In support of Instant.minus(Instant)

[Louis Wasserman]

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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