<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 11, 2017, at 9:49 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="">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></div></blockquote><br class=""></div><div>Some of us who work on the Swift compiler have talked about this extension to the model before, and we *think* it dodges some of the concerns about introducing more-general higher-rank types in Swift while enabling reasonable use cases like the one you provide. It seems like a reasonable direction.</div><div><br class=""></div><div>Way out of scope for Swift 4 at this point, of course ;)</div><div><br class=""></div><div><span class="Apple-tab-span" style="white-space:pre"> </span>- Doug</div><div><br class=""></div><br class=""></body></html>