Re: RFC: explicit empty right-hand side of a rule

[Akim Demaille]

Sorry, I used an obsolete address for Joel.

Le 30 janv. 2013 à 14:40, Akim Demaille <address@hidden> a écrit :

> Hi all, (Hi Joel, happy new year!)
> 
> We have been toying with the introduction of a means to explicitly
> mark empty right-hand sides of rules.  It's on the explicit-empty
> branch on gnu.org's repository.  It allows to write
> 
>       exp: $empty { … }
> 
> and it's an exact synonym for
> 
>       exp: /* nothing. */ { … }
> 
> except that:
> 
> - bison does check that there is just $empty and nothing else
> - it will provide a warning to report implicit empty rhs
> - the various reports (e.g., foo.output, foo.dot) will use
>  $empty (or maybe ε where possible) instead of nothing (or
>  /* empty. */ as is done in some cases).
> 
> I would be happy to receive opinions about this feature, and also
> suggestions for the name for this pseudo-symbol.
> 
> It is far from being bullet proof, for instance I'm pretty
> confident you can give this $empty a name-for-references, which
> is absurd.
> 
> There is one corner case whose treatment is currently unclear:
> mid-rule actions.  Currently
> 
>       exp: $empty { $$ = foo(); } $empty { $$ = bar(); }
> 
> is rejected, because I don't think accepting $empty in this situation
> actually helping.
> 
> /tmp/foo.y:2.6-11: error: $empty must be alone on the right-hand side of the 
> rule
> exp: $empty { $$ = foo(); } $empty { $$ = bar(); }
>      ^^^^^^
> /tmp/foo.y:2.29-34: error: $empty must be alone on the right-hand side of the 
> rule
> exp: $empty { $$ = foo(); } $empty { $$ = bar(); }
>                             ^^^^^^
> 
> Cheers!
> 
>