[antlr-interest] Java API for checking ambiguity in grammars using ANLTR

[Stephen Gaito]

Sathay,

An interesting puzzle ;-)

So AntlrWorks detects ambiguous grammars in the analyze method in the
> org/antlr/works/grammar/antlr/ANTLRGrammarEngineImpl.java
class. (see:
> https://github.com/antlr/antlrworks/blob/master/src/org/antlr/works/grammar/antlr/ANTLRGrammarEngineImpl.java
 and search for "analyze" ).

It does this by setting an error listener to catch all errors coming
from the ANTLR tool's compilation of the grammar. Essentially across the
following code:
> Grammar g = getDefaultGrammar();
> ...
> List rules = g.checkAllRulesForLeftRecursion();
> ...
> if ( g.nfa==null ) {
>   g.composite.createNFAs();
> }
> g.createLookaheadDFAs();
> if(engine.isCombinedGrammar()) {
>   // If the grammar is combined, analyze also the lexer
>   if(lexerGrammar != null) {
>     lexerGrammar.composite.createNFAs();
>     lexerGrammar.createLookaheadDFAs();
>   }
> }
AntlrWorks reports the ambiguous grammar messages by looking for the
following messages/exceptions which have been caught and stored in a
list by the ErrorListener:
> private void buildError(Object o) {
>   if(o instanceof *GrammarUnreachableAltsMessage*)
>    
> errors.add(buildUnreachableAltsError((GrammarUnreachableAltsMessage)o));
>   else if(o instanceof *GrammarNonDeterminismMessage*)
>     
> errors.add(buildNonDeterministicError((GrammarNonDeterminismMessage)o));
>   else if(o instanceof *NonRegularDecisionMessage*)
>     
> errors.add(buildNonRegularDecisionError((NonRegularDecisionMessage)o));
> }
*SO....*

If you wanted to programatically check for ambiguous grammars, you could
simulate the code in this ANTLRGrammarEngineImpl class in a try { }
"block" and catch the above messages.

You could also infer this on the command line by looking at the output
of the antlr -Xdfa command.... but this is rather harder as there is no
/*explicit*/ statement that the grammar is ambiguous....

I hope this helps!

Regards,
Stephen Gaito

On 12/06/12 04:13, sathya moorthy wrote:
> I want to know whether the grammar is ambiguous by calling some ANTLR API
> but I want to know is there such API?
> The purpose is not to generate files from the grammar if it is ambiguous.
>
> Antlr generates files even if it is ambiguous(in some cases)
>
> On Mon, Jun 11, 2012 at 2:53 PM, Loring Craymer <lgcraymer at yahoo.com> wrote:
>
>> You want code generated even if the grammar is reported to be ambiguous.
>>  ANTLR tells you how it handles ambiguity (some alternatives are disabled);
>> if this is not what you want, fix your grammar.
>>
>> --Loring
>>
>>
>>   ------------------------------
>> *From:* sathya moorthy <brsmoorthy at gmail.com>
>> *To:* antlr-interest at antlr.org
>> *Sent:* Monday, June 11, 2012 1:54 AM
>> *Subject:* Re: [antlr-interest] Java API for checking ambiguity in
>> grammars using ANLTR
>>
>> Guys any help would be great
>> thanks
>> Sathya
>>
>> On Sat, Jun 9, 2012 at 1:40 PM, sathya moorthy <brsmoorthy at gmail.com>
>> wrote:
>>
>>> Hi,
>>>
>>> When i use ANTLR IDE, it suggests that the grammar is ambiguous but still
>>> i am able to generate code using that grammar.
>>> Is there a specific API in ANTLR which will tell whether the grammar is
>>> ambiguous?
>>> If the grammar is ambiguous i dont want to generate the code for that
>>> specific file.
>>>
>>> Regards,
>>> Sathya
>>>
>>>
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