Re: FYI: default %printer/%destructor

[Paolo Bonzini]

[Joel, I added some clarifications at the bottom]
>   grammar(): defs() rules() epilogue(!) {
>     $grammar = new_grammar ($defs, $rules);
>   }
>   ;
Here you won't break grammar source compatibility by omitting the () 
altogether.
>   %destructor(!) { printf ("A symbol was discarded.\n"); } <!>
>
> Here, symbols with no type tags have no values but still have a 
> %destructor.
>
> Of course, if no symbols in your grammar have type tags, or if you 
> plan to use $<tag>$ extensively for untagged symbols, it might be 
> reasonable to have <!> without (!) in a %destructor.
>   
I still don't see much similarity with (!) and Then, why not having

%destructor BLOCK

implement a <*> destruction, and something like

%destructor(!) BLOCK
%destructor BLOCK %pragma(unused-value)

implement a <!> destruction?  Going for the latter, of course, would 
imply the possibility to do

%destructor BLOCK %pragma(unused-value) <foo>

even if foo is not untagged.

For now, this would mean having only the semantics of <*> available.  
But besides debugging code, why would <!> functionality be useful?

Paolo