Re: no $$ in %destructor = no unset value warnings?

[Joel E. Denny]

On Thu, 14 Sep 2006, Joel E. Denny wrote:

> On Thu, 14 Sep 2006, Akim Demaille wrote:
> 
> > It is inconsistent, you are right, but I would have chosen the other
> > possibility: just like we invite users to YYUSE (or whatever the
> > macro) in rule actions, we should do that in destructors and printers.
> > Not because it is always wrong not to use the value, but because it's
> > always possibly an error.

I'm thinking of implementing this warning for no $$ in %destructor or 
%printer.

> I was thinking of a user-defined YYSTYPE (no %union) with no <...> usage.  
> In this case, Bison would have no idea what symbols truly have semantic 
> values.  So, I think we need the ability to remove the destructor for 
> specific symbols.  There might even be cases where nearly all symbols of a 
> particular type need a destructor, but a few don't.  Finally, I still like 
> the idea of a %destructor that doesn't access $$ but perhaps just prints a 
> message, and I don't think Bison should warn about rules that don't use 
> such a $$/$n.

I still agree with the above.

> You once discussed the possibility of named semantic values in grammar 
> rules:
> 
>   exp(sum): exp(term1) '+' exp(term2) {
>     $sum = $term1 + $term2
>   }
> 
> And instead of USE($1), you suggested empty parens, for which I still 
> prefer (!).  I think empty parens make a nice default value name:
> 
>   grammar(): defs() rules() epilogue(!) {
>     $grammar = new_grammar( $defs, $rules );
>   }

I don't plan on implementing the above any time soon...
 
> Maybe something similar is useful with %destructor/%printer:
> 
>   %destructor(!) { printf ("A SYM was discarded.\n"); } SYM
> 
> Because the user has explicitly declared that SYM does not need to destroy 
> its value, Bison would not warn about unused SYM values in %destructor or 
> semantic actions.

... but I'm thinking about implementing this now as a solution to the 
issues I discussed at the top of this email.  Before I get too far into 
this, what do people think?