Re: Proposal: Constructors, Assignment [LONG]

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

Robert A Duff wrote:

>> 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.

We will have numerous problems with combining variable-sized objects and 
by-reference parameter passing. This only would add unnecessary complexity 
to the language. It also would raise a question, why OUT parameters of 
procedures and entry points cannot act this way.

> To specify what to return, assign into this special object.
> Or, if limited, *initialize* this object.

The problem is to ensure that this would happen exactly once, for *some* of 
result types.

> 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.

But exactly this ensures that you cannot do it twice.

> 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.

I disagree. IMO it is the basic idea of a subroutine having a result. If you 
want to separate "set" and "return", there are procedures for this.

> 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;".

But in a comparable case:

"X := Y;" is a shorthand for "Finalize (X); Copy_constructor (X, Y);"

the decision made is directly opposite!

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

-- 
Regards,
Dmitry A. Kazakov
www.dmitry-kazakov.de