Dear Jim,
I agree with you. I am on what I believe to be the last "hurdle" of my
grammar:
(simplified)
grammar conjunctive; //using ANTLRWorks
englishSentence : (adverbPhrase ','?)* simpleSentence;
adverbPhrase: 'con' simpleSentence | prepPhr;
simpleSentence : nounPhrase verbPhrase;
nounPhrase: ('noun' prepPhr)=> 'noun' prepPhr |
'noun';
prepPhr: 'prep' nounPhrase;
verbPhrase: 'verb' complement? adverbPhrase?;
complement: (object object)=> object
object | object;
object:
nounPhrase;
Problem sentence: "con noun verb noun verb" SHOULD give adverbPhrase
simpleSentence (but doesn't, instead consuming the second "noun" as a
*complement* to the verbPhrase, leaving an impossible simpleSentence)
My proposed solution was: Make sure that for something to be an "object", it
must not be followed by a verbPhrase. This is so that it is not consumed by
a possible *complement?*, IF it is part of a new simpleSentence (i.e. if it
is the subject of a sentence).
I believe my entire grammar is completed, as soon as this problem is fixed.
2009/10/19 Jim Idle <j...@temporal-wave.com>
> I really think you are beginning to make this way more complicated than
> it really is. You need to step away from your current pursuit of predicates
> and so on and go back to fundamentals. You are trying to get the parser to
> do way more than parser should do. I know it is tempting because it seems
> that the parser will do a lot of validation for you, but it is (generally)
> the wrong way. You want your parser to be as simple as possible, accepting
> anything that is good syntax from a pure string together of tokens point of
> view, THEN apply semantics to see if anything fails. This way you r parser
> rules can accept all the clauses that might be correct (as in all
> syntactically sound combinations), then reject the incorrect ones by
> semantic analysis. Your parser will be simpler, your users will thank you
> for the improved error messages and the whole thing will be easier to
> maintain.
>
>
>
> Basically if you find yourself in the nitty-gritty like this, you are
> probably heading down the wrong path or flogging a dead horse ;-) You could
> use backtracking mode for this, but then your error messages will be even
> more arbitrary.
>
>
>
> For the predicate, don’t forget that the prediction will still select a
> rule and try the predicate. Perhaps you could use gated predicates on the
> ‘invocation’ of the rule so that it is not ‘called’ if it won’t match, but
> once you are in a rule you have to do something like this;
>
> r : (pred1)=>rule1
>
> | (pred2)=>rule2
>
> | // neither of the above, match nothing
>
> ;
>
>
>
> But then you start need to set state flags, test the flags, etc. You soon
> end up with a lot more work than just using a semantic analysis phase after
> the initial parse. You will also have to deal with the absence of flags due
> to syntax errors and so on, whereas if you defer to semantic analysis as a
> separate phase, then you will be able to rely on a sound syntax.
>
>
>
> So:
>
>
>
> 1) Don’t try to influence the paths the parser takes – let the tokens
> do that;
>
> 2) Afterwards, in the tree walk, determine whether the various
> combinations of clauses are correct, issue good errors here if not;
>
> 3) Generate code (or whatever it is you are doing) in the knowledge
> that it is all good from here.
>
>
>
> Jim
>
>
>
> *From:* antlr-interest-boun...@antlr.org [mailto:
> antlr-interest-boun...@antlr.org] *On Behalf Of *Naveen Chawla
> *Sent:* Saturday, October 17, 2009 10:45 PM
> *To:* antlr-inter...@antlr.org
> *Subject:* [antlr-interest] How to make a syntactic predicate exit a rule
> completely
>
>
>
> Ok, so even if all syntactic predicates have failed in a particular
> rule, that rule is still included as "matched" in that analysis! Is this
> true or are my tests wrong? This is not what I hoped. I was hoping that upon
> complete failure of all syntactic predicates in a rule, the rule becomes
> disregarded, and that "disregarding" is cascaded as far up as possible, and
> an alternative path is attempted. Is this how it's works, or am I missing
> something? Or is there a way, using actions, to "force" the exiting of a
> particular rule as if it had not been matched at all?
>
>
>
> Many regards,
>
> N
>
>
> List: http://www.antlr.org/mailman/listinfo/antlr-interest
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>
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