Re: proposal: simplify prologue alternatives into %code

[Hans Aberg]

On 9 Jan 2007, at 12:21, Paolo Bonzini wrote:

> > I agree in that. Though important to get a nice syntax, getting  
> > the other stuff together is more important. :-)
> > In order to get the C++ class polymorphy working with the Bison  
> > static type system, would need an additional feature, and I am not  
> > sure exactly how it relates to %define and %code, though some  
> > additional macro is needed:
>
> You can set up the skeleton like this:
>
> %define "cast" "dynamic_cast"
> %define "cast" "static_cast"
> %define "cast" "" (or default) -> union

Or perhaps a Bison command %cast, like:

%cast(type, variable) {dynamic_cast<type>(*(variable).data())}

Then a special directive %typed triggering the Bison type system may  
not be required, as this if %cast appears, one knows the .y file is  
typed. Another way would that %cast can appear, but is not used,  
unless a %typed also appears. This way, one can put in in, just case.

The implementation of %typed would be the same as %union, only that  
the semantic type isn't defined to anything. I think it would only  
trigger a flag in Bison, saying that typing is used.

This would provide for different solutions.

> > In the C++ class polymorphy variations, I had to write something like
> >   dynamic_cast<type>(*$k.data())
> >   static_cast<type>(*$k.data())
>
> ... if you are ok with
>
> dynamic_cast<type>(*$k)
>
> that is, instead of
>
> struct container {
>   top_class *member;
>   top_class *data () { return member; }
> }
>
> you have to write this:
>
> struct container {
>   top_class *member;
>   top_class &operator *() { return *member; }
> }

So then one can play around with different solutions, and need to  
rely in any particular syntax.

  Hans Aberg