Haskell time tutorial - #152

Is this specific to Ruby on Rails or does Ruby also has the time utilities? I'm getting results for Ruby on Rails, but can you confirm if that's the case? Just curious.

Yes as I far as I know this was only available in Ruby on Rails, @sestrella could you help us clarifying this?

@felixminom yes, this is a feature only available in the context of Rails since by default Ruby does not provide these methods:

Rails (REPL)

➜  bundle exec rails c
Running via Spring preloader in process 77374
Loading development environment (Rails 6.1.4.1)
irb(main):001:0> Date.today.all_month
=> Mon, 01 Nov 2021..Tue, 30 Nov 2021

Ruby (REPL)

➜  irb
irb(main):001:0> require "date"
=> true
irb(main):002:0> Date.today.all_month
Traceback (most recent call last):
        4: from /nix/store/7nja5rmprz8sb72zvjbc1v85rswwsi4j-ruby-2.7.4/bin/irb:23:in `<main>'
        3: from /nix/store/7nja5rmprz8sb72zvjbc1v85rswwsi4j-ruby-2.7.4/bin/irb:23:in `load'
        2: from /nix/store/7nja5rmprz8sb72zvjbc1v85rswwsi4j-ruby-2.7.4/lib/ruby/gems/2.7.0/gems/irb-1.2.6/exe/irb:11:in `<top (required)>'
        1: from (irb):2
NoMethodError (undefined method `all_month' for #<Date: 2021-11-26 ((2459545j,0s,0n),+0s,2299161j)>)
irb(main):003:0>

Rails monkey patch some types in the std library

Note that the introduction for this example (and the Haskell code) is for months, but the Ruby on Rails example is for weeks. Also, I don't think this is comparing the same thing: the Haskell one is the implementation of the function, but the Ruby on Rails one is the call to that function.

To make it clearer, I guess the Haskell one could include how it looks using GHCi:

> monthBoundaries (read "2021-12-05")
(2021-12-01,2021-12-31)

(I'm not sure if that's exactly how it would look like.)

Good catch I will somehow highlight the implementation with the new approach

Can you add a link to the time library in Hackage?

I think this is still missing something to make the Haskell and Ruby codes comparable. The Haskell one shows the implementation (source), but the Ruby one shows how to use the implementation.

Assuming you have the Haskell implementation (which is the one in the example), then the Haskell equivalent would be something like this:

> today <- utctDay <$> getCurrentTime
> monthBoundaries today
(2021-11-01,2021-11-30)

Which is not that different compared to the Ruby one (ignoring that I skipped over the next month part, but the important thing is that the things being compared should match).

I think that the issue that you're trying to highlight is that with Haskell you have to implement the code because it's not part of the time library.

This makes a lot of sense in that case I will rename the function. The RoR version actually returns a range so I will name the Haskell function monthAllDays and I will make it return a whole month (I was returning just starting and ending days). Also latter on, we can compare this with the periodAllDays that was implemented in the library, wdyt ?

Can you add a link to the documentation?

I think this title needs to be reworded. Maybe “Examples of some new time features”?

What happens if you indent this block so that it's part of the item? I think it should look better.

It worked :)

@felixminom I'm more than OK about not mentioning my name here since it was a team effort

@felixminom since this is a short code snippet I think it is clear that the first element of the tuple corresponds to the first day and the second one to the last day, in that sense, maybe we could get rid of MonthFirstDay and MonthLastDay type aliases? @jpvillaisaza what do you think?

Yes, monthBoundaries :: Day -> (Day, Day) sounds better for the tutorial.

As I mentioned before I'm going to make both functions to exactly match, so now it will be something like this: allMonth :: Day -> [Day]

Maybe it is just me, but moving the let statement to the next line makes the code more readable?

monthBoundaries :: Day -> (Day, Day)
monthBoundaries day = 
  let (y, m , _) = toGregorian day
  in (fromGregorian y m 1, fromGregorian y m 31)

I'm not quite sure about the intention of foo in this context, however, if you are trying to highlight the fact that November and 11 are equivalent, I think could do something like this perhaps:

> November == 11
True

Since you already introduced patterns before, I think we could use it here to deconstruct a Day:

dayToHoliday :: Day -> String
dayToHoliday (YearMonthDay _ January _) = ...
dayToHoliday (YearMonthDay _ November 1) = ...
dayToHoliday (YearMonthDay _ December 25) = ...
...

For showcasing the usage of a function I think is useful to use doctest syntax or something similar like a REPL output:

> dateToHoliday 12 25
"Merry Christmas jojojo"

Since the order changed (from Haskell then Ruby on Rails to Ruby on Rails then Haskell), can you make it clearer that the first one is Ruby on Rails? Here's a possible way to make that clearer:

Or invoke.

Also, can you either mention both ghci and stack repl or just say ghci? I guess Stack is not required for this.

I'm guessing you meant now here.

I don't think this is being used for other examples, but I think switching to colon before code snippets should be better.

@jpvillaisaza answering since it does not let me to do it directly in your comment. Regarding the laws all the ones you mentioned are required, the ones I added are more like properties that some instances should satisfy. Not really sure how to proceed with this one.

Okay, that sounds good.

Let's try to write all the sentences in the third-person point of view.

@sestrella Can you elaborate on this comment? I'm not sure if you mean something about the “I” or the “we” here. Just making sure this is clear.

s/latter/then (but perhaps rephrasing should be better, just something other than latter, which is a typo)

Or wants, I don't know which one you meant.

that really don't know

Something's missing here (that don't have a clear purpose?).

Also, why doesn't fromGregorian have a clear purpose? If it doesn't, is the contribution or the YearMonthDay the one adding something clearer (I'm only asking about fromGregorian)?

There are two functions and a pattern to build a Day in the library. Those are fromGregorian, fromJulian and the YearMonthDay pattern. When we started working on this we were using the fromJulian function that has some really weird behavior, specially when the Date needs to be clipped. So I wanted to somehow highlight that using this function is not as straightforward as the way it is right now.

Is it YearMonthDay instead of MonthOfYears here?

Actually are both, I will rephrase this.

typo here: aate -> date

Let's capitalize the first letter here

lenght -> length

Let's add a space after the comma (,)

duplicated word in