Add support for DisplayContext

[sffc]

ICU has a concept called "DisplayContext", which controls the capitalization rules for display names, dates, and numbers. We should consider adding support for this across ECMA 402.

Discussion with regard to Intl.DisplayNames: tc39/proposal-intl-displaynames#13

[littledan]

Does this apply to other things, such as NumberFormat and DateFormat, as well?

[sffc]

The short answer is yes; the casing, mood, etc., of all types of formatted output might depend on the context in which the output is displayed.

This feature will require investigating the variables that could affect formatted output, and then adding the option/options across most Intl formatters. We may also want to consider making this more of a first-class feature such that developers don't overlook it. Ideally, any call site to an affected formatter will declare its display context.

[sffc]

@younies @hugovdm This could be something to consider along with the inflections work you are doing ("Project Morpheus").

[sffc]

+1 to this feature from an internal client. The client is using Closure Templates and goog.i18n.DateTimeFormat to format dates. They reported:

During the localization testing, we found that the months are not capitalized in NL, PL and FR. ... Our linguist says that even though the months in these languages should not be capitalized when they appear within a sentence, they should be capitalized when they appear stand-alone in a list, where other words are capitalized.

internal cross-reference: http://yaqs/eng/q/5813676027281408

[zbraniecki]

During the localization testing, we found that the months are not capitalized in NL, PL and FR. ... Our linguist says that even though the months in these languages should not be capitalized when they appear within a sentence, they should be capitalized when they appear stand-alone in a list, where other words are capitalized.

Interesting. I can only speak for Polish but I'd challenge that linguist. In Polish, names of months are always not capitalized. Standalone and in the middle of the sentence. They should only be capitalized at the beginning of a sentence.

That, of course, may be different for other affected locales and doesn't discount the value of displayContext - just pointing out that standalone/sentence is not all, we'll likely need at least standalone/middle-of-the-sentence/beginning-of-the-sentence.

[littledan]

In this case, let's consider which parts of display context might be handled by a separate title case proposal.

[sffc]

The main problem that DisplayContext solves is that which capitalization scheme to apply is function- and locale-dependent. For example, in some locales, you might titlecase month names, but in other locales, you display them in lowercase.

@macchiati has been a big advocate for using DisplayContext more universally across the board, and bubbling it through the hierarchy of formatters.

[littledan]

@sffc This seems important to understand. I'm missing background in this area. Do you have any recommendations for where to read about the motivation for this effort, and why it doesn't "decompose" into case conversion routines?

[sffc]

TG2 discussion: https://github.com/tc39/ecma402/blob/main/meetings/notes-2024-10-24.md#displaycontext