<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><div dir="auto" style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><br class=""><div><br class=""><blockquote type="cite" class=""><div class="">On Nov 24, 2017, at 3:11 PM, Matthew Johnson 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="">As mentioned in my prior message, I currently have a PR open to update the generics manifesto (<a href="https://github.com/apple/swift/pull/13012" class="">https://github.com/apple/swift/pull/13012</a>). I removed one topic from that update at Doug Gregor’s request that it be discussed on the list first. </div><div class=""><br class=""></div><div class="">The idea is to add the ability to make default arguments conditional (i.e. depend on generic constraints). It is currently possible to emulate conditional default arguments using an overload set. This is verbose, especially when several arguments are involved. Here is an example use case using the overload method to emulate this feature:</div><div class=""><br class=""></div><div class="">```swift</div><div class="">protocol Resource {</div><div class=""> associatedtype Configuration</div><div class=""> associatedtype Action</div><div class="">}</div><div class="">struct ResourceDescription<R: Resource> {</div><div class=""> func makeResource(with configuration: R.Configuration, actionHandler: @escaping (R.Action) -> Void) -> R {</div><div class=""> // create a resource using the provided configuration</div><div class=""> // connect the action handler</div><div class=""> // return the resource</div><div class=""> }</div><div class="">}</div><div class=""><br class=""></div><div class="">extension ResourceDescription where R.Configuration == Void {</div><div class=""> func makeResource(actionHandler: @escaping (R.Action) -> Void) -> R {</div><div class=""> return makeResource(with: (), actionHandler: actionHandler)</div><div class=""> }</div><div class="">}</div><div class=""><br class=""></div><div class="">extension ResourceDescription where R.Action == Never {</div><div class=""> func makeResource(with configuration: R.Configuration) -> R {</div><div class=""> return makeResource(with: configuration, actionHandler: { _ in })</div><div class=""> }</div><div class="">}</div><div class=""><br class=""></div><div class="">extension ResourceDescription where R.Configuration == Void, R.Action == Never {</div><div class=""> func makeResource() -> R {</div><div class=""> return makeResource(with: (), actionHandler: { _ in })</div><div class=""> }</div><div class="">}</div><div class=""><br class=""></div><div class="">```</div><div class=""><br class=""></div><div class="">Adding language support for defining these more directly would eliminate a lot of boilerplate and reduce the need for overloads. </div></div></div></blockquote><div><br class=""></div><div>If one could refer to `self` in a default argument (which is not a big problem), you could turn the default into a requirement itself… although it doesn’t *quite* work with your example as written because it would always need to be implemented somehow:</div><div><br class=""></div><div><blockquote type="cite" class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><div class="">protocol Resource {</div><div class=""> associatedtype Configuration</div><div class=""> associatedtype Action</div></div></blockquote> func defaultConfiguration() -> Configuration</div><div> func defaultHandler() -> ((R.Action) -> Void)<br class=""><blockquote type="cite" class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><div class="">}</div></div></blockquote></div><div><br class=""></div><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=""> Doug mentioned that it may also help simplify associated type inference (<a href="https://github.com/apple/swift/pull/13012#discussion_r152124535" class="">https://github.com/apple/swift/pull/13012#discussion_r152124535</a>).</div></div></div></blockquote><br class=""></div><div>Oh, I thought this was something related to choosing a defaults for associated types, which might have helped with my current associated-type-inference quandary. The topic you actually wanted to discuss is disjoint (sorry).</div><div><br class=""></div><div><span class="Apple-tab-span" style="white-space:pre"> </span>- Doug</div><div><br class=""></div><br class=""></div></body></html>