Re: some trivial questions?

["Randy Brukardt" <randy@rrsoftware.com>]

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
news:ov3dah$rb$1@gioia.aioe.org...
> On 22/11/2017 01:56, Randy Brukardt wrote:
>> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
>> news:ov0v0f$663$1@gioia.aioe.org...
>>> On 2017-11-20 23:03, Randy Brukardt wrote:
>> ...
>>>>> Sure, nothing useful going to happen for Ada.
>>>>
>>>> I agree that this is annoying. The problem is that an incompatible Ada
>>>> would
>>>> be starting over from scratch (as existing libraries would not be
>>>> usable),
>>>
>>> There are lots of changes which can be made while keeping everything
>>> compatibility. The problem is with small increments. That does not work.
>>> If we want to do something real this must be done in one large jump. 
>>> This
>>> worked for Ada 95 once, it can work again.
>>
>> The problem with this sort of thing is *proving* that the changes are in
>> fact compatible. In the absence of that, the forces of FUD would have a
>> field day, and it would be very hard to convince people that there was no
>> problem.
>
> The idea which is floating around for a long while is to introduce a more 
> general type system in which present standard would be expressed as-is 
> without any changes. Compatibility problems arise when doing incremental 
> changes at the same abstraction level, Once you move up or down there is 
> no problem anymore.
>
>> That has plagued every Ada update since Ada 95. (Did you know that Ada 95
>> was supposed to have a mechanism where operators were always visible? 
>> That
>> got killed off in favor of "use type" in part because the Ada 9x team
>> couldn't convince the MRT that there wasn't any possible 
>> incompatibility.)
>
> That is exactly the solution like above but at a narrower scale - you can 
> have operators either visible or not. The method was actually not to 
> decide that and simply implement both variants. The first variant was made 
> default the second received the syntax "use type".
>
> At a larger scale the problem is that the language may fall apart if we 
> keep on adding arbitrary syntax constructs like "use type". This is why it 
> must be one major change.

I agree with your points, but how do you prove compatibility of whatever 
model is adopted? Given that Ada is currently defined with English wording, 
it is very hard to prove if any change is compatible other than the most 
trivial ones. When you change a lot of that English text (as would be 
necessary if a new underlying abstraction was adopted), how do you convince 
skeptics that the results are the same (or at least very close to the same)? 
That's been the problem for any radical changes in the type system or in 
resolution.

                                         Randy.