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Subject: Re: [isocpp-lib-ext] Review of P2093R2: Formatted output
From: Tom Honermann (tom_at_[hidden])
Date: 2020-12-11 23:43:13


On 12/9/20 9:57 AM, Victor Zverovich wrote:
> > In the former case, the programmer has wide latitude for choosing an
> encoding and knows that the content is being written to a file.  In
> the latter case, the programmer doesn't (in general) know whether the
> output is redirected to a file, pipe, or some other character device.
> > For #2, it is the user that is making the choice to write the output
> to a text file, not the programmer.
>
> I don't think this is correct. In general, when you have a file stream
> object you cannot tell whether it was redirected by the user or not in
> both cases. You can of course distinguish between a file and a pipe,
> but not whether it was redirected by the user or the programmer. Even
> if it was possible it would be weird to have lossy transcoding into a
> legacy codepage in one case and not the other.

The scenario I intended to contrast with was an inherited file stream vs
one obtained by a call to fopen() or similar. In the latter case, the
programmer is in control.  I agree that, for inherited file streams,
there is more uncertainty.

>
> > that choice is historically distinct from the run-time encoding used
> by the environment the program runs in.
>
> This is exactly what we are trying to fix because as we can see it
> results in mojibake in common scenarios.

The proposal addresses the specific scenario where the output is known
to be directed to a device that can be independently controlled and I am
in favor of that.  The problem is larger than that though.  The
environment the program runs in (in real deployments outside the
abstract machine) consists of more than just a console.  I would like to
address the wider problem with this facility as well.  We aren't at
liberty to change the environment a program runs in, but we can design
for adaptation to that environment.

>
> > I believe it would be a reasonable choice for a z/OS programmer to
> use UTF-8 as the execution/literal encoding and still run that program
> in an EBCDIC environment.
>
> Is the desired behavior for z/OS to have string literals compiled to
> UTF-8 in the binary and do runtime transcoding into EBCDIC instead of
> having string literals compiled to EBCDIC and avoiding runtime
> transcoding? The latter is already supported by the paper. The former
> is a somewhat strange and inefficient approach but if it is the
> desired behavior I'd be happy to tweak the wording to make this
> possible (suggestions are welcome). In any case I think we should
> avoid making transcoding lossy when possible or having it controlled
> at runtime without very good reasons.

Yes.  The idea is that it should be possible to write a portable program
that uses UTF-8 as the execution/literal encoding and have it run to the
best of its abilities in any environment with the acknowledged
limitation that, if an environment can't support all Unicode characters,
then some data loss is inevitable.  Note that such data limitations
exist independently of what encoding is used for the execution/literal
encoding; there is no character for 🚀 (U+1F680) in EBCDIC.

I certainly agree with the goal of avoiding data loss.  My perspective
is that avoiding mojibake (as your proposal does for the
console/terminal) is more important than avoiding introduction of
substitution characters.

>
> > My suggestion was that, when writing to a stream known (e.g., via
> _isatty()
> <https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/isatty?view=msvc-160>)
> to be directly connected to the Windows console, that the Unicode path
> be taken regardless of what the execution encoding is, with
> transcoding to UTF-16 performed as necessary.
>
> Sounds reasonable. I suggest polling this in SG16.
That sounds good.  Per discussion at our telecon this week, we can do so
when we look at the next revision of this paper.  I think having a
better understanding of what other languages do will be helpful (and
thank you for the research you already started doing for that).
>
> > I think the relevant question for this paper, given that it does
> intend to specify encoding conversions in at least some cases, is how
> output such as filenames that may have content that is not well-formed
> according to the execution encoding, can be incorporated.
>
> OK, I'll look into it and add a report back in the next revision of
> the paper.
Excellent, thank you.
>
> > I don't find throwing an exception to be acceptable, but attempted
> conversion with U+FFFD substitution as suggested by Peter seems ok
>
> I agree. Again this is a good candidate for a poll.

Sounds good.  Other considerations could include some method to specify
a callback, an alternate substitution character, or alternate error
handling, but I suspect little motivation for any of that for this
facility.  Perhaps the further research on what is done for other
languages will provide some additional perspective.

Just to reiterate, thank you for bringing this paper forward.  I very
much want this facility!

Tom.

>
> Thanks,
> Victor
>
>
> On Sat, Dec 5, 2020 at 1:16 PM Tom Honermann <tom_at_[hidden]
> <mailto:tom_at_[hidden]>> wrote:
>
> On 11/28/20 11:33 AM, Victor Zverovich wrote:
>> To keep this thread manageable here are my answers to Tom and
>> Jens (thanks for the feedback!) in one convenient wall of text.
>> *
>> *
>> *Answers to Tom:*
>>
>> > My choice of Windows-1251 for an example scenario was motivated
>> solely by the use of Russian characters in the example in the paper.
>>
>> Sure but it nevertheless a great choice that clearly
>> demonstrates that ACP is definitely the wrong thing when files
>> are involved. That said, EBCDIC and other encodings are still
>> supported via the non-Unicode path.
> I don't feel that same level of clarity.  There is a distinction
> to be made regarding writing to a file vs writing to stdout.  In
> the former case, the programmer has wide latitude for choosing an
> encoding and knows that the content is being written to a file. 
> In the latter case, the programmer doesn't (in general) know
> whether the output is redirected to a file, pipe, or some other
> character device.
>>
>> > I don't think the Notepad example is particularly relevant.
>>
>> It is relevant for #2 because it shows that when a Russian user
>> creates a text file on Windows it will most definitely be encoded
>> in UTF-8 and not "ANSI" encoding (and definitely not the terminal
>> encoding). This is true for Notepad and other popular editors.
>> Same with files obtained from the Internet. We should understand
>> the common encoding for text files in order for our text
>> facilities to be useful and consistent.
>
> I think this misses the concern to some degree.  For #2, it is the
> user that is making the choice to write the output to a text file,
> not the programmer.  I believe the programmer should have the
> ability to choose the encoding used (preferably with the ability
> for the user to influence the choice). but I'm (so far)
> uncomfortable with the behavior being tied to the
> execution/literal encoding chosen at compile time; that choice is
> historically distinct from the run-time encoding used by the
> environment the program runs in.
>
> For example, I believe it would be a reasonable choice for a z/OS
> programmer to use UTF-8 as the execution/literal encoding and
> still run that program in an EBCDIC environment.  This is how Java
> works in such environments (using UTF-16 internally of course). 
> This is the Unicode sandwich model.
>
>>
>> > There is no particular expectation that a .txt file was
>> produced by a program running on the local machine, so the local
>> code page isn't a particularly good default in any case.
>>
>> Exactly.
>>
>> > If I write a version of the Windows 'type' command as you used
>> it above, call it 'cat', compile it without Microsoft's /utf-8
>> option, then I would like it to still do the right thing; not the
>> behavior you illustrated above.
>>
>> I misunderstood your suggestion. Are you suggesting for the
>> non-Unicode path (print_nonunicode) to do the transcoding to the
>> encoding determined by ACP and for the Unicode path
>> (print_unicode) to produce UTF-8? Note that using ACP won't solve
>> the mojjibake problem because the terminal encoding (CP866) is
>> separate from the ACP encoding, at least for Russian. Confusing
>> those two is a common misconception and source of problems (see
>> e.g.
>> https://stackoverflow.com/questions/49259502/windows-console-codepage-866).
>> Using the terminal encoding would produce completely useless
>> output for anything but interaction with legacy command-line
>> programs via pipes (and even there the usefulness of the result
>> of the pipeline is questionable).
>
> No.  My suggestion was that, when writing to a stream known (e.g.,
> via _isatty()
> <https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/isatty?view=msvc-160>)
> to be directly connected to the Windows console, that the Unicode
> path be taken regardless of what the execution encoding is, with
> transcoding to UTF-16 performed as necessary.  My expectation is
> that writes to the console would be performed using the
> WriteConsoleW() function; that is where UTF-16 comes in.  So, when
> the execution encoding is UTF-8, the implementation would convert
> to UTF-16 and then call WriteConsoleW() and, for other execution
> encodings, the implementation would transcode to UTF-16 and then
> call WriteConsoleW(). This approach bypasses the console encoding
> entirely (the console encoding is only relevant for the ANSI
> implementation of the console APIs and for reads/writes to the
> console via ReadFile() and WriteFile()).
>
>>
>> > That is true only if the execution/literal encoding and the
>> run-time encoding do not match
>>
>> Yes and if we use ACP they will likely not match.
> For Windows, that is true, but also a reality of that
> environment.  For other platforms, the likelihood of a mismatch is
> far lower (though not 0; the LANG environment variable is still
> used in POSIX environments and can still be set to select an
> encoding other than UTF-8).
>>
>> > Assuming test.txt is UTF-8 encoded, that is correct; this is a
>> straightforward case of mojibake.
>>
>> test.txt is CP1251 encoded. This example illustrates that using
>> ACP doesn't solve mojibake.
>
> Perhaps we are focused on different instances of mojibake.  I
> think you are pointing out that the output of the findstr command
> will fail to present properly because the console encoding doesn't
> match.  The mojibake I was alluding to is that findstr will fail
> to find a match in the file because the encoding of the pattern
> string (as entered from the console on the command line) doesn't
> match the encoding of the file (unless findstr consults the
> wide/UTF-16 variant of its command line).
>
>>
>> > Perhaps a 'formatter' specialization should be provided for
>> std::filesystem::path?  Proposing something like that is likely
>> subject matter for a different paper, but I think it would be
>> helpful for this paper to discuss it.
>>
>> I think that providing such specialization would be useful but it
>> is out of scope of the current paper since it has nothing to do
>> with I/O integration.
> I think the relevant question for this paper, given that it does
> intend to specify encoding conversions in at least some cases, is
> how output such as filenames that may have content that is not
> well-formed according to the execution encoding, can be
> incorporated.  I think my preference is to have some method to
> opt-out of implicit conversions; probably via a per-field format flag.
>>
>> > What happens if the UTF-8 input is ill-formed?
>>
>> Good question. The current implementation throws an exception on
>> transcoding error but the error handling mechanism is open for
>> discussion.
> I don't find throwing an exception to be acceptable, but attempted
> conversion with U+FFFD substitution as suggested by Peter seems ok
> (perhaps with an opt-out as suggested above); I prefer a loss of
> precision over a loss of output.
>>
>> *Answers to Jens:*
>>
>> > Doing std::format without necessarily creating a std::string is
>> useful functionality, but unrelated to the transcoding issues.
>> Thus, this facility should be separate.
>>
>> Such a facility already exists in C++20 (format_to, format_to_n).
>> The current paper only integrates it with I/O without adding any
>> new functionality on the formatting level.
>>
>> > Apparently, there is some OS-dependent magic going on to
>> determine whether output is to a console and, if so, which
>> encoding the console might prefer. I'm fine with such magic
>> existing, but it should be a distinct facility.
>>
>> Sure, I will extract it into a separate API in the next revision
>> of the paper.
>>
>> > And then there is the facility of converting the C++ literal
>> encoding to the console encoding, if necessary.  Again, this
>> should be a separate facility, preferably offering a generic
>> transcoding facility that can be specialized for the console-only
>> use case.
>>
>> While I agree that such a transcoding facility would be useful I
>> think it is out of scope of the current paper. The latter
>> requires only minimal transcoding facilities for the Unicode case
>> and only on some platforms where dedicated system APIs exist.
> I agree that distinct interfaces should be provided for each of
> these concerns, but I also think each can be pursued separately
> and need not hold up the proposed feature.  We can always
> re-specify the proposed behavior in terms of new interfaces via
> as-if in the future.
>
> Also, progress is being made on these; JeanHeyd is continuing to
> work on general transcoding facilities.  See WG14 N2595
> <http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2595.pdf> for
> his most recent work (we'll be discussing this paper in SG16 early
> next year).
>
> Tom.
>
>>
>> Cheers,
>> Victor
>>
>>
>> On Fri, Nov 27, 2020 at 11:15 AM Jens Maurer via SG16
>> <sg16_at_[hidden] <mailto:sg16_at_[hidden]>> wrote:
>>
>> On 23/11/2020 06.33, Tom Honermann via Lib-Ext wrote:
>> > SG16 began reviewing P2093R2 <https://wg21.link/p2093r2> in
>> our recent telecon
>> <https://github.com/sg16-unicode/sg16-meetings#november-11th-2020>
>> and will continue review in our next telecon scheduled for
>> December 9th.
>> >
>> > The following reflects my personal thoughts on this proposal.
>>
>> Ditto.
>>
>> As I've already said in the SG16 review, I'd like to see
>> smaller bits and pieces offered, instead of or at least in
>> addition to hiding them behind a non-trivial "printf"-style
>> wrapper.
>>
>>  - Doing std::format without necessarily creating a std::string
>> is useful functionality, but unrelated to the transcoding issues.
>> Thus, this facility should be separate.
>>
>>  - Apparently, there is some OS-dependent magic going on to
>> determine whether output is to a console and, if so, which
>> encoding the console might prefer.  I'm fine with such magic
>> existing, but it should be a distinct facility.
>>
>>  - And then there is the facility of converting the C++ literal
>> encoding to the console encoding, if necessary. Again, this
>> should be a separate facility, preferably offering a generic
>> transcoding facility that can be specialized for the console-only
>> use case.  (Only supporting that single transcoding might save
>> binary size.)
>>
>>
>> Jens
>>
>>
>> --
>> SG16 mailing list
>> SG16_at_[hidden] <mailto:SG16_at_[hidden]>
>> https://lists.isocpp.org/mailman/listinfo.cgi/sg16
>>
>



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