On 5/2/24 6:35 PM, Corentin Jabot via SG16 wrote:
>
>
> On Thu, May 2, 2024 at 11:25 PM Tom Honermann <tom_at_[hidden]> wrote:
>
> On 4/30/24 2:31 AM, Corentin Jabot via SG16 wrote:
>>
>>
>> On Tue, Apr 30, 2024 at 12:45 AM Tom Honermann
>> <tom_at_[hidden]> wrote:
>>
>> On 4/29/24 4:11 PM, Peter Dimov via SG16 wrote:
>> > Tom Honermann wrote:
>> >> I'm not entirely sure that cout << std::format("{}",
>> u8"...") is that much
>> >> easier
>> >> to specify and support.
>> >>
>> >> But I'll be glad to be proven wrong, of course. :-)
>> >>
>> >> There is a relevant SO comment
>> >>
>> <https://stackoverflow.com/questions/58878651/what-is-the-printf-
>> >> formatting-character-for-char8-t/58895428#58895428> .
>> >>
>> >> std::format() and std::print(), to some extent, improve
>> the likelihood that an
>> >> implementation selected encoding will be a good match for
>> the programmer's
>> >> intent. This is because:
>> >>
>> >> 1. std::format() and std::print() are not implicitly
>> locale dependent; that
>> >> rules out selection of a locale dependent execution encoding.
>> >> 2. std::format() returns a std::string; that rules out
>> selection of an I/O
>> >> dependent encoding.
>> >> 3. std::print() writes to an I/O stream, but has special
>> behavior for writes
>> >> to a terminal; that rules out selection of a terminal
>> encoding (as unnecessary,
>> >> at least in important cases).
>> >> 4. std::format() and std::print() are both strongly
>> associated with the
>> >> ordinary/wide literal encoding.
>> >> 5. std::format() and std::print() should have the same
>> behavior (other than
>> >> that std::print(...) may produce a better result than
>> std::cout <<
>> >> std::format(...) when the output is directed to a terminal).
>> >> 6. std::format() and std::print() have additional
>> guarantees when the
>> >> ordinary/wide literal encoding is a UTF encoding.
>> >>
>> >>
>> >> Due to those characteristics, we have good motivation for
>> implicit use of the
>> >> ordinary/wide literal encoding as the target for
>> transcoding for std::format()
>> >> and std::print().
>> > I'm afraid that I don't quite understand.
>> >
>> > What does std::format( "{}", u8"..." ) actually do? I
>> suppose it transcodes
>> > the UTF-8 input into the narrow literal encoding (replacing
>> irrepresentable
>> > characters with '?' instead of throwing, I presume, or it
>> would be not very
>> > usable)?
>>
>> We'll have to see what Corentin proposes :)
>>
>> But yes, something very much like that.
>>
>> Note that we could also support std::format("{:L}", u8"...")
>> to enable a
>> programmer to explicitly request transcoding to a locale
>> dependent
>> encoding (either now or at some future point).
>>
>> (Corentin, at a minimum, we should reserve the L option in
>> your paper).
>>
>>
>> We have an opportunity to not conflate locale and encodings here.
> As much as I would like that to be the case, I don't think it is.
>> u8"" is a known quantity here, it's utf-8.
>> But the target is also a known quantity, we very clearly decided
>> it to be the literal encoding, because we need to parse it, and
>> we wisely decided to assume a literal encoding. So the target
>> encoding is also a known quantity
>
> Unfortunately, that isn't the case when a programmer opts in to
> use of a locale. Consider the following when the literal encoding
> is any ASCII derived encoding and the global locale encoding is
> EUC-JP (ujis).
>
> #include <chrono>
> #include <format>
> #include <iostream>
> #include <locale>
> int main() {
> std::locale::global(std::locale(""));
> std::cout << std::format("{:L}\n", std::chrono::August);
> }
>
> The resulting string will be formed from the literal encoding (for
> the '\n' character) and the name of the month provided by the
> /formatting locale <http://eel.is/c++draft/time.format#2>/.
> Nothing ensures that the latter is converted to the literal
> encoding. Further, a validly encoded string is produced so long as
> the characters used in the format string are from the basic
> literal character set.
>
> In my environment (Linux, using a pre-release build of Clang 19
> and libc++), compiling the above with the default literal encoding
> (UTF-8) and running it with LANG=ja_JP.ujis produces output in
> EUC-jp as expected; note the iconv invocation.
>
> $ clang++ -std=c++23 -stdlib=libc++ t.cpp -o t
> $ LANG=ja_JP.ujis ./t | iconv -f ujis -t utf-8
> 8月
>
> (yes, that is the right output, it is convention for some
> translation of month names to include the month number before the
> localized name).
>
> Long time SG16 participants will recall P2373R3 (Fixing locale
> handling in chrono formatters) <https://wg21.link/p2372r3> and LWG
> 3547 <https://wg21.link/lwg3547>. There was relevant discussion
> during the 2021-04-28 SG16 meeting
> <https://github.com/sg16-unicode/sg16-meetings/blob/master/README-2021.md#april-28th-2021>.
>
> I have vague recollections of discussions about requiring that
> locale dependent translations be provided in the literal encoding
> when it is a UTF one, but I haven't been able to identify any such
> recorded discussion. I don't see anything in the current WP that
> would require this.
>
> Based on the above, I think that, at a minimum, the "L" option
> should be reserved.
>
>
> I'm not sure what you are arguing about because "L" can only be
> applied to things that can be "localized" (i.e. mangled horribly by
> POSIX).

You are right that I wasn't very clear about what I'm suggesting. I'll
try to clarify.

The ASCII and EBCDIC code page based locale programming model used on
POSIX and Windows systems is not broken. It does have sharp edges.
Unicode and its associated encodings have enabled a new programming
model with fewer constraints and pitfalls, but that has not completely
displaced the code page based programming model nor do I think it ever
will. The code page based programming model requires the following:

 1. Since C and C++ programs start with the global locale set to "C", it
    is necessary to opt-in to locale dependent behavior by calling
    std::locale::global() and/or std::setlocale().
 2. Such programs, in order to avoid mojibake, must constrain the use of
    compile-time selected characters encoded in the ordinary literal
    encoding to those that have an invariant representation in all
    supported locale dependent encodings

There has been a lot of code written over the last 40 or so years that
adheres to this model. Many such programs are effectively locale
agnostic though full localization requires translations provided by
message catalogs (that themselves rely on locale; GNU gettext
<https://www.gnu.org/software/gettext/> and POSIX catopen
<https://pubs.opengroup.org/onlinepubs/9699919799/functions/catopen.html>
are relevant). In my opinion, these programs should continue to work and
continue to benefit from C++ standard library enhancements.

Let's look at that example from above again:

    std::cout << std::format("{:L}\n", std::chrono::August);

Regardless of what the ordinary literal encoding is, if LANG is
ja_JP.sjis, then valid Shift-JIS output will be produced. Likewise, if
it is ja_JP.utf8, zh_CN.gb18030, or zh_TW.big5, valid output will be
produced in those encodings. This is portable code that works on all
platforms (with the right platform dependent locale names; those sadly
are not portable).

Let's now assume a hypothetical message catalog of translated strings
that works similarly to gettext, but that provides UTF-8 encoded
translations in char8_t.

    std::cout << std::format("{} {:L}\n", u8msg("In the month of"),
    std::chrono::August);

If we unconditionally require the char8_t argument to be transcoded to
the ordinary literal encoding, then mojibake will be produced unless the
ordinary literal encoding happens to match the locale encoding.

I strongly agree that, for std::format(), the default behavior should be
that char8_t strings are transcoded to the ordinary literal encoding.

What I am arguing for is that there should also be an option for the
programmer to opt-in to locale based transcoding of arguments that
potentially require transcoding. Thus:

    std::cout << std::format("{:L} {:L}\n", _u8("In the month of"),
    std::chrono::August);

would portably produce correct locale dependent output (and transcoding
would be reduced to a byte copy when the locale encoding is UTF-8).

For the short term however, I'm content to just reserve the 'L' option;
actually doing the work to support this can await further motivation and
standard transcoding facilities.

> std::format("{:L}\n", ""); is ill-formed, so would be
> std::format("{:L}\n", u8"");
> https://eel.is/c++draft/format#string.std-17 (it's also used in chrono)
> https://compiler-explorer.com/z/58bsTaf3o
> Beside, reservation is not necessary, users cannot write formatters
> for types that do not depend on user-defined types (or, if you prefer,
> it's already reserved)

Victor can correct me if I'm mistaken, but my understanding has been
that changes to std::formatter specializations might cause (sometimes?)
an ABI break. The following is recorded in the 2023-11-29 SG16 meeting
<https://github.com/sg16-unicode/sg16-meetings/blob/master/README-2023.md#november-29th-2023>
notes from discussion of - P3045R0 (Quantities and units library)
<https://wg21.link/p3045r0>.

"Victor recommended reserving an 'L' option specifier in the format
specification that would render the code ill-formed for now so as to
allow extension later without an ABI break."

>
> But the issues with the whole scenario you are describing is that:
>
> 1.
> We keep trying to give meaning to programs where the execution
> encoding is not a superset of the literal encoding even though the
> encoding is generally not part of the type system
> So for 2 arbitrary strings a and b, concatenating them might not
> produce a good result, and we can't solve it.
Such programs already have a meaning and have for the last 40 years or
so. I agree there are sharp edges here that we can't fix.
> You will note that format makes the assumption that everything is in
> the literal encoding and it's working wonderfully well.

std::format() does not ensure that the output produced is an any
particular encoding. We spent a lot of time talking about whether
std::format() produces text and eventually concluded that it is not
required to do so.

I am not arguing for a change in direction; in fact, I'm arguing with
preserving consistent behavior with regard to its existing locale
dependent behavior so that there is an option for _not_ producing mojibake.

> It's certainly not perfect - i.e. we taught people to compile with
> /utf8 on windows but the system is still not defaulting to UTF-8, but
> it's as good as we can reasonably get.
Agreed. And for those that are able to use /utf8, that is great. I have
no data, but I would bet a good deal of cash that the vast majority of
code that is compiled with MSVC is not compiled with /utf-8.
>
> 2.
> When you ask for the name of August in Japanese, as a user, you
> probably don't expect part of your program to be encoded in some weird
> encoding that is different to the rest of the program.
> We try to patch that in format/chrono, but it's certainly not perfect
> https://eel.is/c++draft/time#format-3.sentence-3
Thank you! That is the wording I was looking for with regard to my
"vague recollections of discussions" statement above.
>
> Anyway, I'm not sure how that is relevant to the u8 discussion, L
> affects individual arguments, not the formatting string (the literal
> encoding is the ground truth for encoding as far as format is concerned)

I hope the above better explains the relevance.

Tom.

>
>
>
>
>>
>>
>>
>> >
>> > And then we just fall back to std::cout << "...", where the
>> "..." is in the
>> > narrow literal encoding and hence we assume works, more or
>> less.
>> Correct.
>> >
>> > And we don't want to make std::cout << u8"..." do that,
>> because it can,
>> > in principle, do better?
>> Not because it can do better, but because there is more
>> uncertainty
>> about what the user might expect. If the user writes
>> std::cout <<
>> std::format(...), then that is an explicit opt in to the
>> behavior that
>> std::format() exhibits. But they might also want to just
>> write UTF-8
>> bytes unmodified regardless of what the ordinary literal
>> encoding is. Or
>> they might expect implicit transcoding to either the current
>> locale or
>> the environment locale or even the terminal locale. By not
>> providing a
>> default behavior, we give the programmer the opportunity to
>> think about
>> what they are actually trying to do.
>>
>>
>> I don't quite buy this argument.
>> When cout << 42.0; outputs "42,0", the text nature, locale and
>> encodings were made for us.
>> If the programmer wants to be creative, one can consider io
>> manipulators.
>
> Consider printing of other localized names as in the example above.
>
> #include <chrono>
> #include <format>
> #include <iostream>
> #include <iomanip>
> #include <locale>
> int main() {
> std::cout << "Default locale: '" << std::cout.getloc().name() <<
> "'\n";
> std::cout << std::chrono::August << "\n";
> std::cout.imbue(std::locale(""));
> std::cout << "Environment locale: '" <<
> std::cout.getloc().name() << "'\n";
> std::cout << std::chrono::August << "\n";
> std::cout.imbue(std::locale("ja_JP.utf8"));
> std::cout << "Explicit locale: '" << std::cout.getloc().name()
> << "'\n";
> std::cout << std::chrono::August << "\n";
> }
>
> I get the following output running that locally with
> LANG=ja_JP.ujis. Note the mojibake and corresponding substitution
> of replacement characters.
>
> Default locale: 'C'
> Aug
> Environment locale: ''
> 8��
> Explicit locale: 'ja_JP.utf8'
> 8月
>
> The (well recognized) problem with iostreams is the implicit use
> of the imbued locale. The consistent behavior for iostreams would
> be that inserters and extractors for charN_t would transcode to
> the encoding of the imbued locale. But that doesn't work well at
> all in the common case where no locale has been explicitly imbued.
>
> Making a choice for std::format() is simpler because the
> programmer chooses the locale behavior on a per-argument basis;
> there is a good default.
>
>> >
>> > But let me get back to your list.
>> >
>> >> 1. std::format() and std::print() are not implicitly
>> locale dependent; that
>> >> rules out selection of a locale dependent execution encoding.
>> > What is in a locale-dependent execution encoding in
>> std::cout << u8"..."?
>> iostreams implicitly consults either an imbued locale facet
>> or the
>> global locale for formatting operations. Think about
>> std::cout <<
>> std::chrono::Sunday. Depending on the current locale, this
>> might print
>> "Sun" or a localized weekday name in a locale dependent encoding.
>>
>>
>> But again, the only thing we care about for u8 is the encoding.
>> And I am not aware of std::locale ever impacting that.
> I hope the above examples are motivating.
>>
>> >
>> >> 2. std::format() returns a std::string; that rules out
>> selection of an I/O
>> >> dependent encoding.
>> > Same question. Where is the I/O dependent encoding in
>> std::cout << u8"..."
>> > (that is not also present in std::cout << some_std_string)?
>> In the latter case, we have to assume that some_std_string
>> holds text in
>> the encoding expected on the other end of the stream. We
>> can't do that
>> for u8"...", so we have to transcode to something (or have
>> some other
>> assurance that UTF-8 is intended and expected).
>> >
>> >> 3. std::print() writes to an I/O stream, but has special
>> behavior for writes
>> >> to a terminal; that rules out selection of a terminal
>> encoding (as unnecessary,
>> >> at least in important cases).
>>
>> > This doesn't apply here, because we're using std::format.
>>
>>
>> Right, this is one of the reasons I feel less compelled to pursue
>> iostream surgery.
>> Output behavior is suboptimal on windows, and unlikely to be fixed.
>
> I am likewise not compelled to pursue iostream support.
>
> Agreed with later remarks below.
>
> Tom.
>
>> >> 5. std::format() and std::print() should have the same
>> behavior (other than
>> >> that std::print(...) may produce a better result than
>> std::cout <<
>> >> std::format(...) when the output is directed to a terminal).
>> > OK... but this isn't relevant.
>> The above two are relevant because we wouldn't want to
>> differentiate
>> behavior for formatting a u8"..." argument for std::format() vs
>> std::print(). The latter helps to constrain the reasonable
>> options for
>> the former.
>>
>> Right, print just does format and output the result
>>
>> >
>> >> 6. std::format() and std::print() have additional
>> guarantees when the
>> >> ordinary/wide literal encoding is a UTF encoding.
>> > What additional guarantees, and how do they help here?
>>
>> We specify additional constraints for fill characters,
>> display width
>> (well, normative encouragement), and formatting of escaped
>> strings. None
>> of these are relevant for reflection purposes; they help to
>> reinforce a
>> choice to depend on the ordinary/wide literal encoding for
>> behavior of
>> these functions. We don't have such precedent for iostreams.
>>
>>
>> And you know, the format string is parsed in the ordinary
>> encoding and copied as-it
>>
>>
>> Tom.
>>
>>
>