Re: Your wish list for Ada 202X

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

On Mon, 14 Apr 2014 18:39:24 -0500, Randy Brukardt wrote:

> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
> news:9cm2e094hvj7.sj0t2sh2komn.dlg@40tude.net...
>> On Fri, 11 Apr 2014 16:44:13 -0500, Randy Brukardt wrote:
>>
>>> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
>>> news:16388p09ph28u$.1mglp0rm7pli9$.dlg@40tude.net...
>>
>>>> Your example refers to overloading possible for operations on unrelated
>>>> types. I don't see how this is scenario could be possible for a 
>>>> primitive
>>>> MD operation. One of the requirements put on MD is that *all* 
>>>> combinations
>>>> of tags up to the root of the inheritance tree be defined and checked at
>>>> compile time. This requirement automatically precludes appearance of C 
>>>> from
>>>> air. It cannot be primitive and thus the preference rules would not 
>>>> apply
>>>> to it.
>>>
>>> Doesn't matter. First, you'd have to prevent adding or removing new
>>> primitive operations to a hierarchy. That seems like a nasty limitation.
>>
>> What? This limitation is with us since Ada 95! You cannot add a primitive
>> operation after the freezing point in Ada. You cannot remove a primitive
>> operation at all.
> 
> This is an issue with program maintenance, not with an unchanging 
> declaration -- so freezing is completely irrelevant. If someone decides 
> there is a need to add a primitive operation to the root (for instance), 
> then a preference rule towrd the root could potentially silently change the 
> behavior of a otherwise tested/proved program. That's the sort of thing that 
> needs to be prevented, lest maintenance become too dangerous to allow in 
> existing systems.

It is not prevented now and cannot be prevented in the future. If you
change the package specification, which is the only way to add an operation
you potentially change anything. This requires recompilation of all
clients, which is OK, as you said earlier. Thus the problem does not exist.

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