<html><head><meta http-equiv="Content-Type" content="text/html; charset=us-ascii"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class="">I would also love to have generic associated types in the language, I have a lot of uses for them and, IIUC, supertype constraint would enable me to express the following:<div class=""><br class=""></div><div class=""><div style="margin: 0px; font-stretch: normal; line-height: normal; font-family: Menlo; color: rgb(30, 148, 33); background-color: rgb(255, 255, 255);" class=""><span style="color: #c42275" class="">protocol</span><span style="color: #000000" class=""> Service {}</span></div><div style="margin: 0px; font-stretch: normal; line-height: normal; background-color: rgb(255, 255, 255); min-height: 14px;" class=""><br class=""></div><div style="margin: 0px; font-stretch: normal; line-height: normal; font-family: Menlo; background-color: rgb(255, 255, 255);" class=""><span style="color: #c42275" class="">protocol</span> WikiService: <span style="color: #539aa4" class="">Service</span> {} <span style="color: rgb(30, 148, 33);" class="">// methods not shown for conciseness</span></div><div style="margin: 0px; font-stretch: normal; line-height: normal; font-family: Menlo; background-color: rgb(255, 255, 255);" class=""><span style="color: #c42275" class="">class</span> DefaultWikiService: <span style="color: #539aa4" class="">WikiService</span> {}</div><div style="margin: 0px; font-stretch: normal; line-height: normal; font-family: Menlo; background-color: rgb(255, 255, 255);" class=""><span style="color: #c42275" class="">class</span> DemoWikiService: <span style="color: #539aa4" class="">WikiService</span> {}</div><div style="margin: 0px; font-stretch: normal; line-height: normal; background-color: rgb(255, 255, 255); min-height: 14px;" class=""><br class=""></div><div style="margin: 0px; font-stretch: normal; line-height: normal; font-family: Menlo; background-color: rgb(255, 255, 255);" class=""><span style="color: #c42275" class="">class</span> DataServiceManager {</div><div style="margin: 0px; font-stretch: normal; line-height: normal; background-color: rgb(255, 255, 255); min-height: 14px;" class=""><br class=""></div><div style="margin: 0px; font-stretch: normal; line-height: normal; font-family: Menlo; background-color: rgb(255, 255, 255);" class=""> <span style="color: #c42275" class="">private</span> <span style="color: #c42275" class="">var</span> registry = [<span style="color: #703daa" class="">String</span>: <span style="color: #539aa4" class="">Service</span>]()</div><div style="margin: 0px; font-stretch: normal; line-height: normal; background-color: rgb(255, 255, 255); min-height: 14px;" class=""><br class=""></div><div style="margin: 0px; font-stretch: normal; line-height: normal; font-family: Menlo; background-color: rgb(255, 255, 255);" class=""> <span style="color: #c42275" class="">func</span> register<S>(<span style="color: #c42275" class="">_</span> service: <span style="color: #539aa4" class="">Service</span>, ofType type: <span style="color: #539aa4" class="">S</span>.Type) <span style="color: #c42275" class="">where</span> <span style="color: #539aa4" class="">S</span>: <span style="color: #539aa4" class="">Service</span> {</div><div style="margin: 0px; font-stretch: normal; line-height: normal; font-family: Menlo; background-color: rgb(255, 255, 255);" class=""> <span style="color: #c42275" class="">let</span> key = <span style="color: #c81b13" class="">"</span>\<span style="color: #c81b13" class="">(</span>Swift.type(of: type)<span style="color: #c81b13" class="">)"</span></div><div style="margin: 0px; font-stretch: normal; line-height: normal; font-family: Menlo; background-color: rgb(255, 255, 255);" class=""> <span style="color: #539aa4" class="">registry</span>[key] = service</div><div style="margin: 0px; font-stretch: normal; line-height: normal; font-family: Menlo; background-color: rgb(255, 255, 255);" class=""> }</div><div style="margin: 0px; font-stretch: normal; line-height: normal; background-color: rgb(255, 255, 255); min-height: 14px;" class=""><br class=""></div><div style="margin: 0px; font-stretch: normal; line-height: normal; font-family: Menlo; background-color: rgb(255, 255, 255);" class=""> <span style="color: #c42275" class="">func</span> service<S>(ofType type: <span style="color: #539aa4" class="">S</span>.Type) -> <span style="color: #539aa4" class="">S</span> <span style="color: #c42275" class="">where</span> <span style="color: #539aa4" class="">S</span>: <span style="color: #539aa4" class="">Service</span> {</div><div style="margin: 0px; font-stretch: normal; line-height: normal; font-family: Menlo; background-color: rgb(255, 255, 255);" class=""> <span style="color: #c42275" class="">let</span> key = <span style="color: #c81b13" class="">"</span>\<span style="color: #c81b13" class="">(</span>Swift.type(of: type)<span style="color: #c81b13" class="">)"</span></div><div style="margin: 0px; font-stretch: normal; line-height: normal; background-color: rgb(255, 255, 255); min-height: 14px;" class=""><br class=""></div><div style="margin: 0px; font-stretch: normal; line-height: normal; font-family: Menlo; color: rgb(30, 148, 33); background-color: rgb(255, 255, 255);" class=""><span style="color: #000000" class=""> </span>// It is a programmer error to expect a value for a not yet registered type</div><div style="margin: 0px; font-stretch: normal; line-height: normal; font-family: Menlo; background-color: rgb(255, 255, 255);" class=""> <span style="color: #c42275" class="">guard</span> <span style="color: #c42275" class="">let</span> service = <span style="color: #539aa4" class="">registry</span>[key] <span style="color: #c42275" class="">as</span>? <span style="color: #539aa4" class="">S</span> <span style="color: #c42275" class="">else</span> {</div><div style="margin: 0px; font-stretch: normal; line-height: normal; font-family: Menlo; color: rgb(200, 27, 19); background-color: rgb(255, 255, 255);" class=""><span style="color: #000000" class=""> </span><span style="color: #3e1e81" class="">fatalError</span><span style="color: #000000" class="">(</span>"Service of type <span style="color: #000000" class="">\</span>(<span style="color: #000000" class="">type</span>) cannot be found. Please register a service for that type before accessing it."<span style="color: #000000" class="">)</span></div><div style="margin: 0px; font-stretch: normal; line-height: normal; font-family: Menlo; background-color: rgb(255, 255, 255);" class=""> }</div><div style="margin: 0px; font-stretch: normal; line-height: normal; font-family: Menlo; background-color: rgb(255, 255, 255);" class=""> <span style="color: #c42275" class="">return</span> service</div><div style="margin: 0px; font-stretch: normal; line-height: normal; font-family: Menlo; background-color: rgb(255, 255, 255);" class=""> }</div><div style="margin: 0px; font-stretch: normal; line-height: normal; background-color: rgb(255, 255, 255); min-height: 14px;" class=""><br class=""></div><div style="margin: 0px; font-stretch: normal; line-height: normal; font-family: Menlo; background-color: rgb(255, 255, 255);" class="">}</div><div class=""><br class=""></div><div class=""><div style="margin: 0px; font-stretch: normal; line-height: normal; font-family: Menlo; color: rgb(60, 130, 139); background-color: rgb(255, 255, 255);" class=""><span style="color: #c42275" class="">let</span><span style="color: #000000" class=""> manager = </span>DataServiceManager<span style="color: #000000" class="">()</span></div><div style="margin: 0px; font-stretch: normal; line-height: normal; font-family: Menlo; color: rgb(83, 154, 164); background-color: rgb(255, 255, 255);" class=""><span style="color: #c42275" class="">if</span><span style="color: #000000" class=""> </span>isDemoMode<span style="color: #000000" class=""> {</span></div><div style="margin: 0px; font-stretch: normal; line-height: normal; font-family: Menlo; color: rgb(30, 148, 33); background-color: rgb(255, 255, 255);" class=""><span style="color: #000000" class=""> </span><span style="color: #539aa4" class="">manager</span><span style="color: #000000" class="">.register(</span><span style="color: #3c828b" class="">DemoWikiService</span><span style="color: #000000" class="">(), ofType: </span><span style="color: #539aa4" class="">WikiService</span><span style="color: #000000" class="">.</span><span style="color: #c42275" class="">self</span><span style="color: #000000" class="">) </span>// Currently: error: in argument type 'WikiService.Protocol', 'WikiService' does not conform to expected type 'Service'</div><div style="margin: 0px; font-stretch: normal; line-height: normal; font-family: Menlo; color: rgb(196, 34, 117); background-color: rgb(255, 255, 255);" class=""><span style="color: #000000" class="">} </span>else<span style="color: #000000" class=""> {</span></div><div style="margin: 0px; font-stretch: normal; line-height: normal; font-family: Menlo; color: rgb(30, 148, 33); background-color: rgb(255, 255, 255);" class=""><span style="color: #000000" class=""> </span><span style="color: #539aa4" class="">manager</span><span style="color: #000000" class="">.register(</span><span style="color: #3c828b" class="">DefaultWikiService</span><span style="color: #000000" class="">(), ofType: </span><span style="color: #539aa4" class="">WikiService</span><span style="color: #000000" class="">.</span><span style="color: #c42275" class="">self</span><span style="color: #000000" class="">) </span>// Currently: error: in argument type 'WikiService.Protocol', 'WikiService' does not conform to expected type 'Service'</div><div style="margin: 0px; font-stretch: normal; line-height: normal; font-family: Menlo; background-color: rgb(255, 255, 255);" class="">}</div></div><div class=""><br class=""></div><div class="">If that's right, I'm also +1 on this :)</div><div class=""><br class=""></div><div class="">- Dennis</div><div class=""><div><br class=""><blockquote type="cite" class=""><div class="">On Nov 25, 2017, at 12:13 AM, Adrian Zubarev 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=""><div id="bloop_customfont" style="font-family: Helvetica, Arial; font-size: 13px; 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;" class="">In general this is more then welcome, so +1 for me.</div><div id="bloop_customfont" style="font-family: Helvetica, Arial; font-size: 13px; 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;" class=""><br class=""></div><div id="bloop_customfont" style="font-family: Helvetica, Arial; font-size: 13px; 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;" class="">However I have one question:</div><div id="bloop_customfont" style="font-family: Helvetica, Arial; font-size: 13px; 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;" class=""><br class=""></div><div id="bloop_customfont" style="font-family: Helvetica, Arial; font-size: 13px; 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;" class="">Could this allow support, or at least be a first step towards Swift allowing the following behaviour?</div><div id="bloop_customfont" style="font-family: Helvetica, Arial; font-size: 13px; 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;" class=""><br class=""></div><div id="bloop_customfont" style="font-family: Helvetica, Arial; font-size: 13px; 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;" class="">```<br class=""><div id="bloop_customfont" style="margin: 0px;" class="">extension MyProtocol where Self : SomeClass {</div><div id="bloop_customfont" style="margin: 0px;" class=""><span class="Apple-tab-span" style="white-space: pre;"> </span>static func getSubtypes<T>(ofType _: T.Type = T.self) -> [T] where T : Self { ... }</div><div id="bloop_customfont" style="margin: 0px;" class="">}</div>```</div><div style="font-family: Helvetica, Arial; font-size: 13px; 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><span style="font-family: Helvetica, Arial; font-size: 13px; 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="">I would like to be able to upgrade `Self` to a class constraint, which then will allow me to only accept subtypes from T at compile time.</span><br style="font-family: Helvetica, Arial; font-size: 13px; 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 id="bloop_sign_1511565056096249088" class="bloop_sign" style="font-family: Helvetica, Arial; font-size: 13px; 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><br style="font-family: Helvetica, Arial; font-size: 13px; 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=""><p class="airmail_on" style="font-family: Helvetica, Arial; font-size: 13px; 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;">Am 25. November 2017 um 00:03:23, Matthew Johnson via swift-evolution (<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>) schrieb:</p><blockquote type="cite" class="clean_bq" style="font-family: Helvetica, Arial; font-size: 13px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px;"><span class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><div class=""></div><div class="">One of the most frequent frustrations I encounter when writing generic code in Swift is the requirement that supertype constraints be concrete. When I mentioned this on Twitter (<a href="https://twitter.com/anandabits/status/929958479598534656" class="">https://twitter.com/anandabits/status/929958479598534656</a>) Doug Gregor mentioned that this feature is smaller and mostly straightforward to design and implement (<a href="https://twitter.com/dgregor79/status/929975472779288576" class="">https://twitter.com/dgregor79/status/929975472779288576</a>).<div class=""><div class=""><br class=""></div><div class="">I currently have a PR open to add the high-level description of this feature found below to the generics manifesto (<a href="https://github.com/apple/swift/pull/13012" class="">https://github.com/apple/swift/pull/13012</a>):</div><div class=""><br class=""></div><div class=""><div class="">Currently, supertype constraints may only be specified using a concrete class or protocol type. This prevents us from abstracting over the supertype.</div><div class=""><br class=""></div><div class="">```swift</div><div class="">protocol P {</div><div class=""> associatedtype Base</div><div class=""> associatedtype Derived: Base</div><div class="">}</div><div class="">```</div><div class=""><br class=""></div><div class="">In the above example `Base` may be any type. `Derived` may be the same as `Base` or may be _any_ subtype of `Base`. All subtype relationships supported by Swift should be supported in this context including, but not limited to, classes and subclasses, existentials and conforming concrete types or refining existentials, `T?` and `T`, `((Base) -> Void)` and `((Derived) -> Void)`, etc.</div><div class=""><br class=""></div><div class="">Generalized supertype constraints would be accepted in all syntactic locations where generic constraints are accepted.</div></div><div class=""><br class=""></div><div class="">I would like to see generalized supertype constraints make it into Swift 5 if possible. I am not an implementer so I will not be able to bring a proposal forward alone but am interested in collaborating with anyone interested in working on implementation.</div><div class=""><br class=""></div><div class="">I am also interested in hearing general feedback on this feature from the community at large. Have you also found this limitation frustrating? In what contexts? Does anyone have reservations about introducing this capability? If so, what are they?<div class=""><br class=""></div><div class=""><div class="">Matthew</div><div class=""><div class=""><br class=""></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></div></span></blockquote><span style="font-family: Helvetica, Arial; font-size: 13px; 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="">_______________________________________________</span><br style="font-family: Helvetica, Arial; font-size: 13px; 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=""><span style="font-family: Helvetica, Arial; font-size: 13px; 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="">swift-evolution mailing list</span><br style="font-family: Helvetica, Arial; font-size: 13px; 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=""><a href="mailto:swift-evolution@swift.org" style="font-family: Helvetica, Arial; font-size: 13px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px;" class="">swift-evolution@swift.org</a><br style="font-family: Helvetica, Arial; font-size: 13px; 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=""><a href="https://lists.swift.org/mailman/listinfo/swift-evolution" style="font-family: Helvetica, Arial; font-size: 13px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px;" class="">https://lists.swift.org/mailman/listinfo/swift-evolution</a></div></blockquote></div><br class=""></div></div></body></html>