<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 Oct 17, 2017, at 10:54 AM, Adam Kemp <<a href="mailto:adam_kemp@apple.com" class="">adam_kemp@apple.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div 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; -webkit-text-stroke-width: 0px;" class=""><br class="Apple-interchange-newline"><br class=""><blockquote type="cite" class=""><div class="">On Oct 17, 2017, at 10:41 AM, Kevin Nattinger via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:</div><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><br class=""><div class=""><blockquote type="cite" class=""><div class="">On Oct 17, 2017, at 10:36 AM, Adam Kemp <<a href="mailto:adam_kemp@apple.com" class="">adam_kemp@apple.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space;"><br class=""><div class=""><blockquote type="cite" class=""><div class="">On Oct 17, 2017, at 10:00 AM, Kevin Nattinger <<a href="mailto:swift@nattinger.net" class="">swift@nattinger.net</a>> wrote:</div><div class=""><div class="" style="font-family: Helvetica; font-size: 24px; 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; -webkit-text-stroke-width: 0px;"><div class=""><br class=""></div><span style="font-size: 14px;" class="">Once we allow covariant functions to satisfy protocol requirements and have generalized existentials and recursive protocol requirements, wouldn't we be able to update thusly:</span></div><div class="" style="font-family: Helvetica; font-size: 14px; 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; -webkit-text-stroke-width: 0px;"><br class=""></div><div class="" style="font-family: Helvetica; font-size: 14px; 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; -webkit-text-stroke-width: 0px;">protocol Unordered {</div><div class="" style="font-family: Helvetica; font-size: 14px; 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; -webkit-text-stroke-width: 0px;"><span class="Apple-tab-span" style="white-space: pre;"> </span>func map<T>(…) -> Any<U: Unordered where U.Element == T></div><div class="" style="font-family: Helvetica; font-size: 14px; 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; -webkit-text-stroke-width: 0px;">}</div><div class="" style="font-family: Helvetica; font-size: 14px; 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; -webkit-text-stroke-width: 0px;">protocol Ordered: Unordered {</div><div class="" style="font-family: Helvetica; font-size: 14px; 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; -webkit-text-stroke-width: 0px;"><span class="Apple-tab-span" style="white-space: pre;"> </span>func map<T>(…) -> Any<O: Ordered where O.Element == T></div><div class="" style="font-family: Helvetica; font-size: 14px; 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; -webkit-text-stroke-width: 0px;">}</div><div class="" style="font-family: Helvetica; font-size: 24px; 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; -webkit-text-stroke-width: 0px;"><br class=""></div></div></blockquote><div class=""><br class=""></div><div class="">Now apply that to every order-preserving function that takes a Sequence and returns another Sequence. You’ve moved the burden from users of API to implementers of API. It reminds me of the const/non-const split that C++ developers have to deal with, where a lot of functions end up being implemented twice so that you can have a const version and a non-const version (generally one just calls the other). It’s a pain. I don’t want that when working with Sequences. I don’t think it’s worth it. And FWIW, when I was programming in C# I wrote functions that took an IEnumerable<T> and return another IEnumerable<T> very often. It’s a powerful feature that would have been ruined by a change like this.</div></div></div></div></blockquote><div class=""><br class=""></div>The idea is that covariance would mean you only need to implement the function once.</div></div></div></blockquote><div class=""><br class=""></div><div class="">In the example you showed above map is written twice. Maybe the two protocols can share an implementation, but you still have to have two versions declared somewhere.</div></div></div></blockquote><div><br class=""></div><div>Perhaps I'm wrong, but (once we allow covariant conformance) wouldn't a single implementation of the `Ordered` version be covariant and thus satisfy the `Unordered` requirement without even having a dummy implementation forwarding to it? That's what I'm aiming for. </div><br class=""><blockquote type="cite" class=""><div class=""><div 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; -webkit-text-stroke-width: 0px;" class=""><div class=""><br class=""></div><div class="">What does it look like if you’re just writing a single function somewhere that takes in a Sequence and returns another Sequence? How do you make that function take both ordered and unordered Sequences? To make it concrete, say you write a function that just wraps map:</div><div class=""><br class=""></div></div><blockquote class="" 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; -webkit-text-stroke-width: 0px; margin: 0px 0px 0px 40px; border: none; padding: 0px;"><div class=""><div class="">func firstNames(ofPeople people: Sequence<Person>) -> Sequence<Person> {</div><div class=""> return people.map { $0.firstName }</div><div class="">}</div></div></blockquote></div></blockquote><div><br class=""></div><div>Hmm, I'm not sure that would work with the covariant requirement. The associated type one could:</div><div>func firstNames<U: Unordered>(ofPeople people: U<MapResultType: Person>) -> U.MapResultType<Element: String> {</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>return people.map { $0.firstName }</div><div>}</div><div><br class=""></div><div>If the sequence you put in maps to an ordered sequence, you get the ordered sequence out. </div><div>That said, I can see how the generics there could get out-of-hand as you add more operations… -> U.MapResultType.FilterResultType.MapResultType…<Element: String></div><div><br class=""></div><div>I'm planning on playing with this all before opening a real proposal, if/when I can figure out how, so I'm sure I'll have to deal with these and similar issues. Definitely good things to keep an eye out for, thanks.</div><br class=""><blockquote type="cite" class=""><div class=""><div 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; -webkit-text-stroke-width: 0px;" class=""><br class=""></div><div 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; -webkit-text-stroke-width: 0px;" class="">I want that function to work on both ordered and unordered Sequences, and if you start with an ordered Sequence you should end up with an ordered Sequence. How do I make that work?</div><div 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; -webkit-text-stroke-width: 0px;" class=""><br class=""><blockquote type="cite" class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class="">If there are no real-world problems, why do we feel the need to change the function name in the first place?</div></div></blockquote><div class=""><br class=""></div></div><span 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; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">To quote myself from an earlier email: "I’m not even sure a name change is necessary for this method at all, but I’m not at all in favor of anything beyond that.”</span><div class="" 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; -webkit-text-stroke-width: 0px;"><br class=""></div><div class="" 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; -webkit-text-stroke-width: 0px;">To extend that, I’m content with doing nothing. I’m not sure there’s cause for doing anything at all, and I’m very sure that no one on this list has demonstrated any need for a major change to the library, let alone new language features.</div></div></blockquote></div><br class=""></body></html>