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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Bug in Ada - Latin 1 is not a subset of UTF-8
Date: Tue, 18 Oct 2016 09:41:39 +0200
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Date: 2016-10-18T09:41:39+02:00
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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. String of numeric characters is not an encoding, it is a 
constraint = (def) each instance of numeric string is a string. [An 
example of encoding (= representation) is IEEE 754 vs IBM 360 float.]

UTF-8 string is not a constrained string and conversely string is not a 
constrained UTF-8 string.

These are two distinct types which values (some of them) overlap and can 
be converted into each other. The latter allows making them subtypes, 
but Ada language lacks means for that.

In Ada a subtype can either be a constraint (AKA "Ada subtype") or class 
member / class-wide. UTF-8 is not a constraint and String is not tagged.

The decision to force UTF-8 string and string [Latin-1 string to be 
precise] to be subtypes in Ada sense is the least of two evils. It is 
bad and wrong, but the alternative would be only worse.

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