<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="">This, which is generic sub-typing variance, has already been discussed elsewhere on the list. One problem with what you suggest is that it’s not in general safe to assume that because A : B then X<A> : X<B>. Sometimes it’s the other way around, and often neither applies. For example, it’s not safe for arrays in general (although it might be for Swift arrays due to their value type semantics).</div><div class=""><br class=""></div><div class="">-Sune</div><div class=""><br class=""></div><div><blockquote type="cite" class=""><div class="">On 16 Feb 2016, at 04:46, Cao Jiannan via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><meta http-equiv="Content-Type" content="text/html charset=gb2312" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><br class=""><div class=""><span class="" style="font-family: inherit; color: rgb(51, 51, 51); font-size: 10pt; font-style: inherit;">Hi all,</span></div><div class=""><span class="" style="font-family: inherit; color: rgb(51, 51, 51); font-size: 10pt; font-style: inherit;">I </span><font color="#333333" face="Helvetica Neue" size="2" class="" style="font-family: 'Helvetica Neue';">want to discuss on optional generic sub-typing problem.</font><br class=""><div class=""><br class=""></div><div class=""><div class="">This is my suggesion.</div><div class=""><br class=""></div></div><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class=""><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class="">if B is subclass of A</div><div class="">Either<B,B> is subclass of Either<A,A>, Either<A,B>, Either<B,A></div><div class="">Either<B,A> is subclass of Either<A,A></div><div class=""><div class="">Either<A,B> is subclass of Either<A,A></div><div class=""><br class=""></div></div></div></div></div></div><blockquote type="cite" class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class=""><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class=""></div><div class=""></div></div></div></div></div></blockquote></div><div class="">Why? Let’s see an example code in a real project:</div><div class=""><br class=""></div><div class=""><div class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div style="margin: 0px 0px 0px 6px; line-height: normal;" class=""><br class=""></div><div style="margin: 0px 0px 0px 6px; line-height: normal; font-family: 'Helvetica Neue';" class=""><span style="color: rgb(51, 51, 51); font-family: inherit; font-size: 10pt; font-style: inherit;" class=""><br class=""></span></div><div style="margin: 0px 0px 0px 6px; line-height: normal; font-family: 'Helvetica Neue';" class=""><span style="color: rgb(51, 51, 51); font-family: inherit; font-size: 10pt; font-style: inherit;" class="">Here is a protocol type for some UIViewController,</span></div><div class=""><div class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class=""><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class=""><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class=""><div class="dp-highlighter" style="box-sizing: border-box; padding: 1px 0px 0px; border: none; font-family: Consolas, 'Courier New', Courier, mono, serif; vertical-align: baseline; background-color: rgb(249, 249, 249); width: auto; overflow: visible; color: rgb(51, 51, 51); margin: 0px !important;"><div class="bar" style="box-sizing: border-box; margin: 0px; padding: 0px 0px 0px 45px; border: 0px; font-style: inherit; font-family: inherit; vertical-align: baseline;"><div class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-style: inherit; font-family: inherit; vertical-align: baseline;"></div></div><ol start="1" class="dp-j" style="box-sizing: border-box; padding: 0px; border: none; font-style: inherit; font-family: inherit; vertical-align: baseline; list-style-position: initial; list-style-image: initial; background-color: rgb(255, 255, 255); color: rgb(92, 92, 92); margin: 0px 0px 1px 45px !important;"><li class="alt" style="box-sizing: border-box; border-left-width: 3px; border-style: none none none solid; border-left-color: rgb(255, 255, 255); font-style: inherit; font-size: 1em; font-family: inherit; vertical-align: baseline; line-height: 12pt; list-style: decimal-leading-zero outside; color: inherit; height: 12pt; position: relative; white-space: nowrap; margin: 0px !important; padding: 0px 3px 0px 10px !important;"><span class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-style: inherit; vertical-align: baseline; background-color: inherit; font-size: 9pt !important;">protocol SegueHandlerType { </span></li><li class="" style="box-sizing: border-box; border-left-width: 3px; border-style: none none none solid; border-left-color: rgb(255, 255, 255); font-style: inherit; font-size: 1em; font-family: inherit; vertical-align: baseline; line-height: 12pt; list-style: decimal-leading-zero outside; background-color: rgb(248, 248, 248); height: 12pt; position: relative; white-space: nowrap; margin: 0px !important; padding: 0px 3px 0px 10px !important;"><span class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-style: inherit; vertical-align: baseline; background-color: inherit; font-size: 9pt !important;"> var tableView: UITableView! { get } </span></li><li class="alt" style="box-sizing: border-box; border-left-width: 3px; border-style: none none none solid; border-left-color: rgb(255, 255, 255); font-style: inherit; font-size: 1em; font-family: inherit; vertical-align: baseline; line-height: 12pt; list-style: decimal-leading-zero outside; color: inherit; height: 12pt; position: relative; white-space: nowrap; margin: 0px !important; padding: 0px 3px 0px 10px !important;"><span class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-style: inherit; vertical-align: baseline; background-color: inherit; font-size: 9pt !important;">} </span></li></ol></div><div class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-size: 16px; font-family: 'Myriad Set Pro', 'Lucida Grande', 'Helvetica Neue', Helvetica, Arial, Verdana, sans-serif; vertical-align: baseline; line-height: 1.4737em; color: rgb(51, 51, 51);"><br class=""></div><div class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-size: 16px; font-family: 'Myriad Set Pro', 'Lucida Grande', 'Helvetica Neue', Helvetica, Arial, Verdana, sans-serif; vertical-align: baseline; line-height: 1.4737em; color: rgb(51, 51, 51);"><span class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-style: inherit; font-size: 10pt; font-family: inherit; vertical-align: baseline;">so the UITableViewController can conform to the protocal</span></div><div class="dp-highlighter" style="box-sizing: border-box; padding: 1px 0px 0px; border: none; font-family: Consolas, 'Courier New', Courier, mono, serif; vertical-align: baseline; background-color: rgb(249, 249, 249); width: auto; overflow: visible; color: rgb(51, 51, 51); margin: 0px !important;"><div class="bar" style="box-sizing: border-box; margin: 0px; padding: 0px 0px 0px 45px; border: 0px; font-style: inherit; font-family: inherit; vertical-align: baseline;"><div class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-style: inherit; font-family: inherit; vertical-align: baseline;"></div></div><ol start="1" class="dp-j" style="box-sizing: border-box; padding: 0px; border: none; font-style: inherit; font-family: inherit; vertical-align: baseline; list-style-position: initial; list-style-image: initial; background-color: rgb(255, 255, 255); color: rgb(92, 92, 92); margin: 0px 0px 1px 45px !important;"><li class="alt" style="box-sizing: border-box; border-left-width: 3px; border-style: none none none solid; border-left-color: rgb(255, 255, 255); font-style: inherit; font-size: 1em; font-family: inherit; vertical-align: baseline; line-height: 12pt; list-style: decimal-leading-zero outside; color: inherit; height: 12pt; position: relative; white-space: nowrap; margin: 0px !important; padding: 0px 3px 0px 10px !important;"><span class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-style: inherit; vertical-align: baseline; background-color: inherit; font-size: 9pt !important;">extension UITableViewController : SegueHandlerType { </span></li><li class="" style="box-sizing: border-box; border-left-width: 3px; border-style: none none none solid; border-left-color: rgb(255, 255, 255); font-style: inherit; font-size: 1em; font-family: inherit; vertical-align: baseline; line-height: 12pt; list-style: decimal-leading-zero outside; background-color: rgb(248, 248, 248); height: 12pt; position: relative; white-space: nowrap; margin: 0px !important; padding: 0px 3px 0px 10px !important;"><span class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-style: inherit; vertical-align: baseline; background-color: inherit; font-size: 9pt !important;">} </span></li></ol></div><div class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-size: 16px; font-family: 'Myriad Set Pro', 'Lucida Grande', 'Helvetica Neue', Helvetica, Arial, Verdana, sans-serif; vertical-align: baseline; line-height: 1.4737em; color: rgb(51, 51, 51);"><span class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-style: inherit; font-size: 10pt; font-family: inherit; vertical-align: baseline;"><br class="" style="box-sizing: border-box;"></span></div><div class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-size: 16px; font-family: 'Myriad Set Pro', 'Lucida Grande', 'Helvetica Neue', Helvetica, Arial, Verdana, sans-serif; vertical-align: baseline; line-height: 1.4737em; color: rgb(51, 51, 51);"><span class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-style: inherit; font-size: 10pt; font-family: inherit; vertical-align: baseline;">It's great!</span></div><div class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-size: 16px; font-family: 'Myriad Set Pro', 'Lucida Grande', 'Helvetica Neue', Helvetica, Arial, Verdana, sans-serif; vertical-align: baseline; line-height: 1.4737em; color: rgb(51, 51, 51);"><span class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-style: inherit; font-size: 10pt; font-family: inherit; vertical-align: baseline;">What if the tableView is a subclass UITableView?</span></div><div class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-size: 16px; font-family: 'Myriad Set Pro', 'Lucida Grande', 'Helvetica Neue', Helvetica, Arial, Verdana, sans-serif; vertical-align: baseline; line-height: 1.4737em; color: rgb(51, 51, 51);"><span class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-style: inherit; font-size: 10pt; font-family: inherit; vertical-align: baseline;">like:</span></div><div class="dp-highlighter" style="box-sizing: border-box; padding: 1px 0px 0px; border: none; font-family: Consolas, 'Courier New', Courier, mono, serif; vertical-align: baseline; background-color: rgb(249, 249, 249); width: auto; overflow: visible; color: rgb(51, 51, 51); margin: 0px !important;"><div class="bar" style="box-sizing: border-box; margin: 0px; padding: 0px 0px 0px 45px; border: 0px; font-style: inherit; font-family: inherit; vertical-align: baseline;"><div class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-style: inherit; font-family: inherit; vertical-align: baseline;"></div></div><ol start="1" class="dp-j" style="box-sizing: border-box; padding: 0px; border: none; font-style: inherit; font-family: inherit; vertical-align: baseline; list-style-position: initial; list-style-image: initial; background-color: rgb(255, 255, 255); color: rgb(92, 92, 92); margin: 0px 0px 1px 45px !important;"><li class="alt" style="box-sizing: border-box; border-left-width: 3px; border-style: none none none solid; border-left-color: rgb(255, 255, 255); font-style: inherit; font-size: 1em; font-family: inherit; vertical-align: baseline; line-height: 12pt; list-style: decimal-leading-zero outside; color: inherit; height: 12pt; position: relative; white-space: nowrap; margin: 0px !important; padding: 0px 3px 0px 10px !important;"><span class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-style: inherit; vertical-align: baseline; background-color: inherit; font-size: 9pt !important;">class MyTableView : UITableView {</span></li><li class="alt" style="box-sizing: border-box; border-left-width: 3px; border-style: none none none solid; border-left-color: rgb(255, 255, 255); font-style: inherit; font-size: 1em; font-family: inherit; vertical-align: baseline; line-height: 12pt; list-style: decimal-leading-zero outside; color: inherit; height: 12pt; position: relative; white-space: nowrap; margin: 0px !important; padding: 0px 3px 0px 10px !important;"><span class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-style: inherit; vertical-align: baseline; background-color: inherit; font-size: 9pt !important;">}</span></li><li class="alt" style="box-sizing: border-box; border-left-width: 3px; border-style: none none none solid; border-left-color: rgb(255, 255, 255); font-style: inherit; font-size: 1em; font-family: inherit; vertical-align: baseline; line-height: 12pt; list-style: decimal-leading-zero outside; color: inherit; height: 12pt; position: relative; white-space: nowrap; margin: 0px !important; padding: 0px 3px 0px 10px !important;"><span class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-style: inherit; vertical-align: baseline; background-color: inherit; font-size: 9pt !important;"><br class=""></span></li><li class="alt" style="box-sizing: border-box; border-left-width: 3px; border-style: none none none solid; border-left-color: rgb(255, 255, 255); font-style: inherit; font-size: 1em; font-family: inherit; vertical-align: baseline; line-height: 12pt; list-style: decimal-leading-zero outside; color: inherit; height: 12pt; position: relative; white-space: nowrap; margin: 0px !important; padding: 0px 3px 0px 10px !important;"><span class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-style: inherit; vertical-align: baseline; background-color: inherit; font-size: 9pt !important;">MyTableViewController { </span></li><li class="" style="box-sizing: border-box; border-left-width: 3px; border-style: none none none solid; border-left-color: rgb(255, 255, 255); font-style: inherit; font-size: 1em; font-family: inherit; vertical-align: baseline; line-height: 12pt; list-style: decimal-leading-zero outside; background-color: rgb(248, 248, 248); height: 12pt; position: relative; white-space: nowrap; margin: 0px !important; padding: 0px 3px 0px 10px !important;"><span class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-style: inherit; vertical-align: baseline; background-color: inherit; font-size: 9pt !important;"> <span class="annotation" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-style: inherit; vertical-align: baseline; color: rgb(100, 100, 100); background-color: inherit; font-size: 9pt !important;">@IBOutlet</span><span class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-style: inherit; vertical-align: baseline; background-color: inherit; font-size: 9pt !important;"> var tableView: MyTableView! </span></span></li><li class="" style="box-sizing: border-box; border-left-width: 3px; border-style: none none none solid; border-left-color: rgb(255, 255, 255); font-style: inherit; font-size: 1em; font-family: inherit; vertical-align: baseline; line-height: 12pt; list-style: decimal-leading-zero outside; background-color: rgb(248, 248, 248); height: 12pt; position: relative; white-space: nowrap; margin: 0px !important; padding: 0px 3px 0px 10px !important;"><span class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-style: inherit; vertical-align: baseline; background-color: inherit; font-size: 9pt !important;">} </span></li></ol></div><div class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-size: 16px; font-family: 'Myriad Set Pro', 'Lucida Grande', 'Helvetica Neue', Helvetica, Arial, Verdana, sans-serif; vertical-align: baseline; line-height: 1.4737em; color: rgb(51, 51, 51);"><br class=""></div><div class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-size: 16px; font-family: 'Myriad Set Pro', 'Lucida Grande', 'Helvetica Neue', Helvetica, Arial, Verdana, sans-serif; vertical-align: baseline; line-height: 1.4737em; color: rgb(51, 51, 51);"><span class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-style: inherit; font-size: 10pt; font-family: inherit; vertical-align: baseline;">Then </span></div><div class="dp-highlighter" style="box-sizing: border-box; padding: 1px 0px 0px; border: none; font-family: Consolas, 'Courier New', Courier, mono, serif; vertical-align: baseline; background-color: rgb(249, 249, 249); width: auto; overflow: visible; color: rgb(51, 51, 51); margin: 0px !important;"><div class="bar" style="box-sizing: border-box; margin: 0px; padding: 0px 0px 0px 45px; border: 0px; font-style: inherit; font-family: inherit; vertical-align: baseline;"><div class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-style: inherit; font-family: inherit; vertical-align: baseline;"></div></div><ol start="1" class="dp-j" style="box-sizing: border-box; padding: 0px; border: none; font-style: inherit; font-family: inherit; vertical-align: baseline; list-style-position: initial; list-style-image: initial; background-color: rgb(255, 255, 255); color: rgb(92, 92, 92); margin: 0px 0px 1px 45px !important;"><li class="alt" style="box-sizing: border-box; border-left-width: 3px; border-style: none none none solid; border-left-color: rgb(255, 255, 255); font-style: inherit; font-size: 1em; font-family: inherit; vertical-align: baseline; line-height: 12pt; list-style: decimal-leading-zero outside; color: inherit; height: 12pt; position: relative; white-space: nowrap; margin: 0px !important; padding: 0px 3px 0px 10px !important;"><span class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-style: inherit; vertical-align: baseline; background-color: inherit; font-size: 9pt !important;">extension MyTableViewController:SegueHandlerType { </span></li><li class="" style="box-sizing: border-box; border-left-width: 3px; border-style: none none none solid; border-left-color: rgb(255, 255, 255); font-style: inherit; font-size: 1em; font-family: inherit; vertical-align: baseline; line-height: 12pt; list-style: decimal-leading-zero outside; background-color: rgb(248, 248, 248); height: 12pt; position: relative; white-space: nowrap; margin: 0px !important; padding: 0px 3px 0px 10px !important;"><span class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-style: inherit; vertical-align: baseline; background-color: inherit; font-size: 9pt !important;"> </span></li><li class="alt" style="box-sizing: border-box; border-left-width: 3px; border-style: none none none solid; border-left-color: rgb(255, 255, 255); font-style: inherit; font-size: 1em; font-family: inherit; vertical-align: baseline; line-height: 12pt; list-style: decimal-leading-zero outside; color: inherit; height: 12pt; position: relative; white-space: nowrap; margin: 0px !important; padding: 0px 3px 0px 10px !important;"><span class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-style: inherit; vertical-align: baseline; background-color: inherit; font-size: 9pt !important;">} </span></li></ol></div><div class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-size: 16px; font-family: 'Myriad Set Pro', 'Lucida Grande', 'Helvetica Neue', Helvetica, Arial, Verdana, sans-serif; line-height: 1.4737em;"><span style="font-family: Helvetica; font-size: 12px;" class="">will trigger a compiler error.</span><br style="font-family: Helvetica; font-size: 12px;" class=""><span style="font-family: Helvetica; font-size: 12px;" class=""> </span><br style="font-family: Helvetica; font-size: 12px;" class=""><span style="font-family: Helvetica; font-size: 12px;" class="">So the Optional needs a subclass system.</span><br style="font-family: Helvetica; font-size: 12px;" class=""><span style="font-family: Helvetica; font-size: 12px;" class="">Or to say, that the template system needs a subclass system.</span><br style="font-family: Helvetica; font-size: 12px;" class=""><span style="font-family: Helvetica; font-size: 12px;" class=""> </span><br style="font-family: Helvetica; font-size: 12px;" class=""><span style="font-family: Helvetica; font-size: 12px;" class="">Optional<UITableView> should be the super type of Optional<MyTableView></span><br style="font-family: Helvetica; font-size: 12px;" class=""><span style="font-family: Helvetica; font-size: 12px;" class="">Array<UITableView> should be the super type of Array<MyTableView></span></div></div><div class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; font-size: 16px; font-family: 'Myriad Set Pro', 'Lucida Grande', 'Helvetica Neue', Helvetica, Arial, Verdana, sans-serif; vertical-align: baseline; line-height: 1.4737em; color: rgb(51, 51, 51);"><br class=""></div><div class="" style="box-sizing: border-box; margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; line-height: 1.4737em;"><font color="#333333" face="Myriad Set Pro, Lucida Grande, Helvetica Neue, Helvetica, Arial, Verdana, sans-serif" size="3" class=""><a href="https://forums.developer.apple.com/message/101646#101646" class="">https://forums.developer.apple.com/message/101646#101646</a></font></div></div></div></div></div></div><blockquote type="cite" class=""><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class=""><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"></div></div></div></div></div></blockquote></div><div class=""><br class=""></div><div class=""><div class=""><br class=""></div></div><div class=""><br class=""></div><div class="">Thanks!</div><div class=""><br class=""></div><div class="">Jiannan, Cao</div><div class=""><br class=""></div><div class=""><br class=""></div></div></div></div></div></div></div></div>_______________________________________________<br class="">swift-evolution mailing list<br class=""><a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a><br class="">https://lists.swift.org/mailman/listinfo/swift-evolution<br class=""></div></blockquote></div><br class=""></body></html>