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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Ada 2012 Constraints (WRT an Ada IR)
Date: Tue, 29 Nov 2016 21:44:39 +0100
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Date: 2016-11-29T21:44:39+01:00
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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. 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.

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