<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class="">I vote for (2). Thats what I would have expected. `start` changing from under you is weird.</div><br class=""><div><blockquote type="cite" class=""><div class="">On 21 Jun 2016, at 18:44, Tony Parker via swift-corelibs-dev <<a href="mailto:swift-corelibs-dev@swift.org" class="">swift-corelibs-dev@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><meta http-equiv="Content-Type" content="text/html charset=utf-8" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">Hi Greg,<div class=""><br class=""></div><div class=""><div class=""><blockquote type="cite" class=""><div class="">On Jun 20, 2016, at 9:04 PM, Greg Titus <<a href="mailto:greg@omnigroup.com" class="">greg@omnigroup.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><meta http-equiv="Content-Type" content="text/html charset=utf-8" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class="">One domain where this happens is project planning back from a fixed end date. The most well-known of these would be rocket launches: “T minus 10 seconds” is exactly a negative time interval from an end date.</div><div class=""><br class=""></div></div></div></blockquote><div class=""><br class=""></div><div class="">That makes sense.</div><div class=""><br class=""></div><div class="">Let’s say we added negative interval support to the type. I could see it working in two ways:</div><div class=""><br class=""></div><div class="">1. You initialize with a start and negative duration. After this, the ‘end’ property becomes what you used for ‘start’ and ‘start’ is adjusted according to the interval.</div><div class="">2. We keep the start date to always be what you passed in but allow a negative value for timeInterval and allow for end to be < start.</div><div class=""><br class=""></div><div class="">Which of these would be the least confusing?</div><div class=""><br class=""></div><div class="">- Tony</div><br class=""><blockquote type="cite" class=""><div class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class="">That being said, it’s a fairly uncommon use case and I wouldn’t change the API to support it on an equivalent basis with normal forward date intervals. A separate specific initializer for it might be useful, though.</div><div class=""><br class=""></div><div class=""><span class="Apple-tab-span" style="white-space:pre"> </span>- Greg</div><br class=""><div class=""><blockquote type="cite" class=""><div class="">On Jun 20, 2016, at 4:12 PM, Tony Parker via swift-corelibs-dev <<a href="mailto:swift-corelibs-dev@swift.org" class="">swift-corelibs-dev@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><meta http-equiv="Content-Type" content="text/html charset=utf-8" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">Hi Dave,<div class=""><br class=""></div><div class="">We had some extensive discussion about this ourselves, but we couldn’t come up with a compelling use case for a negative time interval. Can you describe how you wanted to use it?</div><div class=""><br class=""></div><div class="">- Tony</div><div class=""><br class=""><div class=""><blockquote type="cite" class=""><div class="">On Jun 17, 2016, at 2:01 PM, Dave Lyon via swift-corelibs-dev <<a href="mailto:swift-corelibs-dev@swift.org" class="">swift-corelibs-dev@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class=""><p class="">In attempting to use the new DateInterval value type to improve some existing code, I ran in to an issue where DateIntervals cannot be "reverse" intervals, which is contrary to how TimeInterval works, and somewhat confusing.</p><p class="">I would propose that the DateInterval value type should be able to be properly initialized with any TimeInterval, as a reference date and time interval are all that is actually required in order to construct a DateInterval properly.</p><p class="">The simplest solution might be to change the `startDate:interval:` initializer to one allows for negative time intervals, such as:</p><p class="">public init(withInterval: TimeInterval, fromDate: Date) { <br class=""> self.start = Date(timeInterval: interval, since: fromDate)<br class=""> self.duration = abs(interval)<br class="">}</p><p class="">I believe in general it is preferable to avoid preconditions that require a subset of a given input type (in this case, that TimeInterval be positive or 0), and would prefer to see an API where invalid values are properly converted from the given input to the documented output. E.g. the old “Be generous with input, strict with output” idea.<br class=""></p><p class="">I hope this is the right place to bring this up, but if not I’m happy to move the conversation to Radar or elsewhere.</p><p class="">Thanks!</p></div>
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