In support of Instant.minus(Instant)

[Éamonn McManus]

I'd say too that this makes intuitive sense based on algebra. If we have:
*instant1* + *duration* = *instant2*
then we can subtract *duration* from both sides:
*instant1 = instant2 - duration*
or we can subtract *instant1* from both sides:
*duration = instant2 - instant1*

There's no manipulation we can do that would cause us to try to add
instants together, and it's a bit surprising for the API to allow the first
subtraction but not the second.
I also think that if I see instant2.minus(instant1) it's immediately
obvious to me what that means, while instant1.until(instant2) seems both
less discoverable and less obvious.

On Thu, 2 May 2024 at 10:29, Louis Wasserman <lowasser at google.com> wrote:

> That doesn't follow for me at all.
>
> The structure formed by Instants and Durations is an affine space
> <https://en.wikipedia.org/wiki/Affine_space#Definition>, with instants
> the points and durations the vectors.  (An affine space is a vector space
> without a distinguished origin, which of course Instants don't have.)  It
> is 100% standard to use the minus sign for the operation "point - point =
> vector," even when "point + point" is not defined, and to use all the other
> standard idioms for subtraction; the Wikipedia article uses "subtraction"
> and "difference" ubiquitously.
>
> Personally, I'd be willing to live with a different name for the
> operation, but consider "users keep getting it wrong" a strong enough
> argument all by itself for a version with the swapped argument order; it's
> not obvious to me that another API with the same argument order adds enough
> value over Duration.between to bother with.
>
> On Thu, May 2, 2024 at 10:04 AM Stephen Colebourne <scolebourne at joda.org>
> wrote:
>
>> On Thu, 2 May 2024 at 15:58, Kurt Alfred Kluever <kak at google.com> wrote:
>> > instant − instant = duration // what we're discussing
>> > instant + duration = instant // satisfied by instant.plus(duration)
>> > instant - duration = instant // satisfied by instant.minus(duration)
>> > duration + duration = duration // satisfied by duration.plus(duration)
>> > duration - duration = duration // satisfied by duration.minus(duration)
>> > duration × real number = duration // satisfied by
>> duration.multipliedBy(long)
>> > duration ÷ real number = duration // satisfied by
>> duration.dividedBy(long)
>> >
>> > All but the first operation have very clear translations from
>> conceptual model to code. I'm hoping we can achieve the same clarity for
>> instant - instant by using the obvious name: instant.minus(instant)
>>
>> But you can't have
>>  instant + instant = ???
>> It doesn't make sense.
>>
>> This is at the heart of why minus isn't right in this case.
>> Stephen
>>
>
>
> --
> Louis Wasserman (he/they)
>
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