Re: overriding in private part

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

On Sun, 05 Oct 2008 15:57:48 -0400, Robert A Duff wrote:

> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
> 
>> On Sat, 04 Oct 2008 15:47:04 -0400, Robert A Duff wrote:
>>
>>> If you use a compiler that warns on missing "overriding", then you don't
>>> need to say "not overriding", because that's the default -- any
>>> subprogram that doesn't say "overriding" is not overriding.
>>> Saying "not overriding" is just noise.
>>
>> [...] 
>>
>>> I agree that if you want an indicator on every subprogram declaration,
>>> then forbidding "no overriding" on some non-overriding subprograms is a
>>> language design mistake.  But I don't want that -- I want a safe
>>> (non-overriding) default.
>>
>> But non-overriding is unsafe. Taking your example with in and in-out, when
>> the programmer uses in instead of in-out, that does not change the program
>> semantics, so long the program remains legal. Otherwise (if the body
>> actually changes the parameter) it will not compile. This is safe.
> 
> Good point, but I'm only half convinced.  If you get in the habit of
> always saying "overriding" when appropriate, and you use the warnings,
> then you're unlikely to get into trouble.  And the idea of putting "not
> overriding" all over the place seems awfully verbose, to me.

Both are unsafe. A felt verbosity comes from the strange decision to put
[not] overriding in front of the declaration. If it were:

   procedure Foo (X : Boo) is not overriding;

it would not be so offending.

As for me, I think that overriding could be a good default for all
subprograms with at least one controlling argument. Declarations of any new
primitive operation should then be explicit:

   procedure Foo (X : Boo) is [abstract] new; -- I hate "overriding"

Non-primitive operations should be made illegal if any of the arguments is
controlling:

   type T is tagged ...;
   procedure Foo (X : T) is new;

   package Bar is
      procedure Foo (X : T); -- You cannot do this!
      procedure Baz (X : T); -- Neither this!
      procedure Baz (X : T'Class); -- This is OK
   end Bar;

Maybe, that could be relaxed the bodies of the packages which
specifications declare the type:

   package A is
      type X is tagged ...;
   end A;

   package body A is
      procedure Some_Private_Stuff_Without_Redispatch (X : T) is not new;
   end A;

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de