Re: Use of declare blocks

["Randy Brukardt" <randy@rrsoftware.com>]

"Niklas Holsti" <niklas.holsti@nospam.please> wrote in message
news:460198f2$0$24601$39db0f71@news.song.fi...
> Randy Brukardt wrote:
>
> > I use declare blocks extensively to reduce the scope of temporary
variables
> > in subprograms. They're especially important when the object exists just
to
> > take an unused out parameter, because they greatly reduce the need to
come
> > up with names for the objects. I often have code like:
> >
> >     declare
> >             Junk : Index;
> >     begin
> >             Insert_Something (..., Result => Junk);
> >     end;
> >
> > where the result isn't needed by the following code.
>
> How about a language extension to omit unused out parameters, for example
>
>     Insert_Something (..., Result => <>);
>
> Perhaps even allowing declaration of optional out parameters, as in
>
>     procedure Insert_Something (..., Result : out Index := <>);
>
> and then a call of Insert_Something could omit Result entirely.

I think both of these would be a tough sell. I'm sure some people would feel
that having an out parameter that you don't need is a design flaw, and we
shouldn't be encouraging design flaws. They come up a lot of foreign library
interfaces, and I suppose the argument would be that they should be wrapped
and hidden (even if that is too expensive for the particular usage).

After all, once of the good things about Ada is that you have to do
something explicit to ignore returned values (including error returns). If
they're mapped as out parameters, you have to write code to discard them
(like the above). If they're mapped as exceptions, you have to handle and
ignore (with a null handler) the exception (or all exceptions). Both of
these are obvious when reading the code. It's a lot harder to detect an
error of omission than an error of commission.

This is similar in some ways to the arguments against out parameters in
functions, where the possibility of unnoticed order dependencies and tricky
expressions makes some people uncomfortable.

                           Randy.