Re: orthogonal inheritance and extension aggregates

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

On Sat, 20 Jul 2013 00:49:41 -0500, Randy Brukardt wrote:

> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
> news:igo2b70i4ox.t5duix9weguc$.dlg@40tude.net...
> ...
>> P.S. I don't know if Ada 2012 silently inherits from null extension. I
>> heard that there was an intention to add the kludge. Though semantically
>> broken it would save programs like yours.
> 
> That's actually an Ada 2005 feature - see 3.9.3(4/2). One that I was against 
> because it introduces a maintenance problem (null extensions tend to turn 
> into non-null extensions over time, and once that happens, the feature stops 
> working).

Right.

> It appears that GNAT failed to properly implement this feature in the case 
> described by the OP.

But the extension is null only *privately*. How can a public declaration of
the overriding depend on private declaration? It looks totally crazy to me.

> OTOH, as soon as the OP starts adding components to the extension, they'll 
> have the problem again. (I don't think it is practical to extend the 
> predefined containers, particularly because it's impossible to derive a 
> cursor type to go with the extended container -- so you either end up with 
> broken typing or you have do everything twice. And there are a lot of 
> functions that have to be overridden.)

Maintaining parallel types hierarchies is a big type system problem, which
is unclear how to resolve.

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