In P1885, a registered character set is one that is in (at the point when the paper was written) the IANA character set registry. P1885 also provides static functions to query about the encoding used in either the translation or the execution environment. In some cases (involving subsets or supersets), there are questions of when an implementation should return a registered character set as the result of such static functions.
The environment-implements-superset case presents itself in relation to csBig5. The system encodings for "big5" on Windows and AIX contain characters that are not part of the common base of Big5; however, both are also missing characters from Big5-2003:
Big5-2003 has
U+7881 as F9 D6 and U+2460 as C6 A1.
Windows has U+7881 as F9 D6 but not U+2460 as C6 A1.
AIX does not have U+7881 as F9 D6 but does have U+2460 as C6 A1.
So, the environment-implements-superset case can, in practical terms, be generalized as being about divergent implementations of "charsets".
Of course, that generalization could also account for some environment-implements-subset cases; however, in addition to more mundane reasons, the environment-implements-subset case also arises from a technicality: It is questionable whether or not a POSIX environment that uses a UTF-8 encoding paired with a 2-byte (UCS-2) wchar_t can be said to have UTF-8 as the environment text encoding because the characters outside of the BMP cannot (based on wchar_t-representability) be considered members of the character set associated with the environment.
So it seems we have some questions:
Are the design goals better met or not by allowing divergent implementations of "charsets" to be identified as being the same registered character set?
When an implementation indicates a specific environment encoding, do the design goals require that all associated characters or members of the associated code space be wchar_t-representable?
It may be useful to characterize the questions as whether the result of the static functions are meant to be more of a hint (with few guarantees) or more of a promise.
I think we talked about this before, but as you outlined, mapping an encoding name to a specific charset or encoder sometimes
requires out-of-band information about the platform where the text was created.
IANA implies uniqueness and some encodings are registered with a precise mapping (rfc2978) - also in a few cases tracking what that mapping is is difficult.
> Each assigned name MUST uniquely identify a single charset. All
charset names MUST be suitable for use as the value of a MIME content
type charset parameter and hence MUST conform to MIME parameter value
syntax. This applies even if the specific charset being registered
is not suitable for use with the "text" media type.
But that is unfortunately not the case for Big5.
The issue is that these things were registered after being widely deployed by several vendors, so we are left
with minor implementation divergence.
I do not think it needs wording, or special care.
If a vendor considers that their character set maps to a registered IANA character set, they should be able to express it under P1885 - I don't think that will lead to more abuse
as the current situation.
Having the standard written as if the ambiguity does not or should not exist when we fully intend that it does (because we can't practically prevent it) is not helpful. Also, "should be able to" is different from "should".
I believe wording should be present:
An implementation may provide a return value representing a registered character set in lieu of one representing an unregistered variant. When the unregistered variant is the traditional realization of the registered character set in the context of the implementation, an implementation should provide a return value representing the registered character set.
In addition to the encoding used, the implementation may further restrict the set of valid characters. In the absence of a conventional name for the encoding as restricted, implementations should provide a return value without regard for the restriction,
I for some reason missed this email
> When the unregistered variant is the traditional realization of the registered character set in the context of the implementation, an implementation should provide a return value representing the registered character set.
I am struggling to understand what we are allowing here
Can, eg "Big5" be returned on windows? Yes, there is no precise definition of Big5 that can comprehensively account for all code points without considering the platforms. And this is probably the right answer to give to users.
Can, eg "Shift-JIS" be returned on windows? Yes technically. But the Windows implementation of shift-jis is very well documented, with a specific name that is registered. Implementations and users will expect that to be returned instead.
> unregistered variant is the traditional realization of the registered character set.
This is ill-defined. How do you define that when not all-registered charsets have an actual associated charsets - Many times only a name is registered.
What is considered a variant and what is considered traditional?
> an implementation should provide a return value representing the registered character set
Maybe. Depending on the specific encoding, an implementation may want to do something different that more closely matches the expectations of users on that platform.
Given the many many encodings, a lot are only separated by one or two codepoints. How in that context do we define variants?
> In addition to the encoding used, the implementation may further restrict the set of valid characters
I am not sure I understand the goal of this sentence. P1885 is purposefully somewhat removed from precise character sets. For which sets of operations would that restriction apply?
> In the absence of a conventional name for the encoding as restricted, implementations should provide a return value without regard for the restriction,
Again, how do you define what's a conventional name?
Trying to constrain implementation freedom in a field that is plagued by 70+ years of legacy, special cases and exceptions is a minefield.
I would like to better understand
- What useful scenarios are allowed by this wording
- What problematic scenarios are prevented by this wording
Thanks a lot for your feedback,
Corentin
For users it means that implementing a function that would return some kind of transcoder from a name requires special care