In support of Instant.minus(Instant)

[Roger Riggs]

Hi,

I would also reinforce Stephen's early observation that the pattern for 
"until" methods in java.time includes those of the XXXDate classes, with 
a single Temporal parameter.  Period and Duration are similar values 
holding relative TemporalAmounts.

     public Period until(ChronoLocalDate endDateExclusive)

In addition to Instant, the LocalTime class might also benefit from adding:

     public Duration until(LocalTime endExclusive)`

The API design of java.time included an emphasis on consistent naming 
across the packages.

Regards, Roger


On 5/2/24 4:01 PM, Naoto Sato wrote:
> `Temporal` interface is clear that its `minus` methods return objects 
> of the same `Temporal` type, and `until` calculates the amount of time 
> until another `Temporal` type. Introducing `Instant.minus` that 
> returns `Duration` would be confusing to me.
>
> Naoto
>
> On 5/2/24 10:41 AM, Éamonn McManus wrote:
>> 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 
>> <mailto: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 <mailto:scolebourne at joda.org>> wrote:
>>
>>         On Thu, 2 May 2024 at 15:58, Kurt Alfred Kluever <kak at google.com
>> <mailto: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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