RFC: explicit empty right-hand side of a rule

[Akim Demaille]

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!