Re: Lua 5.4: question about label visibility rule

[Arnaud Delobelle <arnodel@...>]

On Wed, 19 Jan 2022 at 12:18, Albert Krewinkel <albert+lua@zeitkraut.de> wrote:
>
>
> Arnaud Delobelle <arnodel@gmail.com> writes:
>
> > In the Lua 5.4 docs it says the following.
> >
> > >   A label should not be declared where a label with the same
> > >   name is visible, even if this other label has been declared
> > >   in an enclosing block.
> >
> > I have found the following test in the Lua 5.4.3 Test Suite (available
> > here: http://www.lua.org/tests/), in the file goto.lua (starting line
> > 223):
> >
> > [...]
> >
> > In this function the label ::l1:: is declared twice, once in the
> > "elseif a == 4" clause and once at the end of the function.  In my
> > understanding, the second declaration of this label is visible at the
> > point where the first declaration is made.  That would contravene the
> > rule I have quoted above.
>
> I believe the cited paragraph from the docs must be read as a
> recommendation rather than a rule: the label *should not* be declared,
> but if people do, the behavior is still well-defined (and tested in the
> test suite).  RFC 2119 phrases it best:
> <https://datatracker.ietf.org/doc/html/rfc2119>
>
> > 4. SHOULD NOT   This phrase, or the phrase "NOT RECOMMENDED" mean that
> >    there may exist valid reasons in particular circumstances when the
> >    particular behavior is acceptable or even useful, but the full
> >    implications should be understood and the case carefully weighed
> >    before implementing any behavior described with this label.

Hi Albert,

It could be an interpretation but I do not think that is the case.
Take for example the following program:

----- testlabel.lua ----
do
    ::foo::
    do
        ::foo::
        goto foo
    end
end
---- end ----

If I run this with Lua 5.4.3 I get the following output:

----
lua: testlabel.lua:5: label 'foo' already defined on line 2
----

If I run it with Lua 5.3 it compiles and run (forever!).  This shows
that Lua 5.4.3 enforces the rule I quoted (to an extent) so I believe
my question remains.

Regards,

-- 
Arnaud