Re: FYI: default %printer/%destructor

[Joel E. Denny]

On Wed, 25 Oct 2006, Paolo Bonzini wrote:

> >   grammar(): defs() rules() epilogue(!) {
> >     $grammar = new_grammar ($defs, $rules);
> >   }
> >   ;
> Here you won't break grammar source compatibility by omitting the ()
> altogether.

If Bison were to complain about value name conflicts (and I think it 
should), you would have trouble with:

  exp: exp '+' exp

> >   %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 (!)

<!> = no tag.  (!) = no value.  Both cases warrant caution.  Those are the 
similarities.  Maybe I don't understand what you're saying.

> and Then, why not having
> 
> %destructor BLOCK
> 
> implement a <*> destruction, and something like

At one time I implemented the empty symbol list notation to indicate a 
default destructor.  Akim and I agreed that was ugly.  To me, it reads 
like a syntactic mistake.  Moreover, one might want the <*> and <!> 
destructor to be the same, and it's nice not to have to repeat code:

  %destructor(!) { printf ("A symbol was discarded.\n"); } <!> <*>

> %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.

It doesn't make sense to me that (!) should mean "no value" in rules but 
"no type tag" in %destructor.  Maybe I don't understand what you're 
saying.

> For now, this would mean having only the semantics of <*> available.

I'm a bit lost.  I thought you just proposed an empty symbol list with (!) 
as a replacement for <!>.  Did I misunderstand?

> But besides debugging code, why would <!> functionality be useful?

You might use @$ in the %destructor to report the locations of discarded 
symbols to the user.

More importantly, if you don't use a union, you might do something like 
this:

  #define YYSTYPE struct node*
  %destructor { node_free ($$); } <!>