Re: Is this expected behavior or not

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

"Anh Vo" <anhvofrcaus@gmail.com> wrote in message 
news:ec2783d9-1ae6-419e-97ad-12cb9e6c3d74@googlegroups.com...
>On Monday, March 11, 2013 1:01:04 PM UTC-7, Robert A Duff wrote:
>>> Anh Vo <anhvofrcaus@gmail.com> writes:
>>> I have read ARM section 3.2.4, Subtype Predicates, I did not see any
>>> rules prohibiting from using attribute 'Succ as contained in the code
>>> nipet below. I would expect Prime'Succ (3) return 5 for next prime
>>> number. However, 4 is returned instead. Is this a correct behavior?
>
>>It is standard-conforming behavior. Prime'Succ is the same as 
>>Prime'Base'Succ.
>>Many other attributes behave the same way. This has been true since Ada 
>>83.
>>For example, Prime'Succ(1000) = 1001, and does not raise an exception.
>>Positive'Image (-10) = "-10".
>
>Thank you for your clarification. My next question is that should ARM 
>mention
>some thing of that effect regarding subtype predicates.

It seems unnecessary, because the behavior is the same as that for 
constraints. Usually, we only talk specifically about predicates when their 
behavior is different than constraints -- otherwise we'd be repeating dozens 
of "obvious" notes. And of course the Standard is not a tutorial for the 
language.

One could argue that 'Pred and 'Succ should have been illegal (like 'First 
and 'Last are), but it's too late to do that. The reason that 'First and 
'Last are illegal is because of potential confusion about the meaning. The 
same is true here - but the effect is less (it's the same as constraints), 
so its a marginal case in any event.

                                            Randy.

P.S. I'm surprised anyone uses those attributes: I almost never do. Now, if 
we had implemented the procedure version that we talked about briefly, that 
would have gotten plenty of use.