Re: Possible Ada deficiency? (goto)

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Peter Hermann <ica2ph@sinus.csv.ica.uni-stuttgart.de> writes:

> Robert A Duff <bobduff@shell01.theworld.com> wrote:
> > The main thing I'm thinking about is that goto labels are implicitly
> > declared.  The Pascal rule requires an explicit label declaration.
> > That makes the code more readable, because you can see up front
> > that "gotos lurk below", which is a useful thing to know.
> 
> maybe
> 
> > Even if you think the extra syntactic overhead is annoying, it can't be
> > *very* annoying, because people don't write a lot of gotos.
> 
> yes. I was astonished after changing from Fortran to Ada
> when I detected to never need any goto.
> There are rare cases where a goto is useful.
> (I am assuming you do not ban goto at all)

True.

> E.g. automatic translation from Fortran spaghetti into Ada.
> The increase of workload of the translator is neglectable
> (when translator=machine, annoying when man)
> but the resulting code would be cluttered with label declarations,
> which IMHO are of no added value.

Well, if I were writing a Fortran-to-Ada translator, it would be smart
enough to generate nearly-goto-free Ada.  At least, if the Ada code is
intended to be read by humans.

> > Also, this rule makes the language a little bit harder to learn, because
> 
> no
> 
> > goto labels don't behave like most everything else in the language,
> > which *do* have to be explicitly declared.
> > 
> > How many Ada programmers know that the following silly example is
> > illegal:
> > 
> >     procedure P is
> >     begin
> >         for Mumble in 1..10 loop
> 
> missing statement in loop

No.  There's a null statement.

> >             <<Mumble>> -- Illegal!
> >             null;
> >         end loop;
> >     end P;

    procedure P is
        --(1)
    begin
        for Mumble in 1..10 loop
            <<Mumble>> --(2)
            null;
        end loop;
    end P;

Here's why the above is illegal:  There is an implicit declaration of
label Mumble at that place marked "--(1)".  At the place marked "--(2)",
this this declaration is not directly visible, because it is hidden by
the loop parameter Mumble.  Thus, the reference to Mumble at "--(2)"
is illegal.  That's just plain weird.

> > but the following equally silly example is legal:
> > 
> >     procedure P is
> >     begin
> >         <<Mumble>>
> 
> why not allow a practical "start over"?
> Although my personal taste is to avoid goto_s
> I welcome this syntactical feature for the comfort of the programer
> in the sense of "let_it_be".
> 
> >         for Mumble in 1..10 loop
> >             null;
> >         end loop;
> >     end P;
> > 
> > and can explain why?
> > 
> > Compiler writers have to work hard to get those little details exactly
> 
> once done, benefit thousandfold.

Thousand times zero = zero.

> > right, and it's of zero benefit to programmers.

> Returning to my precious and almost never used goto :
> It is carved in stone and I like it the way it is :-)

Yes, it's carved in stone.

> > If my passengers bought tickets from New York to Boston, I don't like to
> > have a rule saying I have to drive the bus via Los Angeles!  ;-)
>                     "I"   (-: not "they" :-)
> 
> anyway: thank you for the good laugh which brightens the day.

:-)

- Bob