> -----Original Message----- > From: antlr-interest-boun...@antlr.org [mailto:antlr-interest- > boun...@antlr.org] On Behalf Of Ruslan Zasukhin > Sent: Thursday, October 13, 2011 2:03 AM > To: antlr-interest@antlr.org > Subject: Re: [antlr-interest] Advice on best practice? > > On 10/12/11 9:15 PM, "Jim Idle" <j...@temporal-wave.com> wrote: > > Hi Jim, > > > Avoid backtracking like the plague if you need performance. But if > you > > are careful in the order of your alts and use it on just a few > > decisions/rules, then it might not be so bad (but remember that your > > error messages will be weak). > > Then next question comes to mind: > does ANTLR allow us somehow easy to see that during parsing > was made 2-3 or 4 backtracking. > > Like profiler ... So we can see that and start learn where and why .. > ?
You can turn on the trace rule stuff, but not in the C target as I never implemented that, but I am not sure that it would help as there would be too much information. Some of the other -X options might help. But really, it is better to just turn off backtracking and resolve the ambiguities as that is where all the backtracking will be done. > > > If k=1 on a decision then ANTLR will work that out so you don't need > > to specify but if you want to avoid ANTLR following every possible > alt > > then you can use k=1 on a particular rule or sub rule to avoid > > ambiguity errors. > > Aha, so playing with k=1 option, > We can even kill ambiguity errors/warnings. > > Interesting. > > This is what I ask for -- it will be great to have list of such > tricks/rules from ANTLR experts .. > > > Basically if you know that a decision will be correct at k=1 even > > though ANTLR can see ambiguities, then tell it so. > > > Before 3.4 this would still give a warning unless you added a 1 token > > predicate, but I believe that Ter changed this for 3.4 so that I > could > > remove a lot of those predicates from my own T-SQL grammar. > > So we do not need that one-token predicates now ... Good. > But I do not understand how it looks? > > Can you show example of such predicate just for interest ? In order to turn off a warning, you would have to do this: FRED: FRED ((WAS)=>WAS))? var; Now you can use k=1 to turn that warning off (but experiment on small grammars to get it right). Jim 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 il-antlr-inter...@googlegroups.com. To unsubscribe from this group, send email to il-antlr-interest+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/il-antlr-interest?hl=en.
