<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="">Has the core team or the community considered the possibility of implementing covariant/contravariant generic types? It would really be appreciated.<div class=""><br class=""></div><div class="">I know with Array, vague-ifying or specific-ifying the type ([Int] to [Any]) has help from the compiler—and we can use `map` if all else fails—but that only lessens the impact of the missing functionality. This is my exact use case here, using SceneKit to identify the first-hit Controller object:</div><div class=""><br class=""></div><div class=""><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #008f01" class="">class</span><span style="font-variant-ligatures: no-common-ligatures" class=""> ControllerNode<Controller: AnyObject>: </span><span style="font-variant-ligatures: no-common-ligatures; color: #aa2eb8" class="">SCNNode</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">{</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><span class="Apple-tab-span" style="white-space:pre"> </span></span><span style="font-variant-ligatures: no-common-ligatures; color: #008f01" class="">let</span><span style="font-variant-ligatures: no-common-ligatures" class=""> controller: </span><span style="font-variant-ligatures: no-common-ligatures; color: #498672" class="">Controller</span></div></div><div class=""><font face="Menlo" class=""><span style="font-size: 11px;" class="">}</span></font></div><div class=""><br class=""></div><div class=""><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(146, 146, 146);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">// Ordered front to back, returns the first Controller object.</span></div></div><div class=""><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #008f01" class="">for</span><span style="font-variant-ligatures: no-common-ligatures" class=""> hit </span><span style="font-variant-ligatures: no-common-ligatures; color: #008f01" class="">in</span><span style="font-variant-ligatures: no-common-ligatures" class=""> hitTest</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">{</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(146, 146, 146);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""><span class="Apple-tab-span" style="white-space:pre"> </span></span><span style="font-variant-ligatures: no-common-ligatures" class="">// Determine if this node is part of a controller.</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><span class="Apple-tab-span" style="white-space:pre"> </span></span><span style="font-variant-ligatures: no-common-ligatures; color: #008f01" class="">let</span><span style="font-variant-ligatures: no-common-ligatures" class=""> ancestrySequence = </span><span style="font-variant-ligatures: no-common-ligatures; color: #3d1d81" class="">sequence</span><span style="font-variant-ligatures: no-common-ligatures" class="">(first: hit.</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">node</span><span style="font-variant-ligatures: no-common-ligatures" class="">, next: { $0.</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">parent</span><span style="font-variant-ligatures: no-common-ligatures" class=""> })</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><b class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><span class="Apple-tab-span" style="white-space:pre"> </span></span><span style="font-variant-ligatures: no-common-ligatures; color: #008f01" class="">let</span><span style="font-variant-ligatures: no-common-ligatures" class=""> lastControllerNode: </span><span style="font-variant-ligatures: no-common-ligatures; color: #5faeae" class="">ControllerNode</span><span style="font-variant-ligatures: no-common-ligatures" class=""><</span><span style="font-variant-ligatures: no-common-ligatures; color: #aa2eb8" class="">AnyObject</span><span style="font-variant-ligatures: no-common-ligatures" class="">>? = ancestrySequence.</span><span style="font-variant-ligatures: no-common-ligatures; color: #3d1d81" class="">reduce</span><span style="font-variant-ligatures: no-common-ligatures" class="">(</span><span style="font-variant-ligatures: no-common-ligatures; color: #008f01" class="">nil</span><span style="font-variant-ligatures: no-common-ligatures" class="">)</span></b></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><b class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>{ ($1 </span><span style="font-variant-ligatures: no-common-ligatures; color: #008f01" class="">as</span><span style="font-variant-ligatures: no-common-ligatures" class="">? </span><span style="font-variant-ligatures: no-common-ligatures; color: #5faeae" class="">ControllerNode</span><span style="font-variant-ligatures: no-common-ligatures" class="">) ?? $0 }</span></b></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span class="Apple-tab-span" style="white-space: pre;"> </span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><span class="Apple-tab-span" style="white-space:pre"> </span></span><span style="font-variant-ligatures: no-common-ligatures; color: #008f01" class="">if</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #008f01" class="">let</span><span style="font-variant-ligatures: no-common-ligatures" class=""> cabinet = lastControllerNode?.</span><span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">controller</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #008f01" class="">as</span><span style="font-variant-ligatures: no-common-ligatures" class="">? </span><span style="font-variant-ligatures: no-common-ligatures; color: #5faeae" class="">CabinetController</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>{ </span><span style="font-variant-ligatures: no-common-ligatures; color: #008f01" class="">return</span><span style="font-variant-ligatures: no-common-ligatures" class=""> cabinet }</span></div><p style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; min-height: 13px;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><span class="Apple-tab-span" style="white-space:pre"> </span></span><br class="webkit-block-placeholder"></p><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><span class="Apple-tab-span" style="white-space:pre"> </span></span><span style="font-variant-ligatures: no-common-ligatures; color: #008f01" class="">if</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #008f01" class="">let</span><span style="font-variant-ligatures: no-common-ligatures" class=""> wall = lastControllerNode?.</span><span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">controller</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #008f01" class="">as</span><span style="font-variant-ligatures: no-common-ligatures" class="">? </span><span style="font-variant-ligatures: no-common-ligatures; color: #5faeae" class="">WallController</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><span class="Apple-tab-span" style="white-space:pre"> </span>{ </span><span style="font-variant-ligatures: no-common-ligatures; color: #008f01" class="">return</span><span style="font-variant-ligatures: no-common-ligatures" class=""> wall }</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">}</span></div></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><br class=""></span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><div style="font-family: Helvetica; font-size: 12px;" class="">This compiles, but unfortunately, this will never work. The `reduce` algorithm always ends up trying to convert things like `ControllerNode<WallController> as ControllerNode<AnyObject>`, which—unintuitively—always fails. Without compiler help, so would things like `myIntArray as [Any]` or `Optional<Boy>(Boy()) as Optional<Human>`.</div><div style="font-family: Helvetica; font-size: 12px;" class=""><br class=""></div><div style="font-family: Helvetica; font-size: 12px;" class="">If Swift is supposed to welcome generic programming, this would be a great thing to have.</div></span></div></body></html>