On 23 Jul 2011, at 13:55, John P. Hartmann wrote: > Well, controlling the scanner from the parser is never going to work > in general as the scanner can be way ahead.
You have to put them in special position, like after a token, and verify that Bison does not put in a lookahead in that position. > I'm afraid you'll have to > write a parallel "parser" using exclusive star conditions. However > this "parser" can be vastly simplified as it only has to figure out > whether to return a comment. > > If that becomes cumbersome, there is something wrong with the grammar > (or your understanding of same). > > For example, the REXX language uses blank as both an operator and a > normal token delimiter. So the scanner needs to keep track of > expressions at least to know when it expects an operator. The > alternative would lead to an enormous number of shift/reduce conflicts > (which all would be resolved by shifting), but you'll likely regret > this as you are bound to miss a shift/reduce conflict that you need to > resolve. > > On 23 July 2011 12:43, Hans Aberg <[email protected]> wrote: >> On 23 Jul 2011, at 02:40, uclacasey wrote: >> >>> I'm writing a program that handles comments as well as a few other things. >>> If >>> a comment is in a specific place, then my program does something. >>> >>> Flex passes a token upon finding a comment, and Bison then looks to see if >>> that token fits into a particular rule. If it does, then it takes an action >>> associated with that rule. >>> >>> Here's the thing: the input I'm receiving might actually have comments in >>> the wrong places. In this case, I just want to ignore the comment rather >>> than flagging an error. >>> >>> My question: >>> How can I use a token if it fits into a rule, but ignore it if it doesn't? >>> Can I make a token "optional"? >> >> You can turn start conditions on/off by having a global variable which is >> set in the parser actions. Then the first thing the lexer does is checking >> this variable, setting the correct start condition. >> >> You might check the Usenet newsgroup comp.compilers for other suggestions. >> >> Hans >> >> >> >> _______________________________________________ >> [email protected] https://lists.gnu.org/mailman/listinfo/help-bison >> _______________________________________________ [email protected] https://lists.gnu.org/mailman/listinfo/help-bison
