feat: add comparison function for Duration

[yordis]

Context

I recently discovered that using <= did not work (unless I misunderstood something), so I tried to look for Duration.compare/2 to match other modules such as DateTime.compare/2

iex> remaining_duration = Duration.new!(second: 18)
...> threshold_duration = Duration.new!(second: 10, microsecond: {0, 6})
...> remaining_duration <= threshold_duration
true

[kevinschweikert]

Wouldn't it be better to align with DateTime.compare/2 and return :lt | :eq | :gt?

[v0idpwn]

This is not considering microseconds, probably should. Also has issues with months and years

[maennchen]

I don't think we can compare durations without knowing the date context since months are not always 30 days long.

As an example:

duration_a = Duration.new(month: 1)
duration_b = Duration.new(day: 30)

Duration.compare(duration_a, duration_b) # => :eq
Date.shift(~D[2020-02-01], duration_a)  #=> ~D[2020-03-01]
Date.shift(~D[2020-02-01], duration_b)  #=> ~D[2020-03-02]

[maennchen]

And for DateTime.shift it might even depend on the timezone if there's DST changes.

[yordis]

@maennchen yeah I was wondering if year must be kept outside the comparing; the Kernel.to_timeout/1 already raise an exception based on month and year ... It would be the same with days 😭

[maennchen]

We could have something like this for convenience:

def compare(relative_to) do
  fn a, b ->
    # calc based on relative_to
  end
end

But essentially all you would be doing is:

[Naive]Date[Time].compare(
  [Naive]Date[Time].shift(relative_to, a),
  [Naive]Date[Time].shift(relative_to, b)
)

I'm wondering if the better course it to just add some docs pointing users to compare shifted dates instead of comparing durations directly and explain why.

[nwjlyons]

How will this work with months? The number of days in a month depends on a relative date.

eg.

Duration.compare(Duration.new!(day: 30), Duration.new!(month: 1))

You could propose this in the mailing list for discussion https://github.com/elixir-lang/elixir?tab=readme-ov-file#proposing-new-features

[josevalim]

You cannot compare durations without having a reference point. For example, this says that 12 months is the same as 360 days, but if I add these two durations to the same date, I will get different results, which therefore means they are not the same.

Depending on what you want to compare, it may be best to convert them to timeouts and then compare it.

[yordis]

As I was adding more test I realized why it wasn't included ...

[axelson]

It would be nice to give some suggestions/guidance for how to compare durations in the module doc.

[yordis]

@axelson yeah ... the mistake made of comparing duration as I shown could be a classic situation to happens 😭

[josevalim]

I have pushed some docs. FWIW, the type system should be getting better and better at catching those comparisons too and warning on them.