<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 Mar 12, 2017, at 3:23 PM, Karl Wagner <<a href="mailto:razielim@gmail.com" class="">razielim@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><meta http-equiv="Content-Type" content="text/html charset=utf-8" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><br class=""><div class=""><blockquote type="cite" class=""><div class="">On 12 Mar 2017, at 14:32, Matthew Johnson <<a href="mailto:matthew@anandabits.com" class="">matthew@anandabits.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><meta http-equiv="content-type" content="text/html; charset=utf-8" class=""><div dir="auto" class=""><div class="">This is a really important feature IMO, but as others have pointed out it basically amounts to higher-kinded types. I would love to be wrong about this but I am reasonably sure this is out of scope for Swift 4 (otherwise I would be working on a proposal already).<br class=""><br class="">Sent from my iPad</div><div class=""><br class=""></div></div></div></blockquote><div class=""><br class=""></div><div class="">I’m not an expert on this stuff, but are they still higher-kinded types if we don’t express a relationship between Self and the associated type? I don’t think it’s quite the same conceptual leap as HKT.</div></div></div></div></blockquote><div><br class=""></div><div>I’m no expert either but it sure seems to me like it enables the things usually discussed in the context of higher-kinder types. Maybe someone from the core team can comment on whether there is a meaningful difference and whether this is something that could fit into Swift 4.</div><br class=""><blockquote type="cite" class=""><div class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class=""><div class=""><br class=""></div><div class="">Consider that every associated type must be backed by a typealias (explicit or inferred) in the conforming type. We can already have generic typealiases. This would be a more targeted thing which required those associatedtype-implementing-typealiases to contain generic parameters. It would also extend the constraints from SE-0142 to allow constraints to refer to those parameters and bind them to other associated types.</div><div class=""><br class=""></div><div class="">The workaround is basically to erase and dynamic-cast your way out:</div></div></div></div></blockquote><div><br class=""></div><div>Yes, there are workarounds, none of which are desirable. </div><div><br class=""></div><div>I ran into a case last year where there was a significant performance impact caused by the need to perform type erasure as a workaround. The type erasing wrapper required an allocation and type information that could have been used by the optimizer was lost. This was frustrating and convinced me that we definitely need HKT in Swift eventually. There are very useful generic libraries that cannot be implemented efficiently without them.</div><div><br class=""></div><br class=""><blockquote type="cite" class=""><div class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class=""><div class=""><br class=""></div><div class=""><span style="font-family: Courier;" class=""> //NOTE: dynamic type of ScanPromise.Result *must* be same as closure result. No static enforcement though :(</span></div><div class=""><span style="font-family: Courier;" class=""><br class=""></span></div></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><div class=""><div class=""><font face="Courier" class="">extension Scanner where ScanPromise.Result == Any? {</font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> func scan<T>(from f: Offset, until u: (Offset, Item) -> T?) -> T? {</font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> return withoutActuallyEscaping(u) { _u -> T? in</font></div></div></div></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><div class=""><div class=""><font face="Courier" class=""> return promiseScan(from: f, until: _u).await() as? T // downcast from Any? to T?</font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> }</font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> }</font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class="">}</font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""><br class=""></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class="">class MyPromise<R>: Promise {</font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> typealias Result = R?</font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> let offset: Offset</font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> let block: (Offset, Item) -> R?</font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class="">}</font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""><br class=""></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class="">class MyScanner: Scanner {</font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> typealias ScanPromise = MyPromise<Any> // want this to be “typealias ScanPromise<X> = MyPromise<X>"</font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""><br class=""></font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> func promiseScan<T>(from: Offset, until: @escaping (Offset, Item) -> T?) -> ScanPromise {</font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> return MyPromise(offset: from, block: until) // upcast from T? to Any?</font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class=""> }</font></div></div></div><div class=""><div class=""><div class=""><font face="Courier" class="">}</font></div></div></div></blockquote><div class=""><div class=""><br class=""></div><div class="">- Karl</div><br class=""><blockquote type="cite" class=""><div class=""><div dir="auto" class=""><div class="">On Mar 11, 2017, at 11:49 PM, Karl Wagner via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:<br class=""><br class=""></div><blockquote type="cite" class=""><div class=""><meta http-equiv="Content-Type" content="text/html charset=utf-8" class="">I have a model like this:<div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Courier" class="">protocol Promise {</font></div><div class=""><font face="Courier" class=""> associatedtype Result</font></div><div class=""><font face="Courier" class="">}</font></div><div class=""><font face="Courier" class=""><br class=""></font></div><div class=""><font face="Courier" class="">protocol Scanner {</font></div><div class=""><font face="Courier" class=""> associatedtype ScanPromise: Promise</font></div><div class=""><font face="Courier" class=""><br class=""></font></div><div class=""><font face="Courier" class=""> func promiseScan<T>(from: Offset, until: (Offset, Item) -> T?) -> ScanPromise // where Result == T?</font></div><div class=""><font face="Courier" class="">}</font></div></blockquote><div class=""><br class=""></div><div class="">The thing that I’m trying to express is: whichever type implements the associated type ‘ScanPromise’ must be generic, and that parameter must be its result (i.e. something it got as a result of calling the “until” closure).</div><div class=""><br class=""></div><div class="">Even with SE-0142, this kind of constraint would not be possible. What I would like to write is something like this:</div><div class=""><br class=""></div><div class=""><div class=""><font face="Courier" class="">protocol Promise {</font></div><div class=""><font face="Courier" class=""> associatedtype Result</font></div><div class=""><font face="Courier" class="">}</font></div><div class=""><font face="Courier" class=""><br class=""></font></div><div class=""><font face="Courier" class="">protocol Scanner {</font></div><div class=""><font face="Courier" class=""> associatedtype ScanPromise<T>: Promise // now generic. [SE-0142]: where Result == T</font></div><div class=""><font face="Courier" class=""><br class=""></font></div><div class=""><font face="Courier" class=""> func promiseScan<T>(from: Offset, until: (Offset, Item) -> T?) -> ScanPromise<T></font></div><div class=""><font face="Courier" class="">}</font></div></div><div class=""><br class=""></div><div class="">Thoughts?</div><div class=""><br class=""></div><div class="">- Karl</div></div></blockquote><blockquote type="cite" class=""><div class=""><span class="">_______________________________________________</span><br class=""><span class="">swift-evolution mailing list</span><br class=""><span class=""><a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a></span><br class=""><span class=""><a href="https://lists.swift.org/mailman/listinfo/swift-evolution" class="">https://lists.swift.org/mailman/listinfo/swift-evolution</a></span><br class=""></div></blockquote></div></div></blockquote></div><br class=""></div></div></blockquote></div><br class=""></body></html>