[antlr-interest] Determinig the real ambiguities

[Monty Zukowski]

Ok, with this particular example you need a syntactic predicate, also 
known as "infinite lookahead."  Note that for any value of k this is a 
real ambiguity.

a_must_have_c : ((A)? (B)+ C)=> (A)? (B)+ C D
			  | (B)+ D ;

The other thing to do is to factor the rule, sometimes that is 
practical, sometimes it isn't, depending on your actions & tree 
building.

Monty


On Jul 15, 2004, at 11:42 AM, Bogdanov, Serge wrote:

>> a_must_have_c : (A)? (B)+ C D | (B)+ D ;
>
> How about
> 	a_must_have_c : A (B)+ C D | (B)+ D;
>
>>> Sergey Bogdanov
> intel massachusetts
> M/S HD2-246
> 77 Reed Road,
> Hudson, MA  01749
> Tel: (978)553-2724
>
>> -----Original Message-----
>> From: thrutchy [mailto:eric_mahurin at yahoo.com]
>> Sent: Thursday, July 15, 2004 2:30 PM
>> To: antlr-interest at yahoogroups.com
>> Subject: [antlr-interest] Determinig the real ambiguities
>>
>> Is there a way to turn off the ambiguities warnings that aren't real
>> and only affection the parser prediction (and thus performance)?  Here
>> is an example:
>>
>> a_must_have_c : (A)? (B)+ C D | (B)+ D ;
>>
>> It thinks these alternatives are ambiguous because they both can start
>> with an indeterminate amount of B's, but the ambiguity is resolved
>> right after that.
>>
>> I assume ANTLR will try matching the first alternative and then go to
>> the second if the input matches the second alternative.  Is that
> correct?
>>
>> I know that there are several ways I can turn off the warnings
>> (syntactic predicate, options), but it would be nice if ANTLR could
>> figure out that the alternatives really don't overlap even though it
>> may predict the wrong alternative.  Eventually, it would be good to
>> fix as many of these mispredictions as possible, but initially
>> functionality is what matters.
>>
>> Eric
>>
>>
>>
>>
>>
>>
>> Yahoo! Groups Links
>>
>>
>>
>>
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> Yahoo! Groups Links
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