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<div class="moz-cite-prefix">On 9/8/19 12:02 PM, Steve Downey wrote:<br>
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<div dir="ltr">Character repertoire sounds good, and I will
eventually learn to spell it. Character set is
definitely terminology from the pre-unicode times, and
unfortunately tends to merge the repertoire and encoding, <a
href="https://www.iana.org/assignments/character-sets/character-sets.xhtml"
moz-do-not-send="true">https://www.iana.org/assignments/character-sets/character-sets.xhtml</a><br>
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<p>I think I was a little over zealous earlier in stating that
Unicode uses "character repertoire" as I described. I looked
again and don't find that term formally defined in the standard.
However, "repertoire" is used throughout the standard in ways that
I believe are consistent with my description. I wasn't able to
find an alternative formal term.<br>
</p>
<p>The way I've been thinking about it is that a "character
repertoire" describes a set of <i>abstract characters</i> (a
formal Unicode term) and a "character set" describes a set of <i>encoded
characters</i> (a formal Unicode term) that associate each <i>abstract
character</i> member of a "character repertoire" with a <i>code
point</i> (a formal Unicode term) within a <i>codespace</i> (A
formal Unicode term). See sections 2.4 and 3.4 of Unicode 12 and
uses of the word "repertoire" within those chapters. The Unicode
standard does use the term "character set", but I didn't find a
formal definition.<br>
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Basic source character set is defined in [lex.charset] <a
href="http://eel.is/c++draft/lex.charset#def:character_set,basic_source"
moz-do-not-send="true">http://eel.is/c++draft/lex.charset#def:character_set,basic_source</a><br>
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Yes, and it defines a character repertoire. "Physical source file
characters" is the closest I've found to a term that describes the
actual implementation defined source character set.
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I'd like to get away from "execution encoding" because it
conflates the presumed encoding and the one selected by the
current locale. Now, admittedly, everyone conflates these and
it's a source of error and mojibake, but perhaps with better
words it would be easier to teach. <br>
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I agree. I like "dynamic encoding" because it accurately reflects
the reality that the encoding can be changed dynamically (by calls
to std::setlocale).<br>
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As to UB. I'd like, if possible, to avoid creating new UB
classes. Some things should probably be ill-formed, like
unencodable characters. Others fall into existing UB, like
specifying an inline string literal with two different
encodings. Reading a string with the wrong encoding, I think,
should be at worst unspecified, unless for some reason your
decoder has UB, in which case it's the decoders problem, not the
incorrect or mixed encoding isssue. That said, I'd defer to Core
on this. <br>
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Wherever Core says we can get away with unspecified, I'm all for it.<br>
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Internal encoding is required to preserve distinct universal
character names and treat all representations of the same
universal character the same. So, the standard effectively
requires unicode, but in terms of observables. <br>
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<p>Agreed, I don't think anything is accomplished by trying to
prescribe implementation details.<br>
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<p>Tom.<br>
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<div dir="ltr" class="gmail_attr">On Sun, Sep 8, 2019 at 5:39 AM
Corentin Jabot <<a href="mailto:corentinjabot@gmail.com"
moz-do-not-send="true">corentinjabot@gmail.com</a>>
wrote:<br>
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<div dir="ltr" class="gmail_attr">On Sun, 8 Sep 2019 at
05:46, Tom Honermann <<a
href="mailto:tom@honermann.net" target="_blank"
moz-do-not-send="true">tom@honermann.net</a>>
wrote:<br>
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<div
class="gmail-m_1528546080172065353gmail-m_7194630821368723447moz-cite-prefix">On
9/5/19 9:41 PM, Steve Downey wrote:<br>
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<div dir="ltr">Because I needed to circulate what
I'm doing for Belfast, I've thrown together an
abstract for the paper we've peripherally
discussed about modernizing and tightening the
specification around encodings of characters
generally, and the source and execution character
sets. <br>
<br>
"<br>
This document proposes new standard terms for the
various encodings for character and string
literals, and the encodings associated with some
character types. It also proposes that the wording
used for [lex.charset], [lex.ccon], [lex.string],
and [basic.fundamental] 8 be modified to reflect
the new terminology. This paper does not intend to
propose any changes that would require changes in
any currently conforming implementation.<br>
"<br>
<br>
I'm hoping to have some preliminary work by the
next telecon. The direction I'm thinking is that
both Source and Execution Character Set are
descriptions of the abstract characters, selected
from 10646, that must be present to support C++.
Encodings, both source and execution, are
implementation defined. I would like to introduce
terminology to describe the encoding used when
translating narrow and wide character and string
literals. I'd also like to make it explicit
somewhere up front that there are associated
encodings for some, but not all, character types.
This is mentioned now in filesystem, but should be
moved to a section with wider scope. The encoding
for `char` and `wchar_t` is controlled by
`locale`. The encoding for the unicode character
types is fixed. The encoding used for literals was
chosen at compile time, and is implementation
defined. If locale and that endcoding conflict,
behavior is unspecified. Combining TU with
different encodings is in general unspecified,
unless it results in an ODR violation. <br>
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This all sounds great. My only question is behavior
being unspecified vs undefined. It seems challenging
to get away with making it only unspecified.<br>
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<div>Specifically, I'd like something along the line of:</div>
<div>If a character literal contains a c-char that do not
have the same representation in the character literal
encoding (aka *presumed" execution encoding) and the
execution encoding, the behavior is undefined.</div>
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Some possible terms:<br>
{"",Narrow,Wide} Literal Encoding - encoding on
char and string literals<br>
Dynamic Encoding - encoding implied by locale<br>
*Character Set - A set of abstract characters (
Latin Capital letter A, Digit Zero, Left
Parenthesis ...)<br>
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Unicode uses "character repertoire" for abstract sets
of characters. I favor following suit there.<br>
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<div>+1 to sticking to Unicode terms </div>
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<div dir="ltr">*Basic Character Set - minimum
required to be encoded<br>
*Extended Character Set - what can be encoded<br>
*Source Character Set - must be encodable in C++
source<br>
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I don't think "source character set" is defined
today. The closest we get is "Physical source file
characters" in <a
href="http://eel.is/c++draft/lex.phases#1.1"
target="_blank" moz-do-not-send="true">[lex.phases]p1</a>.<br>
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<div dir="ltr">*Execution Character Set - Source +
control characters<br>
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<div>Be careful not to break that code <a
href="https://stackoverflow.com/questions/5508110/why-is-this-program-erroneously-rejected-by-three-c-compilers"
target="_blank" moz-do-not-send="true">https://stackoverflow.com/questions/5508110/why-is-this-program-erroneously-rejected-by-three-c-compilers</a></div>
<div>More seriously i think it would be beneficial
(necessary even) to have a source character encoding /
character repertoire.</div>
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<div>I wonder if we could specified that the
internal character repertoire is Unicode. It kinda has
to be already make that clearer.</div>
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<div><br>
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<div>I would also propose</div>
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<div>Universal Character Name -> Unicode Code point<br>
</div>
<div>(character name should be reserved to the \N
proposal)</div>
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<div dir="ltr"> <br>
* Current terms, with what I think the actual
meanings are today.<br>
<br>
<br>
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<p>I think these are good. With these, there is no
need for a term like "execution encoding", correct?
At compile-time, "literal encoding" encodes
"execution character set" characters, and at
run-time, "dynamic encoding" encodes "extended
character set" characters, yes?</p>
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<div>I prefer "execution" to dynamic</div>
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<p>I like that this doesn't stray far from the
existing terms.<br>
</p>
<p>Tom.<br>
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