[swift-evolution] Feature proposal: Range operator with step

[Joe Groff]

> On Mar 30, 2016, at 11:42 AM, Dave Abrahams <dabrahams at apple.com> wrote:
> 
> 
> on Wed Mar 30 2016, Joe Groff <jgroff-AT-apple.com> wrote:
> 
>>> On Mar 30, 2016, at 8:50 AM, Dave Abrahams <dabrahams at apple.com> wrote:
>>> 
>>> 
>>> on Tue Mar 29 2016, Joe Groff <jgroff-AT-apple.com> wrote:
>>> 
>> 
>>>>> On Mar 28, 2016, at 5:33 PM, Brent Royal-Gordon <brent at architechies.com> wrote:
>>>>> 
>>>>>>> Floating-point seconds (as NSTimeIntervals) are the natural
>>>>>> Strideable.Stride, but it's not particularly clear to me that you
>>>>>> want 1 second to be a default stride. It's the default you would
>>>> 
>>>>>> guess, but it's not actually a particularly useful default.
>>>>>> 
>>>>>> Any fixed-time-period stride with dates is fraught with peril. Not
>>>>>> every day is 24 hours, not every minute is 60 seconds, etc. Working
>>>>>> with dates requires enough special domain knowledge that I think
>>>>>> it'd be harmful to try to genericize numeric concepts over it.
>>>>> 
>>>>> While this is true, "ten seconds from now" is always ten seconds
>>>>> from now, and "seconds between date1 and date2" is always the same
>>>>> number of seconds. There is a basic level of time measurement and
>>>>> manipulation which is completely independent of time zones and
>>>>> calendars; that's what NSDate and NSTimeInterval represent. They are
>>>>> needed fairly often, and they are perfectly compatible with
>>>>> Strideable's semantics.
>>>> 
>>>> Perhaps, but if you make Date strideable by seconds and automatically
>>>> receive a bunch of utility methods based on that, then it becomes
>>>> really tempting to abuse absolute time periods, or to accidentally
>>>> misuse generic Strideable utilities instead of calendar-aware ones. We
>>>> don't make String a sequence for similar reasons (though perhaps, by
>>>> analogy to String, there could be
>>>> `seconds`/`days`/`solarMonths`/`lunarMonths`/etc. views that are
>>>> Strideable).
>>> 
>>> Except that collections aren't Strideable.  A strideable type is a
>>> value that has an implied unit of measure so that you can offset it
>>> without reference to any collection.
>> 
>> Who said anything about collections?
> 
> That's what those views are.

Sure, but I was making an analogy. String isn't a Collection by itself because operating on pieces of a string usually requires a lot of domain-specific knowledge, yet we provide Collection views that present it as a collection of user-chosen units. Date (IMO) shouldn't be Strideable for similar reasons, since correctly manipulating dates requires a lot of domain-specific knowledge, yet we could provide Strideable views of a Date that advances by user-selected intervals. I wasn't claiming Date was a collection.

-Joe