Hi Sam > Edit: I misread the original email, but I'm still sending this because > it resolves the warning given on the rule y.
;-) Yeah that much is clear, I also fully understand the crazyness of not left-factoring rule "start" into (y C?)+ EOF; Just was wondering why the warning on rule "start" as it stands below, I was expecting ANTLR to "auto-left-factor" with the prediction dfa and don´t quite understand why it does not on this occasion. Thanks Peter > Consider the input 'ba', starting with the rule start. It could parse as > (y y EOF), with the first y matching B without the A, and the second > matching just A. It could also parse as (y EOF), with the first y > matching B and the optional A. > If you mean for y to always include the A when present, you can use > a syntactic predicate, which forces it to choose the second > interpretation above: > y : B ((A) => A)? | A; > Sam -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Peter Kooiman Sent: Thursday, April 07, 2011 7:50 AM To: [email protected] Subject: [antlr-interest] Why is this ambiguous.. Hello, While working on a grammar I had something that boils down to grammar T; start: (y | y C)+ EOF; y: B (A)? | A; A : 'a'; B : 'b'; C : 'c'; This gives an expected warning on rule y, that's fine, ANTLR does the right thing and matches A greedily. However it also gives a warning on rule start: T.g:2:19: Decision can match input such as "B {EOF, B..A}" using multiple alternatives: 1, 2 As a result, alternative(s) 2 were disabled for that input I quite understand the need to left factor this, I was just wondering why ANTLR gives this warning. It must be staring me in the face but I don't see it... How is this non-deterministic on input B EOF? Is ANTLR unable to see past rule y for some reason? Thanks Peter List: http://www.antlr.org/mailman/listinfo/antlr-interest Unsubscribe: http://www.antlr.org/mailman/options/antlr-interest/your-email-address List: http://www.antlr.org/mailman/listinfo/antlr-interest Unsubscribe: http://www.antlr.org/mailman/options/antlr-interest/your-email-address -- You received this message because you are subscribed to the Google Groups "il-antlr-interest" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/il-antlr-interest?hl=en.
