Compute quarter boundaries based on a given day/quarter

[sestrella]

The addition of a module to handler quarters is very useful. As a Rubyist (and Haskeller) I like to come back to Ruby in search of features to implement in Haskell and I found the following one pretty useful:

irb(main):002:0> Date.current.all_quarter
=> Thu, 01 Jul 2021..Thu, 30 Sep 2021

An equivalent Haskell version would look something similar to:

allQuarter :: Day -> (Day, Day)
-- or
allQuarter :: Year -> Quarter -> (Day, Day)

I understand this is a core library and this feature might get rejected, but dealing with quarters seems like a pretty common use case. I'll be more than happy to submit a PR if this feature gets approved.

Reference

• https://api.rubyonrails.org/v6.1.4/classes/DateAndTime/Calculations.html#method-i-all_quarter

[AshleyYakeley]

This or something like it makes sense.

[AshleyYakeley]

Number of ways we could go with this. For example, we currently have:

type DayOfMonth = Int
pattern MonthDay :: Month -> DayOfMonth -> Day -- bidirectional

So we could do this:

type DayOfQuarter = Int
pattern QuarterDay :: Quarter -> DayOfQuarter -> Day -- bidirectional

Alternatively (or additionally), we could do:

class HasDays t where
    firstDay :: t -> Day
    lastDay :: t -> Day

lengthDays :: HasDays t => t -> Int
lengthDays t = succ $ diffDays (lastDay t) (firstDay t)

instance HasDays Year
instance HasDays Quarter
instance HasDays Month
instance HasDays Day

[sestrella]

Great, thank you for the ideas, I'm going to try the first approach

[AshleyYakeley]

Going to do QuarterDay pattern as well I think.

[AshleyYakeley]

OK this is done now.

[sestrella]

@AshleyYakeley thank you for all the follow-up work renaming the type class and moving class instances close to the type definitions, I was in doubt of following that path or creating a separate Instances module like FormatTime and ParseTime. I'm planning to submit a PR adding some property/unit tests for those functions that have DayPeriod as a type constraint if that is OK with you?