Re: How to ignore a token if it doesn't fit in with a rule?

[Hans Aberg]

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 <address@hidden> 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
>> 
>> 
>> 
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