 16:57:59 PST 2001

Actually, I would argue that semantic predicates are the way to go; as  
Monty pointed out 

     ( { n > 0 }? foo { n--; } )*

will parse at most n iterations and you can add a sem pred as a 
termination check, as Ter suggested.  Recursive definitions also work:

{ int n = N; }
       { n > 0 }?  foo { n--; } loop
       | { n == 0}?

will parse exactly n copies of foo or throw an exception.

Unfortunately, syntactic predicates don't behave the same in loops as 
they do in recursive definitions--the recursive solution works as 
ANTLR is currently implemented.  (Maybe I could get the syn preds to 
work with the non-greedy #pragma--I haven't tried that.)  It would be 
nice if this were fixed--the loop expression is rather less verbose, 
and the recursive definition can be inconvenient for tree structuring.

The real advantages of sem preds over a new counter syntax is that 
they support a full range of exit conditions.  Why add a new syntactic 
element which does less than the current machinery?

--Loring Craymer

--- In antlr-interest at y..., Terence Parr <parrt at j...> wrote:
> On Wednesday, December 5, 2001, at 04:01  PM, John D. Mitchell 
> > [...]
> >> Let's see.  Well, it's close, but it wouldn't give you an error 
if you
> >> have less than blockCount.  You need that "did I get enough" 
check at 
> >> the
> >> end of the loop.  Could do with a simple "validating" sem pred 
> >> loop.
> >
> > Hmm... How is that going to be safe in the face of hoisting?
> Hmm...not sure if it's an issue or not.  I just realized though that 
> code gen is easy but the analysis might not be so easy to enhance.  
> There is no concept of repeated grammatical item in my internal data 
> structure.
> Ter
> --
> Chief Scientist & Co-founder, http://www.jguru.com
> Creator, ANTLR Parser Generator: http://www.antlr.org


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