Re: FYI: default %printer/%destructor

[Hans Aberg]

On 24 Nov 2006, at 18:30, Joel E. Denny wrote:

> > > I still wonder if ISO EBNF is the right language.  Aren't most
> > > Lex and Yacc users more familiar with notations like "(...)*",  
> > > "(...)?",
> > > and "(...)+"?
> >
> > Yes, quite likely.  I wouldn't be a slave to ISO EBNF (particularly
> > since we're already incompatible with it :-), but it can't hurt to be
> > inspired by it.
>
> Then we need not fuss over [] being for options anymore.

Not for the sake of this standard.

But otherwise, it seems good to do what Flex is already doing. It  
might be confusing switching syntax.

> > > The argument there isn't about the choice of "/" or "#" or "()"  
> > > or "[]".
> > > It's about the choice of "!" (or "-" in the current discussion)  
> > > to mean
> > > nothing.  I prefer the empty string to mean nothing.
> >
> > OK, how about this idea?  If rules use the syntax S$A to mean that  
> > the
> > symbol S has a value that can be called $A within an action, then
> > let's use plain S to mean the symbol doesn't have a value.
>
> What about default names?  Must the user write?
>
>   grammar$grammar: rules$rules decls$decls epilogue$epilogue {
>     $grammar = new_grammar ($rule, $decls, $epilogue);
>   }
>   ;
>
> I prefer:
>
>   grammar: rules decls epilogue {
>     $grammar = new_grammar ($rule, $decls, $epilogue);
>   }
>   ;

If you think those are needed.

> Also, why $ now instead of #?  $ makes it look like it works for  
> values
> and not locations.

This is why I think OO: The variables indicated in the grammar rules  
really are parser stack locations. From them, one extracts semantic  
value, location, possibly more.

Therefore, I favor "/" or "#". I think though that the ability to put  
rule actions outside the rule will help up the readability of the  
rules. So I arrive at the following (already posted, but possibly  
lost when the mail server went down):

  expression:
      INTEGRAL_NUMBER/x                 #identity
    | '-' expression/x %prec NEGATION   #neg
    | '(' expression/x ')'              #identity
    | expression/x '+' expression/y     #add
    | expression/x '-' expression/y     #sub
    | expression/x '*' expression/y     #mul
    | expression/x '/' expression/y     #div
    | expression/x '^' expression/y     #pow
  ;

identity { $$ = $x; }
neg { $$ = -$x; }
add { $$ = $x + $y; }
sub { $$ = $x - $y; }
mul { $$ = $x * $y; }
div { $$ = $x / $y; }
pow { $$ = pow($x, $y); }

Here, identical actions need only be written once.

  Hans Aberg