null declarative parts (was: Re: Elaboration order handling)

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

Jeffrey Carter <spam.jrcarter.not@spam.not.acm.org> writes:

> On 06/20/2013 08:11 AM, Robert A Duff wrote:
>>
>> P.S. I don't like your convention of putting "-- null;" in empty
>> declarative parts!  Comments should impart useful information,
>> and empty declarative parts are nothing special, and anyway,
>> I can see it's empty without a comment.
>
> The I guess you'd prefer
>
> while Function_With_Side_Effects loop
> end loop;

A loop with no statements is something special and rare; the reader's
attention should be drawn to it.  So I don't mind putting "null;".
Empty declarative parts are not special, and are common.

I wouldn't mind if Ada didn't require "null;" in that loop, but
then I'd leave a blank line to make it stand out:

    while Function_With_Side_Effects loop

    end loop;

Or I'd put a comment:

    while Function_With_Side_Effects loop
        -- Nothing to do here.
    end loop;

Or maybe even a comment explaining why there's nothing to do.

I'm not 100% opposed to functions with side effects, but they should
be rare, and the above loop code is therefore dubious.  So it should
stand out.

Certain languages like C and Pascal have an error-prone syntax:

    while (...);
               ^ There's an empty loop body there, but you can't see it!

There are (at least) two ways to fix that:  (1) Require null statements
to be explicit.  (2) Require "end loop", "end if", and so on.
Ada does both, which is a belts and suspenders solution.
I think (2) is a great idea.  As for (1), I can take it or leave it --
shrug.

On a related subject, I would eliminate the "begin" in the middle of
a procedure body, and allow mixing of declarations and statements
without using block statements.  I would ALLOW block statements,
but their syntax would be something like:

    begin
        ... -- statements and declarations
    end;

Since declarations are executable in Ada, it makes no sense to have
a strong separation between the declarations and the statements
of a procedure body (and similar constructs).  That syntax is
a vestige that was inherited from Pascal for no good reason.

> to the Ada requirement that you have to put a null statement in there? I
> like the idea that something that's empty has to have something special
> in it to show that it's deliberately empty, and think it should apply to
> declarative parts, as well.

I don't expect to convince you.  It's largely a matter of taste.
I tried to explain the reasons for my "taste" above.

You probably like the syntax "(null record)" for an empty aggregate;
I would prefer "[]" (with square brackets).  I might leave a blank,
to emphasize its emptiness -- "[ ]" -- but that's as far as I'd go.

- Bob