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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: GNAT vs UTF-8 source file names
Date: Fri, 07 Jul 2017 12:01:19 +0100
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Date: 2017-07-07T12:01:19+01:00
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"J-P. Rosen" <rosen@adalog.fr> writes:

> Le 06/07/2017 à 20:43, Simon Wright a écrit :
>>> Even if you use Latin-1, the set of allowed characters is defined as
>>> those that belong to NFKC.
>> I don't understand.
>> 
>> If your source has no BOM and you don't say -gnatW8, GNAT expects
>> Latin-1 encoding. If your source has a BOM or you say -gnatW8, GNAT
>> expects UTF8 encoding (I haven't tried what happens if you use NFD).
>> 
>> I haven't tried giving UTF8 coding without BOM or -gnatW8 - ignoring the
>> use in unit names - ARM 2.1(16) says it should be accepted.
>> 
>> (later) UTF8 is accepted in strings but not in identifiers.
>
> This is a common confusion between characters, coded sets, and encodings...
>
> ISO-10646 defines a coded set (code points) for a number of characters
> (identical to the one defined by Unicode). Some of these characters can
> be represented in NFKC. These are the allowed characters.
>
> If you use Latin-1, you have different code points for the same
> characters - and the allowed characters are still those representable in
> NFKC, even with different code points.
>
> UTF8 is an encoding, nothing more than a compression algorithm for
> numerical values. It is generally used to compress Unicode strings, but
> could be used for any numerical values. In any case, it doesn't change
> logical values, just the way they are stored.

I think this is a response to my "I don't understand" - I think I do
understand a little better now, thank you.

The rest is about GNAT's behaviour; to reiterate, ARM 2.1(16/3) says

   "An Ada implementation shall accept Ada source code in UTF-8
   encoding, with or without a BOM (see A.4.11), where every character
   is represented by its code point."

which for GNAT is not met unless either there is a BOM or -gnatW8 is
used.

On the other hand, ARM 2.1(4/3) says "The coded representation for
characters is implementation defined", which seems to conflict with (16)
- but then, the AARM ramification (4.b/2) notes that the rule doesn't
have much force!