Re: Ada 2012 Constraints (WRT an Ada IR)

[Shark8 <onewingedshark@gmail.com>]

On Tuesday, November 29, 2016 at 2:06:21 PM UTC-7, Dmitry A. Kazakov wrote:
> On 2016-11-29 21:51, Shark8 wrote:
> > On Tuesday, November 29, 2016 at 1:44:43 PM UTC-7, Dmitry A. Kazakov wrote:
> >> On 2016-11-29 21:32, Shark8 wrote:
> >>
> >>> The above has a constraint -- that of only holding digits -- but
> >>> still  is unconstrained. If we were doing a formal CS class we'd say something
> >>> like "The type Serial_Number contains all strings that have only the
> >>> decimal digits therein."
> >>
> >> You want convariance, that is a completely different beast.
> >
> > No, I really don't.
> > I'm not talking about Ada-syntax,
> 
> Yes you do, covariance vs. contravariance is semantics, not syntax.

Maybe I'm misunderstanding.
I simply don't see how covariance vs. contravariance is at all germane to representing the definition of a type.

> >> It is a new
> >> type with some operations overridden, e.g. the operation that yields a
> >> value of a discrete type for case statement. It is an implicit operation
> >> in Ada, yet it is an operation. It must be covariant, i.e. returning the
> >> new type rather than the old one for the case be satisfied with the new
> >> set of values. Same with already mentioned 'First, 'Last, 'Range. Or,
> >> for that matter, 'Succ and 'Pred if the new type has a non-contiguous
> >> values set, e.g. Odd_Number. You cannot get there by merely putting some
> >> predicate on the assignment operation (which Ada's predicates do). That
> >> model is too weak to express such things.
> >
> > This is not about defining a type in Ada, it's about abstracting the
> > making of a type [in Ada] and how best to represent _that_.
> 
> What is abstracting making the type?

Perhaps I should have used the word "defining" or "representing" instead of "making".

> In order to have a type you must 
> define it = define the set of values, the set of operations, the 
> semantics of the operations and provide an implementation of these 
> (values representation, bodies of the operations)

Agreed, but restating the motivation of the question isn't often helpful for answering the question. -- Rather like if I were to wake at three in the morning, ask myself "what should I do until sunrise?" and answer with "In a few more hours, the sun will come up."... I mean, it's completely a true statement... but useless for answering the question of what to do until then.