[antlr-interest] Syntactic predicate confusion

[Christopher Nebel]

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

(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

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?


--Chris Nebel



 
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