Re: too many warnings from Bison CVS for Pike

[Hans Aberg]

On 29 Jan 2006, at 20:11, Joel E. Denny wrote:

> > Now, with a code placement in hand, one wants to be able to put  
> > code at
> > various place, including just ahead of the switch statement; call  
> > the macro
> > say 'pre-switch'. Then a fix under C for those that want $$ = $1  
> > would a
> > grammar '.y' file containing:
> >   %define pre-switch { if (yylen >= 1)  $$ = $1; }
>
> This isn't yacc compatible.

That depends on the interpretation of the Yacc specification.

> If the Open Group says we need implicit $$ =
> $1 for explicit actions, bison needs to support it for pure yacc
> specifications.

Sure, but there has not been any clarification there yet.

> Maybe there's still the question of what exactly they mean by:
>
>   By default, the value of a rule is the value of the first element  
> in it.
>
> I feel pretty sure that Paul is right: `By default' does *not* mean  
> `When
> the user doesn't specify the action'; it means `When the user doesn't
> specify the value of the rule'.  In other words, in all cases, $1  
> is the
> default value of $$.  So, this is a yacc issue.

The wording is unclear to me. It needs to be clarified. (And the Yacc  
spec needs to tell that the default value is only valid for rules of  
length >= 1, and what should happen when the rule length is 0.)

In the case of C, one can do both: always do $$ = $1, or just do it  
in the case no explicit rule action has been in the grammar. For C++,  
one should only do the latter. The POSIX specification does not deal  
much with C++ right now, but is moving towards it.

If I look for other uses of the word "default" and the wording "by  
default", it means "in the case no other explicit directions has been  
given". So "By default, the value of a rule shall be the value of the  
first element in it." means that this is the value if no other  
explicit value has been given, and if such an explicit value has been  
given, the default rules does not apply.

This is still unclear, because one can write a rule action explicitly  
as {}. Should then $$ = $1 be applied? - There is a difference  
between explicitly indicating a rule action, and explicitly assigning  
a value to $$.

  Hans Aberg