[antlr-interest] does ANTLR have syntax for negation of syntactic predicates?

[Nikolay Ognyanov]

Can somebody please advise whether it is possible to negate
a syntactic predicate in an ANTLR grammar? This is something
fairly easy to do in the generated code but I am not able to
find ANTLR syntax for it. I googled a couple of threads on
the issue but found no definitive answer.

I am not asking for help with my specific problem because I
have a (kludgey) solution but but will describe it below FYI
and in a hope to motivate implementation of the feature if it
is not available.

The problem is with a rule

like this:

rule
     : alt1
     | alt2
       // ... more alternatives
     | altm
     | altn
     ;

where altm and altn can have (among many others) prefixes
prefm and prefn respectively such that:

   1/ No alternative other than altm and altm can have prefix
      prefm or prefn.
   2/ LL(*) analysis can not distinguish between prefm and prefn
   2/ If the alternatve between prefm and prefm is resolved
      then what remains from altm and altn is resolvable by LL(*)

Now I can not say something like:

rule
     : alt1
     | alt2
       // ... more alternatives
     | (prefm) =>  altm
     | altn
     ;

because prefm is not the only possible start of altm.
It is possible in priciple to say something like this:

rule
     : alt1
     | alt2
       // ... more alternatives
     | (prefm) =>  prefm altmRest
     | altmRest
     | altn
     ;

but prefm and prefn are two of the core nonterminals
in the grammar with so many rules, dependences,
appearances and recursions that the breakdown to
prefix and reminder is not practically feasible.

Moreover I do not want to enable general backtracking for
the rule because it would be quite a bit more expensive
than backtracking over prefn only.

So what I would like to say is something like this:

rule
     : alt1
     | alt2
       // ... more alternatives
     | !(prefn) =>  altm
     | altn
     ;


My kludgey solution is to subclass the parser and to
override the syntactic predicate method to negate its
result. It is somewhat better than directly patching
the generated parser code but there is still the problem
with the need to watch out for change of predicate method
name if/when I add more predicates and the nasty
side effects if/when I fail to do that.

Thank you in advance
Nikolay