Re: too many warnings from Bison CVS for Pike

[Joel E. Denny]

On Sun, 29 Jan 2006, Hans Aberg wrote:

> This way of behaving, with the $$ = $1 always executed seems to have existed
> up to the point I discovered the C++ problem. So probably, many C programs
> rely on it. The problem is that it does not harm under C (except for some
> time), but indeed may harm under C++, and it may be confusing to have
> radically different behavior under C/C++. And it seems strange that a default
> action should be executed when an explicit action has been given.

I agree.

> > In the long run I think Bison needs different flavors of warning
> > options, for users who want to be more or less picky.
> 
> My guess is that perhaps some option is needed. It is perhaps though difficult
> to motivate to C users to not have $$ = $1 always executed, if it does not
> cause any harm in doing it.

Beyond the C++ issue you raise, it seems that an action containing nothing 
but $$ = $2 ought to warn about an unused (typed) $1.

Explicit code is just cleaner.  I doubt seriously that any user is going 
to abandon bison just because he can't bear to add $$ = $1 to his actions.  
Bison will even tell him exactly where to do so.  If he complains that 
he's unsure as to where the $$ = $1 was actually intended, that's the very 
reason he should have made it explicit in the first place.

Couldn't this be a --yacc issue?  That is, --yacc would implement whatever 
the Open Group says.  Without it, bison would be free to behave well and 
consistently for both C and C++: don't execute $$ = $1 before an explicit 
action.

If we're worried that some users won't want --yacc but will still want an 
implicit $$ = $1 for explicit actions, --yacc could accept options to 
specify exactly which yacc features should be implemented.  For example:
  
  --yacc=auto_lhs_value,filenames
  --yacc=filenames
  --yacc

where the last usage implies all yacc features.  This approach makes the 
subtle hint that implicit $$ = $1 before explicit actions is simply an old 
and unfortunate idea.

Joel