How to support multiple week calendars in Elixir?

[josevalim]

@kip said:

Calendar module responsibilities

I think a calendar should encapsulate everything relevant to calculating days, weeks, months, years etc. That is, any configuration should be baked into the module that implements the Calendar behaviour.

This seems obvious when we talk about the iSO calendar versus the Balinese , or Ancient Egyptian or the Islamic calendars. But its less obvious if we consider the myriad of calendars derived from the Gregorian.

For example, in the US, a company may have a fiscal calendar that starts on an arbitrary day of the year. All months, quarters, weeks and days are calculated from this date. Cisco Systems has a financial year calendar that ends on the last Saturday of July. There are a whole class of calendars called 445 calendars and their cousins the 454 and 544 which are common in retail and are designed to make it better for comparing this year to prior year periods.

Although I started thinking this would be best done as configuration passed around in each structurally compatible Date , Time or DateTime struct or map, the variations are too many. I now feel it is better that this detail be backed into a module that implements the Calendar behaviour. This doesn’t make implementing a calendar any more complex. It does potentially have the side effect of having to generate calendar modules at runtime for some use cases.

And then @josevalim said:

I am not sure I agree with this.

The instants are the same and they are represented by the same year, month, day, hour, minute and second, so it does not feel to me they should be different calendars. To me, it is the same calendar, but it wants to be traversed or viewed in different ways. My suggestion would be to explore a WeekTraverser or WeekBuilder or some sort of behaviour that focuses on exploring a given calendar through different interpretations of its weeks. It may be even something that can be implemented on top of iso days (and therefore calendar agnostic)?

Which @kip finally replied:

Totally ok with not agreeing. I just can’t reconcile how to do what you suggest and still use the Date , Time and DateTime standard structs which have a :calendar field only. Also every calendar is basically just a different representation of the same moments in time so by extension aren’t you saying there is only one calendar and multiple different representations that model year, month, day in alternate ways?

I will continue the discussion below.