Re: Problems with 'class, help anyone?

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

Alexandre E. Kopilovitch wrote:

>>> There is another way, with perhaps even less incompatibility: let's
>>> introduce third mode for a formal parameter - "return" mode (in addition
>>> to conventional "in" and "out"). That "return" may be applied to the
>>> procedure's first parameter only, and it means the same as "out" mode,
>>> with single exception: if the first parameter of a procedure is in
>>> "return" mode then the procedure *may* be called without the
>>> corresponding (first) argument, and in such a case it returns value. For
>>> example:
>>> 
>>> procedure Two_Fold(Result : return Boolean; Something : in Integer);
>>> 
>>> X : Boolean;
>>> Two_Fold(X, 0);  -- "return" as "out", procedure does not return value
>>> 
>>> if Two_Fold(0) then  -- first argument omitted, procedure returns value
>>>   ...
>>> end if;
>>
>>But an out parameter and a return value are not the same, i.e. they cannot
>>generally be interchanged!
>>
>>Consider e.g.
>>     function F return String;
>>vs.
>>     procedure P (S: out String);
>>The former has no idea how long the object is that will get the result (if
>>any), the latter takes the length from the parameter.
> 
> Well, I was not writing an AI (perhaps, I should?), therefore I did not
> look at the details. Surely, you are right - there must be a limitation
> for a parameter in "return" mode: if one is of indefinite type then the
> currently undefined attributes (Length, First, etc.) cannot be used for
> that parameter.

If the goal of your proposal is to expose the result as a variable within 
the body, then some equilibristic is indeed required. For instance, you 
could require that such a parameter has to be overridden within the body:

procedure Read (Item : result String) is
   Length : Natural;
begin
   Get (Length); -- Get the actual string length
   declare
      Text : String (1..Length) renames Item; -- Override
   begin
      Get (Text);
   end;
end Read;

Honestly, I do not like it. Further, if you go this way, then you should 
consequently say, OK, let's allow multiple-return functions capable to 
return more than one potentially unlimited result. Maybe then it will worth 
the efforts.

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