<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 5:35 PM, Xiaodi Wu via swift-evolution &lt;<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>&gt; wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class="">On Tue, Jan 31, 2017 at 5:28 PM, David Sweeris <span dir="ltr" class="">&lt;<a href="mailto:davesweeris@mac.com" target="_blank" class="">davesweeris@mac.com</a>&gt;</span> wrote:<br class=""><div class="gmail_extra"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word" class=""><br class=""><div class=""><span class=""><blockquote type="cite" class=""><div class="">On Jan 31, 2017, at 2:04 PM, Xiaodi Wu &lt;<a href="mailto:xiaodi.wu@gmail.com" target="_blank" class="">xiaodi.wu@gmail.com</a>&gt; wrote:</div><br class="m_4871187344016849002Apple-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;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="">On Tue, Jan 31, 2017 at 3:36 PM, David Sweeris via swift-evolution<span class="m_4871187344016849002Apple-converted-space">&nbsp;</span><span dir="ltr" class="">&lt;<a href="mailto:swift-evolution@swift.org" target="_blank" class="">swift-<wbr class="">evolution@swift.org</a>&gt;</span><span class="m_4871187344016849002Apple-converted-space">&nbsp;</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="m_4871187344016849002gmail-"><div class=""><br class=""></div><div class="">On Jan 31, 2017, at 11:32, Jaden Geller via swift-evolution &lt;<a href="mailto:swift-evolution@swift.org" target="_blank" class="">swift-evolution@swift.org</a>&gt; 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="m_4871187344016849002Apple-converted-space">&nbsp;</span><i class="">think</i><span class="m_4871187344016849002Apple-converted-space">&nbsp;</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="m_4871187344016849002Apple-converted-space">&nbsp;</span><i class="">after</i><span class="m_4871187344016849002Apple-converted-space">&nbsp;</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 class=""><br class=""></div></span><div class="">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 class=""><br class=""></div><div class="">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..&lt;“. 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 class=""><div class=""><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&lt;T</span><span style="font-variant-ligatures:no-common-ligatures" class="">&gt; {</span></div></div></div><div class=""><div class=""><div style="margin:0px;line-height:normal;font-family:Menlo" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">&nbsp; &nbsp; </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 class=""><div class=""><div style="margin:0px;line-height:normal;font-family:Menlo" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">&nbsp; &nbsp; </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 class=""><div class=""><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 class=""><div class=""><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 class=""><div class=""><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;" class=""> </span><span style="font-variant-ligatures:no-common-ligatures" class="">Collection</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> {</span></div></span></div></div></div><div class=""><div class=""><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="">&nbsp; &nbsp; </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="">&lt;Index&gt;) -&gt; SubSequence {</span></div></span></div></div><div class=""><div class=""><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="">&nbsp; &nbsp; &nbsp; &nbsp; </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 class=""><div class=""><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="">&nbsp; &nbsp; &nbsp; &nbsp; </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 ..&lt; </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 class=""><div class=""><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="">&nbsp; &nbsp; &nbsp; &nbsp; </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=""> ..&lt; upper]</span></div></span></div></div><div class=""><div class=""><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="">&nbsp; &nbsp; &nbsp; &nbsp; }</span></div></span></div></div><div class=""><div class=""><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="">&nbsp; &nbsp; }</span></div></span></div></div><div class=""><div class=""><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></blockquote><div class=""><br class=""></div><div class="">I understand that you can do this from a technical perspective. But I'm arguing it's devoid of semantics. &nbsp;That is, it's a spelling to dress up a number.</div></div></div></div></div></blockquote><div><br class=""></div><div>It’s not any more devoid of semantics than a partially applied function. &nbsp;It is a number or index with added semantics that it provides a lower (or upper) bound on the possible value specified by its type.</div><br class=""><blockquote type="cite" class=""><div dir="ltr" class=""><div class="gmail_extra"><div class="gmail_quote"><div class=""><br class=""></div><div class="">What is such an `IncompleteRange&lt;T&gt;` other than a value of type T? It's not an upper bound or lower bound of anything until it's used to index a collection. Why have a new type (IncompleteRange&lt;T&gt;), a new set of operators (prefix and postfix range operators), and these muddied semantics for something that can be written `subscript(upTo upperBound: Index) -&gt; SubSequence { ... }`? _That_ has unmistakable semantics and requires no new syntax.</div></div></div></div></blockquote><div><br class=""></div><div>Arguing that it adds too much complexity relative to the value it provides is reasonable. &nbsp;The value in this use case is mostly syntactic sugar so it’s relatively easy to make the case that it doesn’t cary its weight here.</div><div><br class=""></div><div>The value in Ben’s use case is a more composable alternative to `enumerated`. &nbsp;I find this to be a reasonably compelling example of the kind of thing a partial range might enable.</div><div><br class=""></div><div>I also tend to find concise notation important for clarity as long as it isn’t obscure or idiosyncratic. &nbsp;With that in mind, I think I lean in favor of `…` so long as we’re confident we won’t regret it if / when we take up variadic generics and / or tuple unpacking.</div><br class=""><blockquote type="cite" class=""><div dir="ltr" class=""><div class="gmail_extra"><div class="gmail_quote"><div class=""><br class=""></div></div><br class=""></div></div>
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