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Subject: Re: What do we want from source to internal conversion?
From: Tom Honermann (tom_at_[hidden])
Date: 2020-06-15 11:41:12


On 6/15/20 12:30 PM, Corentin wrote:
>
>
> On Mon, 15 Jun 2020 at 17:56, Tom Honermann <tom_at_[hidden]
> <mailto:tom_at_[hidden]>> wrote:
>
> On 6/15/20 11:38 AM, Corentin wrote:
>>
>>
>> On Mon, 15 Jun 2020 at 17:17, Tom Honermann <tom_at_[hidden]
>> <mailto:tom_at_[hidden]>> wrote:
>>
>> On 6/15/20 7:14 AM, Corentin via SG16 wrote:
>>
>> Hubert has specifically requested better support for
>> unmappable characters, so I don't agree with the parenthetical.
>>
>> I don't think that's a fair characterisation. Again there is a
>> mapping for all characters in ebcdic. That mapping is
>> prescriptive rather than semantic, but both Unicode and IBM agree
>> on that mapping ( the codepoints they map to do not have
>> associated semantic whatsoever and are meant to be used that
>> way). The wording trick will be to make sure we don't prevent
>> that mapping.
>
> The claim that Unicode and IBM agree on this mapping seems
> overreaching to me.  Yes, there is a specification for how EBCDIC
> code pages can be mapped to Unicode code points in a way that
> preserves round tripping.  I don't think that should be read as an
> endorsement for conflating the semantic meanings of those
> characters that represent distinct abstract characters
> before/after such a mapping.  I believe there have been requests
> to be able to differentiate the presence of one of these control
> characters in the source input and the mapped Unicode code point
> being written as a UCN.
>
>
> The Unicode characters they map to do no have associated semantic
>
> There are 65 code points set aside in the Unicode Standard for
> compatibility with the C0 and C1 control codes defined in the ISO/IEC
> 2022 framework. The ranges of these code points are U+0000..U+001F,
> U+007F, and U+0080..U+009F, which correspond to the 8- bit controls
> 0016 to 1F16 (C0 controls), 7F16 (delete), and 8016 to 9F16 (C1
> controls), respectively. For example, the 8-bit legacy control code
> character tabulation (or tab) is the byte value 0916; the Unicode
> Standard encodes the corresponding control code at U+0009. The Unicode
> Standard provides for the intact interchange of these code points,
> neither adding to nor subtracting from their semantics. The semantics
> of the control codes are generally determined by the application with
> which they are used. However, in the absence of specific application
> uses, they may be interpreted according to the control function
> semantics specified in ISO/IEC 6429:1992. In general, the use of
> control codes constitutes a higher-level protocol and is beyond the
> scope of the Unicode Standard. For example, the use of ISO/IEC 6429
> control sequences for controlling bidirectional formatting would be a
> legitimate higher-level protocol layered on top of the plain text of
> the Unicode Standard. Higher-level protocols are not specified by the
> Unicode Standard; their existence cannot be assumed without a separate
> agreement between the parties interchanging such data.

Yes, I'm aware, but this point still stands: I believe there have been
requests to be able to differentiate the presence of one of these
control characters in the source input vs the mapped Unicode code point
being written as a UCN.

Tom.



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