<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 Jan 7, 2016, at 8:30 PM, Joe Groff <<a href="mailto:jgroff@apple.com" class="">jgroff@apple.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: 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-stroke-width: 0px;" class=""><blockquote type="cite" class=""><div class=""><br class="Apple-interchange-newline">On Jan 7, 2016, at 11:30 AM, Matthew Johnson <<a href="mailto:matthew@anandabits.com" class="">matthew@anandabits.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><br class=""><div class=""><blockquote type="cite" class=""><div class="">On Jan 7, 2016, at 12:51 PM, Joe Groff <<a href="mailto:jgroff@apple.com" class="">jgroff@apple.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><br class=""><div class=""><blockquote type="cite" class=""><div class="">On Jan 7, 2016, at 10:49 AM, Matthew Johnson <<a href="mailto:matthew@anandabits.com" class="">matthew@anandabits.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><blockquote type="cite" class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: 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-stroke-width: 0px;"><div class=""><br class="Apple-interchange-newline">On Jan 7, 2016, at 12:37 PM, Joe Groff <<a href="mailto:jgroff@apple.com" class="">jgroff@apple.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><br class=""><div class=""><blockquote type="cite" class=""><div class="">On Jan 7, 2016, at 10:32 AM, David Owens II <<a href="mailto:david@owensd.io" class="">david@owensd.io</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: 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-stroke-width: 0px;">And this is more clear than this?</div><div class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: 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-stroke-width: 0px;"><font face="Menlo" class=""><br class=""></font></div><blockquote class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: 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-stroke-width: 0px; margin: 0px 0px 0px 40px; border: none; padding: 0px;"><font face="Menlo" class="">class Foo {<br class=""></font><font face="Menlo" class=""> var x,y,z: Int<br class=""></font><font face="Menlo" class=""> init(x: Int, y: Int, z: Int) {</font></blockquote><blockquote class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: 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-stroke-width: 0px; margin: 0px 0px 0px 40px; border: none; padding: 0px;"><font face="Menlo" class=""> self.x = x</font></blockquote><blockquote class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: 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-stroke-width: 0px; margin: 0px 0px 0px 40px; border: none; padding: 0px;"><font face="Menlo" class=""> self.y = y</font></blockquote><blockquote class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: 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-stroke-width: 0px; margin: 0px 0px 0px 40px; border: none; padding: 0px;"><font face="Menlo" class=""> self.z = z</font></blockquote><blockquote class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: 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-stroke-width: 0px; margin: 0px 0px 0px 40px; border: none; padding: 0px;"><font face="Menlo" class=""> }</font></blockquote><blockquote class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: 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-stroke-width: 0px; margin: 0px 0px 0px 40px; border: none; padding: 0px;"><font face="Menlo" class="">}</font></blockquote></div></blockquote></div><br class=""><div class="">No, it isn't, but Matthew asked… I'm personally not too motivated to support anything more than all-or-nothing memberwise initialization, and tend to agree that anything more specialized deserves an explicit implementation.</div></div></div></blockquote><div class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: 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-stroke-width: 0px;"><br class=""></div><div class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: 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-stroke-width: 0px;">Maybe you would feel differently if you were an app developer. Different kinds of code have different needs. The most important use cases I have in mind are related to UI code, which is often the majority of the code in an app.</div></div></blockquote><br class=""></div><div class="">Do you have any concrete examples in mind?</div><div class=""><br class=""></div><div class="">-Joe</div><br class=""></div></div></blockquote><br class=""></div><div class="">Here is an example where partial memberwise initialization would apply. It is similar to something in a real project:</div><div class=""><br class=""></div><div class="">public class FontPicker: UIControl {</div><div class=""> public let fonts: [UIFont]</div><div class=""><br class=""></div><div class=""> public var fontSize: CGFloat = 22</div><div class=""> public var foregroundColor: UIColor = UIColor.darkGrayColor()</div><div class=""> public var backgroundColor: UIColor = UIColor.whiteColor()</div><div class=""> // A bunch of other appearance attributes here</div><div class=""><br class=""></div><div class=""> private let collectionView: UICollectionView</div><div class=""> private let layout: UICollectionViewLayout</div><div class=""> // other internal state required by the implementation</div><div class=""><br class=""></div><div class=""> public memberwise init(...) {</div><div class=""> // configure the collection view and add it as a subview</div><div class=""> }</div><div class="">}</div><div class=""><br class=""></div><div class="">A couple points are relevant here:</div><div class=""><br class=""></div><div class="">1. Memberwise initialization is very valuable for the appearance attributes, but is useless if it exposes our implementation details.</div><div class=""><br class=""></div><div class="">2. In many custom UI widgets the appearance attributes don’t really need to be mutable post-initialization. At the same time, it is necessary to allow, but not require a value to be specified. It would be ideal if they were `let` properties with a default value, but still able to participate in memberwise initialization. Without that capability we are forced to choose between the advantages of using a `let` property and the advantages of memberwise initialization.</div><br class=""><div class="">UI widgets are a great example. View controllers often have a similar divide between state provided by the user and state related to internal implementation details.</div></div></div></blockquote><br class=""></div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: 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-stroke-width: 0px;" class="">Access control seems like a poor tool for the kind of categorization you want here. The vast majority of code is app code, where there's no reason to use 'public', so 'internal' and 'private' are the interesting visibility layers. Using 'private' to opt fields out of memberwise initialization is too brittle, in my opinion—You've made it much harder to factor the class's functionality into different files in the future, since you can no longer change any of these fields to internal without also breaking all of the memberwise initializers as a second-order effect.</div></div></blockquote><div><br class=""></div></div>That is a reasonable position. In practice it wouldn’t be a problem for projects I’ve worked on, and I suspect others would say the same or may have commented on this already. But I do understand why you want to avoid the possibility of increasing the visibility of a member breaking code. <div class=""><br class=""></div><div class=""><div class="">Nevertheless, I feel that access control rules should be applied to memberwise initializers unless we go with the opt-in model where the programmer explicitly states which properties are relevant to memberwise initialization and which are not. I don’t think exposing more-private members in an more-public initializers without an explicit request to do so is a good idea. And I don’t think the `memberwise` declaration modifier on the initializer itself is enough to constitute and explicit request to do this.<div class=""><br class=""></div><div class="">The safest, most clear, most robust way to determine the correct set of properties to use is to go with the opt-in model. The downside of that is that it requires a declaration modifier on properties to opt-in. I am not opposed to going down this path at all, but I believe Chris thinks it would be too verbose for a convenience feature to require that (although he seemed to be comfortable with offering it as an alternative when requested in a specific case by the programmer).<br class=""><div class=""><br class=""></div></div></div></div><div class="">Matthew</div><div class=""><br class=""></div></body></html>