On Thu, Sep 16, 2021 at 5:04 PM Corentin <corentin.jabot_at_[hidden]> wrote:

>
>
> On Thu, Sep 16, 2021 at 10:23 PM Hubert Tong <
> hubert.reinterpretcast_at_[hidden]> wrote:
>
>> On Thu, Sep 16, 2021 at 1:42 PM Corentin <corentin.jabot_at_[hidden]>
>> wrote:
>>
>>>
>>>
>> Given the many many encodings, a lot are only separated by one or two
>>> codepoints. How in that context do we define variants?
>>>
>>
>> I think you are raising a new question about the design: If a system has
>> more than one implementation of the same encoding, should it return the
>> same registered character set to represent more than one of those encodings?
>>
>
> Well, 2 implementations of the same encoding would be the same, and for
> registered character sets, aliases would cover that use case.
>

Sorry (my fault for not being clear): I meant the question not for the
"same encoding" but for encodings separated by a small percentage of
differences. For example, near matches for a registered character set but
one being modified for the euro sign.


>
>>
>>>
>>> > In addition to the encoding used, the implementation may further
>>> restrict the set of valid characters
>>>
>>> I am not sure I understand the goal of this sentence. P1885 is
>>> purposefully somewhat removed from precise character sets. For which sets
>>> of operations would that restriction apply?
>>>
>>
>> This mainly occurs in the 2-byte wchar_t case. Some implementations take
>> the strategy of using UTF-8 encoding but consider only scalar values in the
>> BMP range to be valid characters.
>>
>
> UTF-8 happens to be one of the encodings that are precisely defined and
> specified.
> An encoding that would not map to all scalar values would not fit the
> definition of UTF-8 - Likewise, WTF-8, CESU-8, BOCU, etc are NOT UTF-8
> Can an implementation still advertise UTF-8? Sure, I don't see a value in
> trying to prevent hostile implementations
> Do I want to specifically bless that behavior? Nope
>

I guess (from later statements below) that we just want to chalk this all
up to a "wchar_t" problem.


>
>
>>
>>
>>>
>>> > In the absence of a conventional name for the encoding as restricted,
>>> implementations should provide a return value without regard for the
>>> restriction,
>>>
>>> Again, how do you define what's a conventional name?
>>>
>>
>> I'm happier with overt handwaving than less obvious handwaving. This
>> sentence is meant to allow "UTF-8" as the result even in implementations
>> where not all Unicode scalar values are supported by mbstowcs.
>>
>
> We do not mention mbstowcs anywhere.
>
> P1885 is not the place to address that the constraints the standard places
> on wchar_t are not representative of existing practice.
> P1885 also puts no requirements of relation between the narrow and wide
> literals nor does it mention representability.
> So returning UTF-8 for narrow and UTS-2 for wide would be perfectly valid,
> with the proposed wording.
>

P1885 does not exist in a vacuum. And the existing wording does place a
requirement between the narrow and wide execution encodings. I am somewhat
convinced that P1885 is not the place to address the wchar_t problems re:
UCS-2 versus UTF-16, but I will point out that P1885 theoretically
exacerbates the problem because the UTF-8 narrow and UCS-2 wide case is not
perfectly valid. Previously, only the UTF-16 case was clearly misaligned
with the standard; with P1886, the UCS-2 case is also misaligned.


>
>>
>>>
>>> Trying to constrain implementation freedom in a field that is plagued by
>>> 70+ years of legacy, special cases and exceptions is a minefield.
>>> I would like to better understand
>>>
>>> - What useful scenarios are allowed by this wording
>>>
>>
>> See above.
>>
>>
>>> - What problematic scenarios are prevented by this wording
>>>
>>
>> Implementations choosing to invent new names because a strict reading
>> says the registered name is not okay.
>>
>
> The wording intent is to allow an implementation to
>
> - Return unknown
> - Return an encoding that is different from that used by mbstowcs, for
> example (the wide environment is rather an environment that you would
> expect wprintf could consume without creating mojibake)
>
> This particular intent has additional limitations: The understanding of
locales with the same name is not consistent in practice on various
platforms between 32-bit and 64-bit processes.

>
> - Return an encoding that does not fit into a single wide code unit
> - Return an encoding that matches that used by other components of the
> platforms and/or the user expectation of that platform.
>
> User expectations of something completely novel is rather hard to guess
at. Should the narrow and wide EBCDIC versions of the same character set be
called the same charset? For cases where there are no multibyte characters,
most indications are "yes". For cases where there are multibyte characters,
it seems to be more up in the air. If the answer is "no", then I imagine we
end up with some "x-" prefixes and "-2byte" or "-4byte" suffixes (the
endianness is always big endian).

I do believe that "implementation-defined encoding" gives us a better
> outcome than trying to constrain either a relation between narrow and wide
> (especially given the state of the standard), or trying to force
> implementation to return a registered name when they'd rather not, or
> return an unregistered name when they'd rather not.
>

The placement of the "implementation-defined" in the currently proposed
wording for environment() is hard for me to read this way. Also, the
wording for literal() does not say "implementation-defined".

Minimal wording (for the general/synopsis section):
How a text_encoding object is determined to be representative of a
character encoding implemented in the translation or execution environment
is implementation-defined.


>
> For example, on windows the implementation will probably want to return
> UTF-16 and we do not want to disallow that.
>
> And I don't think it's necessary, nor possible to add some wording that
> would encourage implementation not to lie,
> because it may be that they have to choose between 2 lies (is windows big5
> exactly the big5 intended by IANA? Maybe not. Is that the answer users
> expect anyway? Maybe!)
>
> There are some historical oddities to contend with and there is a balance
> to be found between portability and existing practice.
>

I would again emphasize that some of the issues are with the novelty of
trying to name wide encodings where there has not been sufficient need that
there is established existing practice. Do you have a list of existing APIs
that provide names for wide encodings out of locale information?


> Especially as the number of problematic scenarios is, thankfully,
> extremely small.
>
>
>
>
>>
>>
>>>
>>> Thanks a lot for your feedback,
>>>
>>> Corentin
>>>
>>>
>>>
>>>> For users it means that implementing a function that would return some
>>>>> kind of transcoder from a name requires special care
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>