Add builder with `MonthMatch`

[vrom911]

It would be helpful to have a builder with which you could use MonthMatch.
For example, in co-log we needed to have months in 3-letter format (like Dec, Jan, etc.), so we implemented a builder manually:
co-log/co-log#166

If you think that it makes sense to have in the chronos library, I could help with the implementation as soon as we agree on the API (to have an additional field in the DatetimeFormat or to explicitly take a Maybe (MonthMatch a) as an argument, or something else?).

Let me know what do you think.

Oh, by the way, while implementing the builder, I have noticed that functions like zeroPadDayOfMonth etc. are not exported. Would you consider exporting them, or there is a reason not to do that? (if this question if offtopic for this issue I totally understand and could create a new one if you think that this idea makes sense at all).

[andrewthad]

I think this is the function I would want:

builder_DmyHMSz_m ::
     OffsetFormat -> SubsecondPrecision -> DatetimeFormat
  -> OffsetDatetime -> MonthMatch -> Builder

This is similar to the builderDmyHMSz function you have in co-log except that the formatting things are configurable.

Concerning exporting zeroPadDayOfMonth, I don't think chronos is the right place for this. I would rather push this into a different library and have a function like:

zeroPaddedTwoDigit :: Int -> Ptr CChar

as a part of its public API, and then chronos would depend on that library.

[vrom911]

Great!

The only concern I had for the builder_DmyHMSz from chronos (apart from the MonthMatch) is that it doesn't have a separator option to separate offset with a space as we do now. Do you think that we can include this option somewhere as well?

[andrewthad]

For anyone browsing this issue, an example of the desired format (taken from co-log) is 03 May 2019 05:23:19.058 +00:00. I had forgotten about the space. What I've been doing for spaces is using underscores in the function names. So rather than builder_DmyHMSz_m, how about chronos having builder_DbyHMS_z? The rational being:

• Underscore before z means there's a space before the offset.
• The letter b is taken from the man page for date(1), which uses %b to mean "locale's abbreviated month name (e.g., Jan)" and %B to mean locale's full month name (e.g., January). Here, we would just use b to mean both of these since MonthMatch can store both abbreviated and unabbreviated localizations.