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Subject: Re: Agreeing with Corentin's point re: problem with strict use of abstract characters
From: Jens Maurer (Jens.Maurer_at_[hidden])
Date: 2020-06-14 13:44:52


Supposed I'm on an EBCDIC-only system.

If I understand correctly, EBCDIC has a host of control characters
that can't be represented in Unicode (while preserving semantics);
the mapping that exists re-uses some Unicode control characters,
but with different semantics.

Suppose one of those EBCDIC control characters is mapped to \u1234.
What if \u1234 also appears as such in my source code, obviously
intending to mean the Unicode semantics?

I think I'd like to have at least the option of getting a
syntax error (I asked for a Unicode control character that
doesn't exist as such on EBCDIC), but it seems the mapping
will give me the EBCDIC control character with different
semantics. (All of this in a string literal, of course,
so it hurts when performing output.)

Hubert, is my understanding above correct?

Jens

On 14/06/2020 20.03, Hubert Tong wrote:
> On Sun, Jun 14, 2020 at 5:03 AM Corentin Jabot <corentinjabot_at_[hidden] <mailto:corentinjabot_at_[hidden]>> wrote:
>
>
>
> On Sun, 14 Jun 2020 at 08:59, Jens Maurer via SG16 <sg16_at_[hidden] <mailto:sg16_at_[hidden]>> wrote:
>
> On 11/06/2020 00.06, Hubert Tong wrote:
> > On Wed, Jun 10, 2020 at 5:39 PM Jens Maurer <Jens.Maurer_at_[hidden] <mailto:Jens.Maurer_at_[hidden]> <mailto:Jens.Maurer_at_[hidden] <mailto:Jens.Maurer_at_[hidden]>>> wrote:
> >
> >     On 10/06/2020 23.23, Hubert Tong via SG16 wrote:
> >     > I agree with Corentin's point that the strict use of abstract characters introduces problems where a coded character set contains multiple values for a single abstract character/contains characters that are canonically the same but assigned different values.
>
> To be clear, the statement I made above is an indication that I now believe that the notion of "coded character set" is necessary for making progress for our purposes.
>  
>
> >
> >     I have a hard time imagining such a thing.  Can you give an example?
> >
> > Yes, U+FA9A as described in https://en.wikipedia.org/wiki/Han_unification has this situation with U+6F22.
> > These characters are distinct as members of a coded character set, but as abstract characters, I do not believe we can easily say the same.
>
> I would expect these to be two different abstract characters in the C++ sense.
> Roughly, anything you can distinguish in the source should be a different
> "abstract character", if only for the benefit of raw string literals.
>
>
> "abstract character" is a notion for humans
>
> +1
>  
>
> and to talk about mapping from one set to another.
>
> After phase 1, C++ deals with code points such that two sequences of code points are identical if and only if they have the same values
>
> This seems to be a source of getting hung up on terminology. I think this could help: The above sentence can be read as a tautology. A "code point" (within a coded character set) is synonymous with the value component of a coded character within that coded character set. Unfortunately, "value in the UCS codespace" is chosen as the "definition" for "code point" in ISO/IEC 10646.
>  
>
>
> I don't think we should entertain any notion of "same character" in C++,
> beyond value comparisons in the execution encoding and "identity" as
> needed for "same identifier".
>
>
> We need to in/before phase 1, but I think we reached the consensus that we otherwise
> shouldn't and wouldn't
>
> To be clear, we need to make sure we are on the same page with respect to the meta (notion of) notion of "same character":
> By "character", do we mean an "abstract character" or a "coded character"?
>  
>
>  
>
>
> For example, if some hypothetical input format differentiates red and
> green letters that are otherwise "the same", I'd still expect a red A
> to be a different abstract character than a green A.  (Ok, that doesn't
> work for the basic source character set, but should work for anything
> beyond that.)
>
>
> It doesn't work as there isn't any culture on earth that make that distinction such that there exist no universal-character-name to make that distinction.
> It is best left to people of letter to decide whether colors carry meaning (and they sometimes do https://en.wikipedia.org/wiki/Ersu_Shaba_script) 
>
> If that means the term "character" or "abstract character" is too loaded
> to be used here, so be it.  (The terminology space is already fairly
> crowded due to Unicode, so it's hard to find unused phrases that give the
> right connotation.)
>
>
> The terminology used by Unicode people isn't Unicode specific. In particular,  "abstract character" is meaningful independently of
> any computer system. 
>
> I think that the relationships between terms represent an ideal that is not met in practice. "Abstract character" is a meaningful notion; however, the ideal that coded character sets are a bijective function between values in a codespace and abstract characters has not been clearly attained.


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