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

[Dave Abrahams]

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.

-- 
Dave