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Newsgroups: comp.lang.ada
Date: Tue, 29 Nov 2016 12:51:42 -0800 (PST)
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Subject: Re: Ada 2012 Constraints (WRT an Ada IR)
From: Shark8 <onewingedshark@gmail.com>
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Xref: news.eternal-september.org comp.lang.ada:32518
Date: 2016-11-29T12:51:42-08:00
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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, I'm talking about how to map underlying ideas uniformly -- IOW, this topic is *NOT* about "how do I do _X_ in Ada?" but rather "How is Ada doing X best [uniformly] represented?".

> 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_.