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Subject: Re: Reminder: SG16 telecon tomorrow (Wednesday, 2020-06-10)
From: Hubert Tong (hubert.reinterpretcast_at_[hidden])
Date: 2020-06-09 20:37:55


On Tue, Jun 9, 2020 at 9:03 PM Corentin Jabot <corentinjabot_at_[hidden]>
wrote:

>
>
> On Wed, 10 Jun 2020 at 01:39, Hubert Tong <
> hubert.reinterpretcast_at_[hidden]> wrote:
>
>> On Tue, Jun 9, 2020 at 7:12 PM Steve Downey <sdowney_at_[hidden]> wrote:
>>
>>> While I understand what you are asking for, and I agree it doesn't seem
>>> unreasonable, I also don't see how that it works with the machinery today?
>>>
>> I am not saying that the C++ wording today works for this by the letter
>> (except for heavy-handed interpretations of phase 1). I consider it to be a
>> bug that it doesn't.
>>
>>
>>> All characters outside the basic source character set are mapped to
>>> universal-character-names that are named by Unicode scalar values.
>>> We'd need a mechanism to get back to the completely untranslated
>>> original source.
>>>
>>
> I think we have that mechanism already.
> We have a mapping source -> universal-character-names (which for your
> interest is specified both by IBM and Unicode), and
> the universal-character-names -> execution mapping, which again is fully
> specified.
> I think that is enough to do, if desired, a direct source -> execution
> which is bytes preserving, as it is not observable whether it was done or
> not.
>
It is round-trippable but at the cost of one-way (during compilation)
conversions that are not semantically preserving. Even these are
justifiable, but I think they deserve to be called out. Which is to say
that the paper should document that these concerns were considered and not
simply dismiss the issue.

'\u0096' becoming '\x36': I suppose this could be justified for the case
where the user application is expected to have its output subjected to
automatic conversion, e.g., via SSH to a non-EBCDIC terminal.

For the much rarer case of u'<0x36>' (character literal that, in the
physical source file, contains the EBCDIC control character) becoming
u'\u0096': I suppose this could be justified for the case where the user
source was originally non-EBCDIC, but subjected to conversion into EBCDIC.

>
>
>> I think this is similar to how raw string literals need some sort of
>> mechanism.
>>
>>
>>>
>>> On Tue, Jun 9, 2020, 18:32 Hubert Tong <hubert.reinterpretcast_at_[hidden]>
>>> wrote:
>>>
>>>> On Tue, Jun 9, 2020 at 5:21 PM Corentin Jabot <corentinjabot_at_[hidden]>
>>>> wrote:
>>>>
>>>>>
>>>>>
>>>>> On Tue, 9 Jun 2020 at 23:06, Hubert Tong <
>>>>> hubert.reinterpretcast_at_[hidden]> wrote:
>>>>>
>>>>>> On Tue, Jun 9, 2020 at 4:59 PM Corentin Jabot <
>>>>>> corentinjabot_at_[hidden]> wrote:
>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> On Tue, 9 Jun 2020 at 22:17, Hubert Tong <
>>>>>>> hubert.reinterpretcast_at_[hidden]> wrote:
>>>>>>>
>>>>>>>> On Tue, Jun 9, 2020 at 1:01 PM Corentin Jabot via SG16 <
>>>>>>>> sg16_at_[hidden]> wrote:
>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> On Tue, 9 Jun 2020 at 18:45, Steve Downey <sdowney_at_[hidden]>
>>>>>>>>> wrote:
>>>>>>>>>
>>>>>>>>>> One thing I have realized while working on identifiers is that
>>>>>>>>>> after conversion from whatever the sources are, lexing and parsing are
>>>>>>>>>> symbolic. That is, 'a' doesn't have a value until it's rendered into a
>>>>>>>>>> literal. That is " The values of the members of the execution
>>>>>>>>>> character sets and the sets of additional members are locale-specific
>>>>>>>>>> . <http://eel.is/c++draft/lex.charset#3.sentence-5>"
>>>>>>>>>> http://eel.is/c++draft/lex.charset#3.sentence-5 really only
>>>>>>>>>> comes into play when rendering the "execution character set" into a
>>>>>>>>>> characters or strings. The execution character set and the source character
>>>>>>>>>> set exist in the same logical space right now, and the "source character
>>>>>>>>>> set" isn't what is in source files today.
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> Yep, and they don't have to have a value either. identifiers are
>>>>>>>>> not sorted etc.
>>>>>>>>> Everything in lex is symbolic anyway the phases don't exist in
>>>>>>>>> practice.
>>>>>>>>> However, the international representation being isomorphic to
>>>>>>>>> Unicode, it would be possible to describe in term of unicode with no
>>>>>>>>> observable behavior change.
>>>>>>>>>
>>>>>>>> I would like to allow characters not present in Unicode within
>>>>>>>> character literals, string literals, comments, and header names. More
>>>>>>>> abstractly, I would like to allow source -> encoding-used-for-output
>>>>>>>> conversion.
>>>>>>>>
>>>>>>>
>>>>>>> Do you have an example of a use case you want to support?
>>>>>>>
>>>>>> I am still evaluating the round-trip mapping for EBCDIC.
>>>>>>
>>>>>
>>>>> I believe Unicode -> EBCDIC round trip perfectly using the process
>>>>> described in https://www.unicode.org/reports/tr16/tr16-8.html
>>>>> The tricky part is the control characters, which this TR maps to the
>>>>> C1 unicode control characters
>>>>>
>>>> I'm not questioning the ability to round-trip. I am questioning the
>>>> ability to avoid conflating certain EBCDIC control characters with certain
>>>> C1 control characters. For example, it seems clear to me that U+0096 START
>>>> OF GUARDED AREA and U+0097 END OF GUARDED AREA are paired in the intended
>>>> usage, but the mapping of these to, respectively, Numeric Backspace and
>>>> Graphic Escape does not retain semantic meaning. If such EBCDIC characters
>>>> appear within a literal that should be encoded in a Unicode encoding, I
>>>> find it potentially questionable if the string is considered well-formed. I
>>>> have similar thoughts for the case where a UCN escape for such a C1 control
>>>> character appears in a string that is to be encoded in EBCDIC.
>>>>
>>>> In other words, I do not consider the mapping (which is useful if you
>>>> track out-of-band whether the data was originally EBCDIC or not) to
>>>> establish the presence of the EBCDIC control characters in Unicode. These
>>>> opinions do not necessarily represent those of IBM.
>>>>
>>>> -- HT
>>>>
>>>



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