<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="">I would love to have this feature.</div><div class=""><br class=""></div><div class="">I've tried to do this before in Swift, assuming it was supported. I'm sure I'm not the only one.</div><div class=""><br class=""></div><div class=""><br class=""></div><br class=""><div><blockquote type="cite" class=""><div class="">On Mar 9, 2016, at 11:47 PM, Chris Lattner 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=utf-8" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class="">Hi All,</div><div class=""><br class=""></div><div class="">I’ve started prototyping generic type aliases in master, but we’d like to run this through the evolution process for discussion. Comments and discussion are welcome. Here’s the start of the email thread for the actual formal proposal:</div><div class=""><br class=""></div><div class=""><b class=""><br class=""></b></div><b class="">Introduction</b><br class=""><br class="">This proposal aims to add generic typealiases to Swift.<br class=""><br class="">Swift-evolution thread: <you are here><br class=""><br class=""><b class="">Motivation</b><br class=""><br class="">Generic typealiases are a somewhat obvious generalization of the existing Swift model for type aliases, which allow you to provide a name for an existing nominal generic type, or to provide a name for a non-nominal type (e.g. tuples, functions, etc) with generic parameters.<div class=""><br class=""></div><div class=""><b class="">Proposed solution</b><br class=""><br class="">The solution solution is straight-forward: allow type aliases to introduce type parameters, which are in scope for their definition. This allows one to express things like:</div><div class=""><br class=""></div><div class=""><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco;" class=""> typealias StringDictionary<T> = Dictionary<String, T></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco;" class=""> typealias IntFunction<T> = (T) -> Int</div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco;" class=""> typealias MatchingTriple<T> = (T, T, T)</div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco;" class=""> typealias BackwardTriple<T1,T2,T3> = (T3, T2, T1)</div><div class=""><br class=""></div>This is consistent with the rest of Swift’s approach to generics, and slots directly into the model.</div><div class=""><br class=""><b class="">Detailed design</b><br class=""><br class="">This is a minimal proposal for introducing type aliases into Swift, and intentionally chooses to keep them limited to being “aliases”. As such, additional constraints are not allowed in this base proposal, e.g. you can’t write:</div><div class=""><br class=""></div><div class=""><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco;" class=""> typealias StringDictionary<T where T : Hashable> = Dictionary<String, T></div><div class=""><br class=""></div><div class="">Otherwise, generic type aliases follow the model of type aliases and the precedent of the other generic declarations in Swift. For example, they allow the usual access control features that type aliases support. Similarly, like non-generic type aliases, generic type aliases cannot be “resilient”.</div><br class=""><b class="">Impact on existing code</b><br class=""><br class="">This is a new feature, so there is no impact on existing code.<br class=""><br class=""></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>