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Subject: Re: Bike shedding for Christmas: P1885 Naming Text Encodings
From: Tom Honermann (tom_at_[hidden])
Date: 2019-12-29 22:17:06

On 12/28/19 6:38 PM, Corentin Jabot via SG16 wrote:
> On Sun, Dec 29, 2019, 00:32 Corentin Jabot <corentinjabot_at_[hidden]
> <mailto:corentinjabot_at_[hidden]>> wrote:
> On Sat, Dec 28, 2019, 22:21 Tom Honermann <tom_at_[hidden]
> <mailto:tom_at_[hidden]>> wrote:
> On 12/27/19 6:28 AM, Corentin Jabot via SG16 wrote:
>> Hello
>> In P1885, I introduce the name "text_encoding"  for the class
>> representing the name of a text encoding.
>> I wonder whether that might conflict or interfere with actual
>> encoding/decoder classes and would like your opinion.
>> Here are a few possible names:
>> * Charset (IANA nomenclature, posix)
>> * text_codec (Qt)
>> * text_encoding
>> * text_encoding_name (encoding is used by posix / python /
>> Unicode nomenclature would favor encoding (Unicode is a
>> charset of which utf-8 and utf-16 are both are encodings)
> I suggest text_encoding_id.  I'd like to preserve
> text_encoding for a tag type (or concept) that can be used at
> compile time to specify a (compile-time) encoding as in a
> template parameter to std::text.
> Tangent 1: the proposed text_encoding is not extensible, at
> least not in a very meaningful way.  I suggest we do one of
> the following:
> 1. Remove the text_encoding(const char*) constructor.  It
> doesn't allow setting the MIB ID, so is unsatisfactory at
> present.
> 2. Allow first class extension by, for example, reserving the
> full range of IANA MIB values, defining a "private use"
> range of values, and modifying the text_encoding(const
> char*) constructor to also accept a MIB value (and perhaps
> make the name parameter optional such that, if specified,
> it would override the internally known name for the
> provided MIB value and if not specified, name() would
> return a suitable default).
> It is extensible in multiple-choice ways:
> - implentation can provide their own aliases for existing mib
> - the other mib + custom name can be used to use a non register
> encoding. Hence the existence of both unknown and other
> I will not support custom mib as it is not in line with the rfc - 
> the mib being a way to standardize names.
I'm not sure whether you are referring to RFC 2978 or 3808 here.
Regardless, the design of both the RFCs and your paper is such that
there are multiple possible names for any encoding.  A program that
wishes to identify an encoding for which the implementation does not
supply a MIB ID would have to be expected to recognize all aliases. 
That doesn't seem realistic to me. Additionally, as specified, the
proposal only includes enumerators for a select subset of the IANA
registered names with no facility (other than implementation provided
extension) for creating a text_encoding object with a MIB ID for any
other IANA registered encoding.

Refined suggestions:

 1. Include all IANA MIB IDs in the set of enumerators of
    text_encoding::id.  This may require either a normative reference to
    (a dated version of) the IANA registry or that we copy from a
    version of the IANA registry and update it for each C++ standard. 
    Note that support for names need not imply support for the named
    encoding; these are just identifiers.
 2. Use the IANA "cs" prefixed names for the names of the enumerators of
    text_encoding::id.  They may not be the prettiest names, but using
    these names will better facilitate automation and they are intended
    to be used as identifiers.
 3. Provide a text_encoding(text_encoding::id) constructor that enables
    creation of an instance with a particular ID.  If the implementation
    doesn't have a registered name for the provided ID, then use a name
    like "MIB#42".

> Encoding have names, mib is very close to an implentation
> details). For that same reason the name cannot be optional. The
> name in parameter _always_ take precedence over the iana name so
> it can roundtrip to iconv or similar APIs

> To rephrase that:
> Different names may map to the same mib but two encoding with the same
> names have to compare equal.

I agree with that, but there is also present in the proposal that two
encodings with different names that map to the same MIB ID compare
equal.  The currently proposed design has the following issue:  IANA
doesn't have a registration for the WTF-8 encoding today, but it is
conceivable that it could be registered in the future.  As proposed,
text_encoding("WTF8") != text_encoding("WTF-8"), but if the encoding
were to be registered in the future with both of those names as aliases,
then they would compare equal despite having different names.  It seems
to me that either 1) text_encoding shouldn't have equality comparison
operators (though text_encoding::id should) or, 2) text_encoding should
be split into two facilities, one that stores an integral ID, and
another that performs name lookups and returns a value of the former. 
The latter would also facilitate dynamic registration of additional names.

> 1.
>> if text_encoding remains the name of that class,
>> encoder/decoder can be used for the class doing the actual
>> conversions.
>> I will further rename "system" to "environment" to be more
>> generic and aligned with POSIX.
> Is text_encoding::system() intended to be equivalent to
> text_encoding::for_locale(std::locale{})? (I think the answer
> is, and should be, no; e.g., on Windows, this would query
> GetACP()).
> It would query getacp which is equivalent to query the user ("")
> locale at the start of the program.
Good, that sounds right.
>> (user, environment and system are, for our purpose synonym
>> and intended to mean "the encoding assumed and expected by
>> whatever launched our program).
>> Environment has the added benefit that it implies neither
>> user or systems which makes it more friendly to
>> embedded platforms
> Since locale settings are generally determined by environment
> (variables), use of the term "environment" may be confusing. 
> I prefer system.
> Tangent 2: I don't recall if we discussed this in Belfast, but
> the paper identifies three sets of encodings to expose
> (literals, system, locale).  A fourth would be
> terminal/console encoding.  This encoding can be easily
> queried on Windows, but not on Linux/UNIX (though terminal
> encoding rarely differs from locale there, so it would be
> reasonable to just return the system encoding).
> I considered that a few days ago. I am not aware of it being a
> thing on other platforms than windows (I am probably wrong) and I
> don't believe we can come up with a nice API in the short term to
> query that nicely.
It is a thing on other platforms in that terminals (typically) have an
encoding setting that specifies how characters are translated for
input/display purposes.  What isn't common is a facility for querying
the terminal for its encoding setting (as far as I can tell, there is no
terminfo capability specified, nor have I found escape sequences that
can request it though there are escape sequences for some terminal types
to provide a character set map).


> Tom.
>> Thanks for your input,
>> Corentin

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