*Proposal summary* Change the callback function signature for `Calendar.strftime/3` options to pass both a number and the first arguments calendar.
*Explanation* Today `Calendar.strftime/3` allows a function to be provided as an option to any of the keywords. This is very helpful when working with localised calendars because a simple function call can return all the data relevant to the localised date. For example: iex> Calendar.strftime ~D[2020-01-30 Cldr.Calendar.US], "%a", MyApp.Cldr.Calendar.strftime_options!() “Fri" But there’s an issue: That date is actually a Thursday, not Friday! So what’s going on? Internally `Calendar.strftime/3` is calling `Date.day_of_week/1`. That function returns a number representing the nth day of the calendar-relative week (so-called ordinal day). There is the same issue with month of year because not all calendars start in January. For example, the Australia tax year calendar starts in July. Since the US official calendar starts its weeks on Sunday - unlike Calendar.ISO which starts its weeks on Monday (like many territories) - the day name lookup failed to show the correct result. This is because the callback didn’t have the calendar context to know what the default start-of-week day is. Now that can be worked around by specifying the calendar in MyApp.Cldr.Calendar.strftime_options!/1 like this: iex> Calendar.strftime ~D[2020-01-30 Cldr.Calendar.US], "%a", MyApp.Cldr.Calendar.strftime_options!(calendar: Cldr.Calendar.US) “Thu" But that feels an error prone. Perhaps it would be better to pass the calendar of the date/datetime/time argument to the callback functions instead? Today those functions receive a number only. This proposal is to have the functions receive both a number and the calendar of the first argument (which might be nil). This would give appropriate context to the 4 out of the 5 callback functions that would benefit from it (2 each for month name and day names). *Precedent* This would be a breaking change for the callback signature - but not for the Calendar.strftime/3 public function. There is some limited precendent. For example when the `Calendar.day_of_week/3` callback changed to be `Calendar.day_of_week/4`. That change affected the implementation of alternative calendars, but it did not affect consumers of those calendars. -- You received this message because you are subscribed to the Google Groups "elixir-lang-core" group. To unsubscribe from this group and stop receiving emails from it, send an email to elixir-lang-core+unsubscr...@googlegroups.com. To view this discussion visit https://groups.google.com/d/msgid/elixir-lang-core/7268adf3-689b-4c6a-acf3-0831350a66b3n%40googlegroups.com.
