<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=""><br class=""><div><blockquote type="cite" class=""><div class="">On Jan 31, 2017, at 2:04 PM, Xiaodi Wu <<a href="mailto:xiaodi.wu@gmail.com" class="">xiaodi.wu@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class="">On Tue, Jan 31, 2017 at 3:36 PM, David Sweeris via swift-evolution<span class="Apple-converted-space"> </span><span dir="ltr" class=""><<a href="mailto:swift-evolution@swift.org" target="_blank" class="">swift-evolution@swift.org</a>></span><span class="Apple-converted-space"> </span>wrote:<br class=""><div class="gmail_extra"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-style: solid; border-left-color: rgb(204, 204, 204); padding-left: 1ex;"><div dir="auto" class=""><span class="gmail-"><div class=""><br class=""></div><div class="">On Jan 31, 2017, at 11:32, Jaden Geller via swift-evolution <<a href="mailto:swift-evolution@swift.org" target="_blank" class="">swift-evolution@swift.org</a>> wrote:<br class=""><br class=""></div><blockquote type="cite" class=""><div class=""><span class="">I think that is perfectly reasonable, but then it seems weird to be able to iterate over it (with no upper bound) independently of a collection). It would surprise me if</span><br class=""><span class="">```</span><br class=""><span class="">for x in arr[arr.startIndex…] { print(x) }</span><br class=""><span class="">```</span><br class=""><span class="">yielded different results than</span><br class=""><span class="">```</span><br class=""><span class="">for i in arr.startIndex… { print(arr[i]) } // CRASH</span><br class=""><span class="">```</span><br class=""><span class="">which it does under this model.</span><br class=""></div></blockquote><br class=""></span><div class="">(I<span class="Apple-converted-space"> </span><i class="">think</i><span class="Apple-converted-space"> </span>this how it works... semantically, anyway) Since the upper bound isn't specified, it's inferred from the context.</div><div class=""><br class=""></div><div class="">In the first case, the context is as an index into an array, so the upper bound is inferred to be the last valid index.</div><div class=""><br class=""></div><div class="">In the second case, there is no context, so it goes to Int.max. Then,<span class="Apple-converted-space"> </span><i class="">after</i><span class="Apple-converted-space"> </span>the "wrong" context has been established, you try to index an array with numbers from the too-large range.</div><div class=""><br class=""></div><div class="">Semantically speaking, they're pretty different operations. Why is it surprising that they have different results?</div></div></blockquote><div class=""><br class=""></div><div class="">I must say, I was originally rather fond of `0...` as a spelling, but IMO, Jaden and others have pointed out a real semantic issue.</div><div class=""><br class=""></div><div class="">A range is, to put it simply, the "stuff" between two end points. A "range with no upper bound" _has to be_ one that continues forever. The upper bound _must_ be infinity.</div></div></div></div></div></blockquote><div><br class=""></div><div>Depends… Swift doesn’t allow partial initializations, and neither the `.endIndex` nor the `.upperBound` properties of a `Range` are optional. From a strictly syntactic PoV, a "Range without an upperBound” can’t exist without getting into undefined behavior territory.</div><div><br class=""></div><div>Plus, mathematically speaking, an infinite range would be written "[x, ∞)", with an open upper bracket. If you write “[x, ∞]”, with a <i class="">closed</i> upper bracket, that’s kind of a meaningless statement. I would argue that if we’re going to represent that “infinite” range, the closest Swift spelling would be “x..<“. That leaves the mathematically undefined notation of “[x, ∞]”, spelled as "x…” in Swift, free to let us have “x…” or “…x” (which by similar reasoning can’t mean "(∞, x]”) return one of these:</div></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div><div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">enum</span><span style="font-variant-ligatures: no-common-ligatures" class=""> IncompleteRange<T</span><span style="font-variant-ligatures: no-common-ligatures" class="">> {</span></div></div></div><div><div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">case</span><span style="font-variant-ligatures: no-common-ligatures" class=""> upperValue(</span><span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">T</span><span style="font-variant-ligatures: no-common-ligatures" class="">)</span></div></div></div><div><div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">case</span><span style="font-variant-ligatures: no-common-ligatures" class=""> lowerValue(</span><span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">T)</span></div></div></div><div><div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">}</span></div></div></div></blockquote><div><div><div class=""><span style="font-variant-ligatures: no-common-ligatures" class="">which we could then pass to the subscript function of a collection to create the actual Range like this:</span></div></div></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div><div><div class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><div style="margin: 0px; line-height: normal; font-family: Menlo; color: rgb(112, 61, 170);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">extension</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> </span><span style="font-variant-ligatures: no-common-ligatures" class="">Collection</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> {</span></div></span></div></div></div><div><div><span style="font-variant-ligatures: no-common-ligatures" class=""><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">subscript</span><span style="font-variant-ligatures: no-common-ligatures" class="">(</span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">_</span><span style="font-variant-ligatures: no-common-ligatures" class=""> ir: </span><span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">IncompleteRange</span><span style="font-variant-ligatures: no-common-ligatures" class=""><Index>) -> SubSequence {</span></div></span></div></div><div><div><span style="font-variant-ligatures: no-common-ligatures" class=""><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">switch</span><span style="font-variant-ligatures: no-common-ligatures" class=""> ir {</span></div></span></div></div><div><div><span style="font-variant-ligatures: no-common-ligatures" class=""><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">case</span><span style="font-variant-ligatures: no-common-ligatures" class=""> .</span><span style="font-variant-ligatures: no-common-ligatures; color: #31595d" class="">lowerValue</span><span style="font-variant-ligatures: no-common-ligatures" class="">(</span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">let</span><span style="font-variant-ligatures: no-common-ligatures" class=""> lower): </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">return</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">self</span><span style="font-variant-ligatures: no-common-ligatures" class="">[lower ..< </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">self</span><span style="font-variant-ligatures: no-common-ligatures" class="">.</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">endIndex</span><span style="font-variant-ligatures: no-common-ligatures" class="">]</span></div></span></div></div><div><div><span style="font-variant-ligatures: no-common-ligatures" class=""><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">case</span><span style="font-variant-ligatures: no-common-ligatures" class=""> .</span><span style="font-variant-ligatures: no-common-ligatures; color: #31595d" class="">upperValue</span><span style="font-variant-ligatures: no-common-ligatures" class="">(</span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">let</span><span style="font-variant-ligatures: no-common-ligatures" class=""> upper): </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">return</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">self</span><span style="font-variant-ligatures: no-common-ligatures" class="">[</span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">self</span><span style="font-variant-ligatures: no-common-ligatures" class="">.</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">startIndex</span><span style="font-variant-ligatures: no-common-ligatures" class=""> ..< upper]</span></div></span></div></div><div><div><span style="font-variant-ligatures: no-common-ligatures" class=""><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> }</span></div></span></div></div><div><div><span style="font-variant-ligatures: no-common-ligatures" class=""><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> }</span></div></span></div></div><div><div><span style="font-variant-ligatures: no-common-ligatures" class=""><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">}</span></div></span></div></div></blockquote><div><div class=""><br class=""></div><div class="">- Dave Sweeris</div></div></body></html>