[antlr-interest] Ambiguous reference

[Prerovsky, Tomas]

Hi Jim,

Yes, you are right, the "(expression)=>" predicate slows the parser down, the line i have posted stems from an earlier version of my grammar at a time i was experimenting with the possibilities of ANTLR.
The final version of my grammar, which by the way describes a small SML like language to define UML Model Transformations,  contains no seemingly awkward constructs . 

But this is not my point here. The line in question is perfectly legal : there is no ambiguity. Therefore ANTLR should not report an error.


Tomas
________________________________________
Von: antlr-interest-bounces at antlr.org [antlr-interest-bounces at antlr.org] im Auftrag von Jim Idle [jimi at temporal-wave.com]
Gesendet: Donnerstag, 18. November 2010 18:23
An: antlr-interest at antlr.org
Betreff: Re: [antlr-interest] Ambiguous reference

It is a little overzealous in its warning but it isn't failing ;-)

The warning is telling you that it is picking the correct interpretation,
but the label of the rule reference should of course suppress the warning.

I should also point out that using an entire expression reference as a
predicate (especially in this case) will be very slow and indicates that the
grammar is poorly formed. I suspect that you (Tomas) are following a written
spec too closely and are really trying to implement right associativity.

I also suspect that overall, you need to start again but this time left
factor as you go. Remember that the spec you are working from is probably
written in LALR terms, that you will need to interpolate before typing in to
an ANTLR grammar spec. If you are not working from a spec then stepping back
and reconsidering the syntax may be in order.

What language are you trying to parse and what spec are you using? I think
that you may be asking the wrong questions :)

Look more at a construct like this (I am guessing at what you are ultimately
trying to do of course):

expression
  : p=paring e=expression
      -> ^(APPLY $p $e)
  | expr2
  ;

expr2
  : expr3 (OPER expr3)*
  ;

etc

Jim

> -----Original Message-----
> From: antlr-interest-bounces at antlr.org [mailto:antlr-interest-
> bounces at antlr.org] On Behalf Of Sam Harwell
> Sent: Thursday, November 18, 2010 8:33 AM
> To: 'Prerovsky, Tomas'; antlr-interest at antlr.org
> Subject: Re: [antlr-interest] Ambiguous reference
>
> Hi Tomas,
>
> This is the first time I've seen someone else mention this. If you look
> at all the ways this could be written, it's clear that there is no
> ambiguity, so ANTLR is incorrectly failing:
>
> VALID: If the rhs expression is not labeled (first example), then its
> reference in a rewrite rule doesn't use a $ sign. If the rhs is labeled
> (second example), then its label is different from the enclosing rule's
> name.
>
> expression : (pairing->pairing) (expression -> ^(APPLY $expression
> expression));
>
> expression : (pairing->pairing) (rhs=expression -> ^(APPLY $expression
> $rhs));
>
> INVALID: The only actual way to introduce an ambiguity is to write the
> rule in the following way. However, it is an error in ANTLR to have the
> name of any rule as a label anywhere in the grammar. This restriction
> guarantees that there can never be an ambiguity when rewriting the rule
> as you were trying to.
>
> expression : (pairing->pairing) expression=expression -> ^(APPLY
> $expression $expression);
>
> Sam
>
> -----Original Message-----
> From: antlr-interest-bounces at antlr.org
> [mailto:antlr-interest-bounces at antlr.org] On Behalf Of Prerovsky, Tomas
> Sent: Thursday, November 18, 2010 9:50 AM
> To: antlr-interest at antlr.org
> Subject: [antlr-interest] Ambiguous reference
>
> Hi,
>
> the following line :
>
> expression
> :
>  (pairing->pairing) ( (expression)=> rhs=expression -> ^(APPLY
> $expression
> $rhs) |  ) ;
>
> causes in the latest build of ANTLR 3.2 (Sep 23, 2009) the error:
>
> "reference $expression is ambiguous; rule expression is enclosing rule
> and referenced in the production (assuming enclosing rule)  |--->
> (pairing->pairing) ( (expression)=> rhs=expression -> ^(APPLY
> $expression
> $rhs) |  ) "
>
>
> ANTLR 3.0.1 accepts the given line without reporting an error.
>
> kind regards
>
> Tomas
>
>
>
>
>
>
>
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