[antlr-interest] Re: The eternal question: Why is this ANTLR grammar ambiguous??

[lgcraymer]

--- In antlr-interest at yahoogroups.com, Harald M. Müller
<harald.m.mueller at b...> wrote:
> 
> 
> [I have posted this to the pccts newsgroup; and on the antlr forum 
> on jguru - I'm sorry if this is "against the rules", but thos two 
> forums seem to have quite low answer rates, and I have found this 
> mailing list only now]
> 
> I have already written a few ANTLR lexers and parsers (for 
> productive systems, with sometimes weird grammars) - but the 
> following puzzles me: 
> Why does the following simple grammar create an ambiguous warning?: 
> 
> 
> class MyLexer extends Lexer;
> 
> options {
> 	charVocabulary = '\3'..'\377';
> 	k=3;
> 	caseSensitive=false;
> }
> 
> COMMENT_COMMAND
>   : '-' '-' '$'
>   ;
>   
> SQL_STATEMENT
>   : ( SYMBOL )+ ';'
>   ;
> 
> protected SYMBOL
>   : ('-' (~ '-')) => '-'   //1
>   | 'a'
>   | '"' (~ '"')* '"'       //2
>   ;
> 
> When I remove either of //1 or //2, the ambiguity goes away. 
> When I replace //2 with 
> 
>   | '"' (~ ('"'|'$'))* '"'
> 
> - i.e., I also exclude $ inside the "string" -, the ambiguity also 
> goes away. 
> But why would a $ after(!!) a " lead to an ambiguity????? 

This is an approximate LLk issue--there isn't really an ambiguity, but
ANTLR sees  --$ as matchable by SYMBOL; SYMBOL is matched as ( 'a' | (
('-' | '"') (~'-' | ~'"') ...

If you take a look at the generated code, it should do the right
thing.  Fortunately, Ter is doing full LLk for ANTLR 3 so we don't
have to be confused by this sort of thing.

--Loring

> 
> Thanks for any help! 
> 
> Harald M.





 
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