<div dir="ltr"><div class="gmail_extra"><div class="gmail_quote">On Mon, Aug 15, 2016 at 7:03 PM, Tim Vermeulen <span dir="ltr"><<a href="mailto:tvermeulen@me.com" target="_blank">tvermeulen@me.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="auto"><div><div style="direction:inherit">If I'm not mistaken, the type of T can be inferred from the `next` closure here. $0 is an optional, so you end up with a sequence of optionals.</div></div></div></blockquote><div><br></div><div>Currently, yes. But if we have both </div><div><br></div><div><div>public func sequence<T>(first: T, next: @escaping (T) -> T?) -> UnfoldFirstSequence<T> { ... }</div><div>public func sequence<T>(first: T?, next: @escaping (T) -> T?) -> UnfoldFirstSequence<T> { ... }</div></div><div><br></div><div>then $0 could be inferred to be of type `T` or `T?`, right?</div><div><br></div><div>And if we replace the first function with the second (instead of having both), then the same code that gives you an UnfoldFirstSequence<T?> today would give you an UnfoldFirstSequence<T> tomorrow, which would be the most pernicious way to break code (subtle, hard to catch unless you know what you're looking for, with a potentially massive difference in behavior).</div><div><br></div><div><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="auto"><div><div class="h5"><div>On 16 Aug 2016, at 00:51, Xiaodi Wu <<a href="mailto:xiaodi.wu@gmail.com" target="_blank">xiaodi.wu@gmail.com</a>> wrote:<br><br></div><blockquote type="cite"><div>Given:<br><br>let foo: T? = something()<br>let bar = sequence(first: foo, next: { $0?.frobnicate() })<br><br>If first could be of type `T` or `T?`, is bar of type `UnfoldSequence<T>` or `UnfoldSequence<T?>`?<br><div class="gmail_quote"><div dir="ltr">On Mon, Aug 15, 2016 at 17:15 Tim Vermeulen via swift-evolution <<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div style="word-wrap:break-word"><div>I doubt that’s it, in no way is an an empty sequence inherently unsafe. The entire standard library is built with empty sequences in mind. I’m more inclined to think it’s an oversight.</div></div><div style="word-wrap:break-word"><br><div><blockquote type="cite"><div>On 15 Aug 2016, at 23:15, Maximilian Hünenberger <<a href="mailto:m.huenenberger@me.com" target="_blank">m.huenenberger@me.com</a>> wrote:</div><br><div><div dir="auto"><div></div><div>Ok, I see. However this could be a potential source of bugs/performance issues where you don't consider the nil case and you do some unnecessary work. By prohibiting to pass nil you have to manually unwrap and you immediately see the "optionality".</div><div><br>Am 15.08.2016 um 22:36 schrieb Tim Vermeulen <<a href="mailto:tvermeulen@me.com" target="_blank">tvermeulen@me.com</a>>:<br><br></div><blockquote type="cite"><div><div>Oh, that’s true, I misunderstood your previous message. It’s not about passing nil, but it’s about passing optionals. The point is to be able to do something like this:</div><div><br></div><div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo"><span style="color:rgb(186,45,162)">let</span><span> number = functionThatReturnsAnOptional(<wbr>)</span></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:Menlo"><span>sequence(first: </span><span style="color:rgb(79,129,135)">number</span><span>, next: { $0 % </span><span style="color:rgb(39,42,216)">2</span><span> == </span><span style="color:rgb(39,42,216)">0</span><span> ? $0 / </span><span style="color:rgb(39,42,216)">2</span><span> : </span><span style="color:rgb(186,45,162)">nil</span><span> })</span></div></div><br><div><blockquote type="cite"><div>On 15 Aug 2016, at 22:26, Maximilian Hünenberger <<a href="mailto:m.huenenberger@me.com" target="_blank">m.huenenberger@me.com</a>> wrote:</div><br><div><div>Probably I didn't understand your proposal. What do you want to change exactly?<br><br>I thought:<br>public func sequence<T>(first: T, next: @escaping (T) -> T?) -> UnfoldFirstSequence<T> { ... }<br><br>To:<br>public func sequence<T>(first: T?, next: @escaping (T) -> T?) -> UnfoldFirstSequence<T> { ... }<br><br><blockquote type="cite">Am 15.08.2016 um 22:17 schrieb Tim Vermeulen <<a href="mailto:tvermeulen@me.com" target="_blank">tvermeulen@me.com</a>>:<br><br>You can’t; the `first` parameter has type `T`, not `T?`.<br><br><blockquote type="cite">On 15 Aug 2016, at 22:10, Maximilian Hünenberger <<a href="mailto:m.huenenberger@me.com" target="_blank">m.huenenberger@me.com</a>> wrote:<br><br>Hi Tim,<br><br>If you pass "nil" to "first" isn't this an empty sequence? So it would be redundant.<br><br>Best regards<br>Maximilian<br><br><blockquote type="cite">Am 15.08.2016 um 01:27 schrieb Tim Vermeulen via swift-evolution <<a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a>>:<br><br>sequence(first:next:) takes a non-optional first argument. Is there a reason for that? sequence(state:next:) allows empty sequences, and I don’t see why sequence(first:next:) shouldn’t. The fix would be to simply add the `?` in the function signature; no other changes are required to make it work.<br><br>I considered just filing a bug report, but since this is a change of the public API...<br>______________________________<wbr>_________________<br>swift-evolution mailing list<br><a href="mailto:swift-evolution@swift.org" target="_blank">swift-evolution@swift.org</a><br><a href="https://lists.swift.org/mailman/listinfo/swift-evolution" target="_blank">https://lists.swift.org/<wbr>mailman/listinfo/swift-<wbr>evolution</a><br></blockquote></blockquote><br></blockquote></div></div></blockquote></div><br></div></blockquote></div></div></blockquote></div><br></div>______________________________<wbr>_________________<br>
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