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From: "G.B." <bauhaus@futureapps.invalid>
Newsgroups: comp.lang.ada
Subject: Re: Bug in Ada - Latin 1 is not a subset of UTF-8
Date: Tue, 18 Oct 2016 12:09:11 +0200
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Xref: news.eternal-september.org comp.lang.ada:32113
Date: 2016-10-18T12:09:11+02:00
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On 18.10.16 10:45, Dmitry A. Kazakov wrote:
> On 18/10/2016 10:23, G.B. wrote:
>> On 18.10.16 09:41, Dmitry A. Kazakov wrote:
>>> On 18/10/2016 01:25, G.B. wrote:
>>>> On 17.10.16 22:18, Lucretia wrote:
>>>
>>>> According to ISO 10646, UTF stands for UCS Transformation
>>>> Format. So, it's a format, suggesting a representation.
>>>>
>>>> On similar grounds, one could define a string subtype for
>>>> other types of objects, for example
>>>>
>>>>   subtype Number_String is String;
>>>
>>> You are wrong.
>>
>> The constraints on either UTF_String or or Number_String are
>> not expressible as simple Ada subtypes. They are given by
>> description and normative reference, respectively.
>
> In the case of UTF-8 it is not a constraint.

Not an Ada constraint, in particular insofar as UTF-8 means
a representation;
still, any UTF-8 encoded "string" of UCS objects is wellformed
and it satisfies a predicate that involves all components x, x', x'', ...
of a UTF_8_String object, by stating that if x matches 2#10......#,
then x' is such-and-such, and so on. I'm not sure this predicate
is easily stated as a stand-alone type invariant, for example, but
that's the idea. It shouldn't have to be visible to Ada programmers.

>
> Numeric character is a constraint expressible in Ada:
>
>    subtype Numeric is Character range '0'..'9';
>
> Numeric string constraint is not expressible, but it still a constraint.

(Although, the Numeric_String subtype described earlier will have
a meaningless constraint on Numeric, since all remainders
are values both in base 256 and in Character. Come to think of it,
the example format is broken. #-)



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