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Subject: Re: Agreeing with Corentin's point re: problem with strict use of abstract characters
From: Corentin Jabot (corentinjabot_at_[hidden])
Date: 2020-06-15 11:17:13


On Mon, 15 Jun 2020 at 17:49, Tom Honermann <tom_at_[hidden]> wrote:

> On 6/15/20 4:18 AM, Corentin Jabot wrote:
>
>
>
> On Mon, Jun 15, 2020, 08:40 Tom Honermann <tom_at_[hidden]> wrote:
>
>> On 6/14/20 6:53 PM, Corentin Jabot wrote:
>>
>>
>>
>> On Mon, 15 Jun 2020 at 00:36, Tom Honermann <tom_at_[hidden]> wrote:
>>
>>> On 6/14/20 6:21 PM, Hubert Tong wrote:
>>>
>>> On Sun, Jun 14, 2020 at 6:03 PM Tom Honermann via SG16 <
>>> sg16_at_[hidden]> wrote:
>>>
>>>> On 6/14/20 4:57 PM, Corentin Jabot wrote:
>>>>
>>>>
>>>>
>>>> On Sun, 14 Jun 2020 at 22:45, Jens Maurer <Jens.Maurer_at_[hidden]> wrote:
>>>>
>>>>> On 14/06/2020 22.19, Corentin Jabot wrote:
>>>>> >
>>>>> >
>>>>> > On Sun, 14 Jun 2020 at 21:55, Jens Maurer <Jens.Maurer_at_[hidden]
>>>>> <mailto:Jens.Maurer_at_[hidden]>> wrote:
>>>>>
>>>>> > No, each code point in a sequence (given Unicode input) is a
>>>>> separate abstract character
>>>>> > in my view (after combining surrogate pairs, of course).
>>>>> >
>>>>> >
>>>>> > For example diatrics, when preceded by a letter are not considered
>>>>> abstract characters of their own.
>>>>>
>>>>> "Abstract character" is defined in https://www.unicode.org/glossary/
>>>>> as follows:
>>>>>
>>>>> "A unit of information used for the organization, control, or
>>>>> representation of textual data."
>>>>> (ISO 10646 does not appear to have a definition in its clause 3.)
>>>>>
>>>>> I'm not seeing a conflict between that definition and my view that a
>>>>> diacritic,
>>>>> preceded by a letter, can be viewed as two different abstract
>>>>> characters.
>>>>> I agree that the alternate viewpoint "single abstract character" is not
>>>>> in conflict with the definition, either.
>>>>>
>>>>> What is your statement "are not considered abstract characters of
>>>>> their own"
>>>>> (which seems to leave little room for alternatives) based on?
>>>>>
>>>>
>>>> Right the glossary, is very much incomplete
>>>>
>>>> The definition is given in Unicode 13. 3.4 (
>>>> http://www.unicode.org/versions/Unicode13.0.0/ch03.pdf )
>>>>
>>>> Abstract character: A unit of information used for the organization,
>>>> control, or representation of textual data.
>>>> • When representing data, the nature of that data is generally symbolic
>>>> as
>>>> opposed to some other kind of data (for example, aural or visual).
>>>> Examples of
>>>> such symbolic data include letters, ideographs, digits, punctuation,
>>>> technical
>>>> symbols, and dingbats.
>>>> • An abstract character has no concrete form and should not be confused
>>>> with a
>>>> glyph.
>>>> • An abstract character does not necessarily correspond to what a user
>>>> thinks of
>>>> as a “character” and should not be confused with a grapheme.
>>>> • The abstract characters encoded by the Unicode Standard are known as
>>>> Unicode abstract characters.
>>>>
>>>> *• Abstract characters not directly encoded by the Unicode Standard can
>>>> often be represented by the use of combining character sequences. *
>>>>
>>>> My reading of that aligns with Jens' interpretation. An abstract
>>>> character can be composed from abstract characters. The emphasized
>>>> statement above appears to reaffirm that.
>>>>
>>>>
>>>> The definition of encoded character is also informative
>>>>
>>>> Encoded character: An association (or mapping) between an abstract
>>>> character and a code point.
>>>> • An encoded character is also referred to as a coded character.
>>>> • While an encoded character is formally defined in terms of the mapping
>>>> between an abstract character and a code point, informally it can be
>>>> thought of
>>>> as an abstract character taken together with its assigned code point.
>>>> •
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> *Occasionally, for compatibility with other standards, a single
>>>> abstract character may correspond to more than one code point—for example,
>>>> “Å” corresponds both to U+00C5 Å latin capital letter a with ring above and
>>>> to U+212B Å angstrom sign. • A single abstract character may also be
>>>> represented by a sequence of code points—for example, latin capital letter
>>>> g with acute may be represented by the sequence <U+0047 latin capital
>>>> letter g, U+0301 combining acute accent>, rather than being mapped to a
>>>> single code point.*
>>>>
>>>> Likewise here, these examples indicate that an abstract character may
>>>> have multiple encoded representations, but I don't read this as precluding
>>>> the indicated code points reflecting abstract characters on their own.
>>>>
>>> It seems that, because we are not looking at a model where we retain
>>> coded characters in their original form for as long as possible, we're
>>> dealing with certain issues in larger scopes than may be strictly
>>> necessary. Are we sure that the same text processing should occur for the
>>> entirety of the source? In other words, should we consider more
>>> context-dependent (e.g., specific to raw strings, specific to identifiers,
>>> etc.) text processing?
>>>
>>> I've been thinking along those lines as well. I've been considering a
>>> model in which an *extended-source-character* is introduced in phase 1
>>> and then, in phase 3, all *extended-source-character*s outside of raw
>>> string literals are converted to *universal-character-name*s.
>>>
>>
>> I would really love it if someone could explain to me the value of
>> introducing *universal-character-name*s (or *extended-source-character*)
>> etc in the internal representation,
>> instead of unicode codepoints, knowing that these things represent
>> unicode codepoints.
>>
>> I find the UCN mechanism to be quite elegant. It simultaneously
>> accomplishes several things:
>>
>> 1. It allows the source language, as seen by all phases of
>> translation after phase 1 (except for the magical revert for raw string
>> literals) to be completely and abstractly defined by characters defined in
>> the standard. An implementation's internal representation only needs to
>> differentiate 96 characters. I don't need to know how to type, write,
>> read, or pronounce ሴ; I just need to know what \u1234 represents
>> (abstractly; from a language perspective, I probably don't care what
>> character it denotes)
>>
>>
> There is no difference between \u1234 and U+1234
>
> I haven't understood your direction as encoding the sequence of characters
> 'U', '+', '1', '2', '3', '4' where UCNs are produced today (if that were
> your direction, then I agree that the only difference here is spelling). I
> think you have a different mental model in mind in which the internal
> representation encodes what today are UCNs in some unspecified encoding
> form that does not serialize the code point value as text. I don't see
> what that fixes from a standard perspective. I think the current design is
> more elegant because it doesn't require translating explicitly written UCNs
> to code points encoded in some unspecified internal representation.
>

My approach would be to not transform verbatim escape sequences until the
tokenization in phase 3, such that we would not have to revert that
operation for string literals
universal escape sequence can then appear
* in character literals
* non raw string literals
* pp-number, pp-identifier
* header names
* UDLs

This avoid losing the distinction in phase 1 and keep phase 1 simple

>
>
>
>> 1.
>> 2. It allows explicit encoding of Unicode code points regardless of
>> the encoding of the source input.
>>
>> I am not arguing against the presence of verbatim escape sequences in
> source
>
> I know that. I'm just listing it as one of my perceived benefits of UCNs
> and how they solve a number of distinct issues.
>
>
>> 1.
>> 2. It enables source input to use extended characters with no special
>> support beyond phase 1 (well, beyond phase 3 because of raw string
>> literals).
>>
>> What does special support means?
>
> It means that the standard does not have to have separate rules for
> handling them post phase 1 (really phase 3).
>
>
>
>> 1. It enables an escape from Unicode should such an escape prove
>> necessary (e.g., to support those EBCDIC control characters, or to encode
>> whether a UCN was explicit in the source or the result of character
>> conversion, or to encode which of the possible Shift-JIS code points a
>> character was written in). Yes, such an escape could always be introduced
>> anyway. And yes, these are edge cases, some of which are probably not
>> deserving of support.
>>
>>
> The standard is explicit about there not being any observable difference
> outside of raw literal
>
> Yes, and Hubert has claimed that the existing wording is deficient in this
> area because it doesn't reflect existing practice (e.g., those EBCDIC
> control characters).
>
>
>> 1.
>>
>>
>> It would be so much simpler to map the source physical characters (using
>> existing terminology so nobody gets confused) to unicode code points in
>> phase 1 and then replace universal character-names that appear as escape
>> sequence in non-raw string and character literals, as well as header names,
>> pp-identifier and pp-number later on when parsing pp-tokens in phase 3
>> (which would allow filtering out raw literals at that point)
>>
>> It seems like we're converging towards do-something-in-phase-1 and then
>> do-something-further-in-phase-3. Good. But mapping to code points assumes
>> a bijective mapping between source input characters and Unicode and we know
>> we don't have that in all cases today. The idea of an
>> *extended-source-character* is that it could carry additional
>> implementation-defined information (e.g., the actual code unit value(s) for
>> a source input character).
>>
>
> I don't think anything in the wording says that today, nor do I think
> there would be any value for that
>
> The standard implies it today by requiring reversion of phase 1 in raw
> string literals. This approach (which, admittedly, is not at all well
> described yet) would provide a way of tracking source input details that
> are not reflected by code point values by themselves.
>
> Tom.
>
> This still doesn't help with the reversion of line splicing in raw string
>>> literals, but a similar approach should work for that.
>>>
>> I think reversing that is fine - ish (see CWG1655, which i hope is one of
>> the issue we can close as a result of all of that)
>>
>> See also CWG1709.
>>
>> Tom.
>>
>
>



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