[antlr-interest] ANTLR Optional statements

[Tom Moog]

Ahhh.  I was misled by the difference in antlr2 and
pccts syntax.  I saw the (...)? and didn't see the
"=>" on the far right side.  My mistake.

Yes, this was pccts fix #196, I think.  You have to
use a special option (-alpha) to get a warning - so
it's a two pass algorithm.  After you know there
is a "(alpha)? beta" problem you can verify it with
-alpha.  A little bit too late, usually.

Tom

On Thu, 11 Apr 2002, Terence Parr wrote:

> 
> On Wednesday, April 10, 2002, at 03:30  PM, Tom Moog wrote:
> 
> >
> > Part 1:
> >
> > The extra EOF in the follow set is a general problem.
> > When antlr2 or pccts analyzes a grammar and finds an
> > orphan rule they assume that it is a start rule.  Since
> > every start rule should have EOF in the follow-set they
> > add one.  They should probably warn the user, but they
> > don't.
> 
> Well, you might have a grammar with lots of start rules...should one 
> really get a warning for that?  Certainly it would be nice to be able to 
> get a list of these suckers.  Not sure a warning is proper since every 
> single grammar will get a warning then ;)
> 
> > There is a warning about this in the "Notes For New Users
> > of pccts" and why pccts has a "-info o" option which
> > lists all orphan rules.
> 
> Ah...you've beaten me to the punch :)  Thanks for all your hard work on 
> PCCTS!
> 
> > Part 2
> >
> > If antlr2 doesn't see class_head, then obviously it will
> > compute the follow set improperly.  Sounds like a bug
> 
> It sees class_head in the predicate but not in the list of rule 
> references apparently.  Therefore a standard FOLLOW will not compute it 
> properly.  I think I remember saying that a predicate is not part of the 
> grammar and is merely there to show a minimum lookahead language that 
> predicts an alternative.  It should not be included in FOLLOW 
> computations.  HOWEVER, it needs to compute FIRST sets properly.  If the 
> lookahead can see past class_head it must include what follows that 
> specific reference since it knws the context of the FOLLOW.
> 
> > to me.  My hypothesis is that it is skipping the block
> > following the syntax predicate in computing first and follow
> > sets.  This is correct for first sets, but incorrect for
> > follow sets.
> 
> ANTLR computes standard static k lookahead to determine if the backtrack 
> is necessary even if you specify one.  I should be computing this on the 
> alternative not the predicate.
> 
> Tom: is this similar to the error you pointed out in PCCTS that you 
> fixed regarding syn preds?
> 
> Ter
> 
> 
>  
> 
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> 
> 
> 


 

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