Re: Feature request

[Fernando Ferreira]

Evan Lavelle wrote:
> Fernando Ferreira wrote:
>
> > The reason for my request is that, as bison generates LR parsers, 
> > there is no way to, for instance, execute an action before a rule is 
> > reduced, only after.
>
> I've never tried an action at the start of a rule, but I'm pretty sure 
> that you can do it. Look up 'embedded actions' in the O'Reilly 
> lex&yacc book, or 3.5.5 'Actions in mid-rule' in the Bison 2.0 manual. 
> It may be difficult to avoid conflicts, though. According to the 
> O'Reilly book, yacc implements embedded actions using the technique 
> described at the end of 3.5.5 ("Another solution is to bury the action 
> inside a nonterminal symbol which serves as a subroutine...").
>
> Evan
>
>
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But that would mean that I would have to add the same action for every 
rule. My intention is to have a way to execute an single action before 
any and every rule gets executed, similarly to YY_USER_ACTION on Flex.

An example would be to print the name of the rule that is about to be 
reduced, using something like yy_reduce_print_ uses, "cout<< 
yytname_[yyr2_[yyn]]". That's a silly example, there are other truly 
marvelous uses for this feature which this margin is too narrow to 
contain (hehe).