Ensure components::Bag will always generate a result

[gregtatum]

I'm modifying this big to be a bit more subtle, which is to ensure that every components::Bag returns a result. The appendItems support is one way of doing that, but currently the components bag may not return a result. This wouldn't be compatible with ECMA-402, so we should have a solution for it. This may be appendItems, or something else.

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Original discussion:

https://unicode.org/reports/tr35/tr35-dates.html#Matching_Skeletons

In case the best match does not include all the requested calendar fields, the appendItems element describes how to append needed fields to one of the existing formats. Each appendItem element covers a single calendar field. In the pattern, {0} represents the format string, {1} the data content of the field, and {2} the display name of the field (see Calendar Fields).

For the skeleton matching code, there is an appendItems list. It is used to add items to the matching patterns. This is the only way to add time zones to the patterns. However, there was some debate on the quality of results from using the appendItems list. In the 2020-02-05 ICU4X meeting, it was determined to not use this appendItems list. However, we clearly need it for time zones.

I'm labeling this issue as discussion, as we have some outstanding issues to figure out:

• Which appendItems must we support? Time zones are the minimum as of now.
• Which appendItems should we not support.
• What happens with Skeleton missing fields?
  • Right now we are actively not planning on supporting appendItems, as it can generate really bad localizations.
  • In the ICU4C implementation, the skeleton matching attempts a few smart heuristics like adding the minutes field if only hour and seconds are in the skeleton. Should we determine and document if we will fill in "holes" in the components::Bag like this?

[sffc]

I like using append items for time zones, since they are their own orthogonal component.

As far as eras, and other things like weekdays and fractional seconds... I would rather see a world where all of those patterns are already present in data, so we don't need to use append items at runtime. Either we get those patterns from translators, or we apply the append items at build time only (in the CLDR transformer) in order to make sure we cover all the key sets of fields.

[gregtatum]

I'm trying wrap my brain around the math of potential combinations. I'm documenting it here.

Combinations for "date, time, era, timezone, weekday"

date, era
date, era, time
date, era, time, timezone
date, era, time, timezone, weekday
date, era, time, weekday
date, era, timezone
date, era, timezone, weekday
date, era, weekday
date, time
date, time, timezone
date, time, timezone, weekday
date, time, weekday
date, timezone
date, timezone, weekday
date, weekday
era, time
era, time, timezone
era, time, timezone, weekday
era, time, weekday
era, timezone
era, timezone, weekday
era, weekday
time, timezone
time, timezone, weekday
time, weekday
timezone, weekday

[gregtatum]

Next, let's assume weekday will be included in either date or time during the initial field symbol matching.

date, era
date, era, time
date, era, time, timezone
date, era, timezone
date, time
date, time, timezone
date, timezone
era, time
era, time, timezone
era, timezone
time, timezone

[sffc]

Era should go inside date. It's strongly coupled with the year, like day period is strongly coupled with the hour.

I think we have either three of four orthogonal components:

1. Date
2. Time
3. Time Zone
4. Weekday (could be part of Date)

Then, within each component, we just need to exhaustively enumerate all valid sets of fields.

[gregtatum]

2020-04-02: Deep dive conclusion: Overall, appendItems for every field is something we do not want to support. Instead, the API should be a clear cartesian product of available inputs. We will still support appendItems as necessary "glue patterns" for certain fields, although it may eventually be rolled up into the data provider to generate the full cartesian product.

From my own perspective, this means that we can use them as needed for things like time zone, but to keep an eye out for ways to factor them out.

[sffc]

@gregtatum: Please file smaller follow-up issues to track the appendItems that we actually want to add. I am putting this issue on the v1 backlog to revisit once we have figured out the larger story for datetime skeletons.