[antlr-interest] ANTLR Optional statements

[Terence Parr]

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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