[antlr-interest] Re: Syntactic predicate confusion

[lgcraymer]

--- In antlr-interest at yahoogroups.com, Christopher Nebel <c.nebel at a...> wrote:
> Apparently I don't quite "get" syntactic predicates.  Does the 
> predicate imply that the attached alternative will fire *only* if the 
> predicate is true, or it only apply the predicate if the lookahead is 
> otherwise ambiguous?  It appears to be the former, which seems lame.  
> Here's my situation:
> 
> 	x: a | b
> 	a: c | b d


Well, actually neither is quite correct.  Using your example (below)

> 
> (The real thing is far more complicated, but this is the basic 
> problem.)  Naturally, I get an ambiguity warning here -- if it sees the 
> stuff predicted by "b", it doesn't know whether to follow x-> a-> b e 
> or just x-> b.  So, I put in a predicate:
> 
> 	x: (b d)=> a | b
> 	a: c | b d

ANTLR reasons as follows
1.)  Could it be a (normal alternative testing precedes synpred testing)?
2.)  If yes, does it match (b d)?
3.)  If and only if yes, it is an a; ignore all other alternatives.
> 
> This doesn't work, however: if I give it something c-like, then it 
> can't parse it -- it just completely ignores the a-> c production.  If 
> I add a duplicate of x-> a without the predicate, I just get more 
> ambiguity warnings.  What to do?  I don't want to break up the "a" 
> rule, since the real thing is rather complicated, not to mention 
> self-referential.  I suppose I could come up with a syntactic predicate 
> for the duplicate x-> a rule, but it's going to be sort of large.  
> Suggestions?

If you can identify a minimal synpred, you can do something like adding a rule a_prime to be used just for the synpred:

a_prime
      :
      c | b B
      ;

and change your rule to

x  :
    (a_prime)=> a
    |   b
    ;

As long as a_prime is not otherwise referenced, you won't have any problems.  One of the nice features of LL parsing is that you can 
have "duplicate" versions of rules--unlike most LR parsers, if you expect an a in an LL parser, you only look for a.  That is useful for 
attaching different sets of actions to the same syntactic phrase to get context-dependent semantics.

--Loring



 
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