Re: Proposal: Constructors, Assignment [LONG]

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

> Dmitry A. Kazakov wrote in message ...

> >function Make_Session (Count: Integer) return Internet_Session is
> >begin
> >   for Index in Integer'Range loop
> >      declare
> >         Result : return Internet_Session (Index);
> >      begin
> >         if HALT (x) then
> >            return Result;
> >         end if;
> >      end;
> >   end loop;
> >end Make_Session;

There were various rules, such as disallowing the "return object" in a
nested block.  I don't remember what they all were.  Look at the AI
and/or ARG minutes if you care.

The idea is basically just like in Pascal, where there's a special
return object, conceptually declared as a local of the function.
To specify what to return, assign into this special object.
Or, if limited, *initialize* this object.

Various syntaxes were discussed for defining the special object --
including allowing it to be implicitly declared and/or denoting it via
an attribute.

Pascal uses the name of the function to denote this special object.
That's a bad idea, because it's ambiguous with a function call
of zero parameters.

IMHO, having a special object is better than the traditional Ada way (a
return statement), and I wish the language had used this mechanism in
the first place.  This is because a return statement causes a transfer
of control.  It's not quite as bad as a goto, but it seems to me that
one should not be required to do a jump when it's not needed.  The usual
(and easiest to understand) case is when the "return" is actually at the
end of execution anyway, and no jump is needed.  When a jump-to-end *is*
needed, that case should look "special" in the code.  Combining "set
result" with "jump to end" is a bad idea.

At least, it's a bad idea if you're forced to use it.  I wouldn't mind
making "return X" be a shorthand for "<result obj> := X; return;".

Also, I find myself declaring a "Result" object anyway, quite often.
It would be convenient to get it for free.

- Bob