<html><head><meta http-equiv="Content-Type" content="text/html charset=us-ascii"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">I'm not sure this is the best design for Swift. In other languages where I've used similar features, particularly Perl and Ruby, the "set a dictionary key to a default value" was by far the most common use. Particularly in Ruby, with its reference-semantics containers, it's useful to pass down 'foo[x] ||= []' as an expression into a function or method that would then populate the array:<div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class="">def populate(array)</div><div class=""> array << [1,2,3]</div><div class="">end</div><div class=""><br class=""></div><div class="">hash = {}</div><div class=""><br class=""></div><div class="">populate(hash["foo"] ||= [])</div><div class=""><br class=""></div></blockquote>Though ugly, that's a handy bit of expressivity you don't get with the proposed '??=' design. Even if '??=' returned the result, it wouldn't be able to return it as an lvalue.<div class=""><br class=""></div><div class="">I think a more expressive alternative approach to this problem would be to extend Dictionary with a `subscript(_:orDefault:)` member:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class="">extension Dictionary {</div><div class=""> subscript(key: Key, orDefault value: Value) -> Value {</div><div class=""> get {</div><div class=""> return self[key] ?? value</div><div class=""> }</div><div class=""> set {</div><div class=""> self[key] = newValue</div><div class=""> }</div><div class=""> }</div><div class="">}</div><div class=""><br class=""></div></blockquote>For the common use case of providing a default value for a dictionary key, this has several advantages. It can be used as an lvalue, as in `populate(&hash["foo", orDefault: []])`. It also avoids storing to the dictionary if the defaulted value is only read. (I think in the fullness of time, the default value probably belongs as a type parameter, e.g. `DefaultDictionary<Key: String, Value: [String], Default: []>`, but we're a ways away from that today.) It's true that this feature is dictionary-centric, whereas `??=` applies generally to any optional, but as others have noted, I'm not sure that generality is a *feature* we want to encourage.<div class=""><br class=""></div><div class="">-Joe<br class=""><div class=""><div class=""><div class=""><br class=""><div><blockquote type="cite" class=""><div class="">On Feb 12, 2016, at 9:15 PM, Douglas Gregor 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=us-ascii" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(51, 51, 51); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class="">Hello Swift community,</p><p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(51, 51, 51); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class="">The review of SE-0024 "Optional Value Setter `??=`" begins now and runs through February 18, 2016. The proposal is available here:</p><blockquote style="box-sizing: border-box; margin: 0px 0px 16px; padding: 0px 15px; border-left-width: 4px; border-left-style: solid; border-left-color: rgb(221, 221, 221); background-color: rgb(255, 255, 255);" class=""><div style="box-sizing: border-box; margin-top: 0px; margin-bottom: 0px;" class=""><font color="#777777" face="Helvetica Neue, Helvetica, Segoe UI, Arial, freesans, sans-serif, Apple Color Emoji, Segoe UI Emoji, Segoe UI Symbol" size="3" class=""><a href="https://github.com/apple/swift-evolution/blob/master/proposals/0024-optional-value-setter.md" class="">https://github.com/apple/swift-evolution/blob/master/proposals/0024-optional-value-setter.md</a></font></div></blockquote><p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(51, 51, 51); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class="">Reviews are an important part of the Swift evolution process. All reviews should be sent to the swift-evolution mailing list at</p><blockquote style="box-sizing: border-box; margin: 0px 0px 16px; padding: 0px 15px; color: rgb(119, 119, 119); border-left-width: 4px; border-left-style: solid; border-left-color: rgb(221, 221, 221); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class=""><div style="box-sizing: border-box; margin-top: 0px; margin-bottom: 0px;" class=""><a href="https://lists.swift.org/mailman/listinfo/swift-evolution" style="box-sizing: border-box; background-color: transparent; color: rgb(64, 120, 192); text-decoration: none;" class="">https://lists.swift.org/mailman/listinfo/swift-evolution</a></div></blockquote><p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(51, 51, 51); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class="">or, if you would like to keep your feedback private, directly to the review manager. When replying, please try to keep the proposal link at the top of the message:</p><blockquote style="box-sizing: border-box; margin: 0px 0px 16px; padding: 0px 15px; border-left-width: 4px; border-left-style: solid; border-left-color: rgb(221, 221, 221); background-color: rgb(255, 255, 255);" class=""><p style="color: rgb(119, 119, 119); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; box-sizing: border-box; margin-top: 0px; margin-bottom: 16px;" class="">Proposal link:</p><blockquote style="box-sizing: border-box; margin: 0px 0px 16px; padding: 0px 15px; border-left-width: 4px; border-left-style: solid; border-left-color: rgb(221, 221, 221);" class=""><div style="box-sizing: border-box; margin-top: 0px; margin-bottom: 0px;" class=""><font color="#777777" face="Helvetica Neue, Helvetica, Segoe UI, Arial, freesans, sans-serif, Apple Color Emoji, Segoe UI Emoji, Segoe UI Symbol" size="3" class=""><a href="https://github.com/apple/swift-evolution/blob/master/proposals/0024-optional-value-setter.md" class="">https://github.com/apple/swift-evolution/blob/master/proposals/0024-optional-value-setter.md</a></font></div></blockquote><p style="color: rgb(119, 119, 119); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; box-sizing: border-box; margin-top: 0px; margin-bottom: 16px;" class="">Reply text</p><blockquote style="color: rgb(119, 119, 119); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; box-sizing: border-box; margin: 0px; padding: 0px 15px; border-left-width: 4px; border-left-style: solid; border-left-color: rgb(221, 221, 221);" class=""><div style="box-sizing: border-box; margin-top: 0px; margin-bottom: 0px;" class="">Other replies</div></blockquote></blockquote><h5 style="box-sizing: border-box; margin-top: 1em; margin-bottom: 16px; line-height: 1.4; font-size: 1em; color: rgb(51, 51, 51); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; background-color: rgb(255, 255, 255);" class=""><a id="user-content-what-goes-into-a-review-1" class="anchor" href="https://github.com/apple/swift-evolution#what-goes-into-a-review-1" aria-hidden="true" style="box-sizing: border-box; background-color: transparent; color: rgb(64, 120, 192); text-decoration: none; display: inline-block; padding-right: 2px; margin-left: -18px; line-height: 1.1;"><svg aria-hidden="true" class="octicon octicon-link" height="16" role="img" version="1.1" viewBox="0 0 16 16" width="16"><path d="M4 9h1v1h-1c-1.5 0-3-1.69-3-3.5s1.55-3.5 3-3.5h4c1.45 0 3 1.69 3 3.5 0 1.41-0.91 2.72-2 3.25v-1.16c0.58-0.45 1-1.27 1-2.09 0-1.28-1.02-2.5-2-2.5H4c-0.98 0-2 1.22-2 2.5s1 2.5 2 2.5z m9-3h-1v1h1c1 0 2 1.22 2 2.5s-1.02 2.5-2 2.5H9c-0.98 0-2-1.22-2-2.5 0-0.83 0.42-1.64 1-2.09v-1.16c-1.09 0.53-2 1.84-2 3.25 0 1.81 1.55 3.5 3 3.5h4c1.45 0 3-1.69 3-3.5s-1.5-3.5-3-3.5z"></path></svg></a>What goes into a review?</h5><p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(51, 51, 51); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class="">The goal of the review process is to improve the proposal under review through constructive criticism and, eventually, determine the direction of Swift. When writing your review, here are some questions you might want to answer in your review:</p><ul style="box-sizing: border-box; padding: 0px 0px 0px 2em; margin-top: 0px; margin-bottom: 16px; color: rgb(51, 51, 51); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class=""><li style="box-sizing: border-box;" class="">What is your evaluation of the proposal?</li><li style="box-sizing: border-box;" class="">Is the problem being addressed significant enough to warrant a change to Swift?</li><li style="box-sizing: border-box;" class="">Does this proposal fit well with the feel and direction of Swift?</li><li style="box-sizing: border-box;" class="">If you have used other languages or libraries with a similar feature, how do you feel that this proposal compares to those?</li><li style="box-sizing: border-box;" class="">How much effort did you put into your review? A glance, a quick reading, or an in-depth study?</li></ul><p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(51, 51, 51); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class="">More information about the Swift evolution process is available at</p><blockquote style="box-sizing: border-box; margin: 0px 0px 16px; padding: 0px 15px; color: rgb(119, 119, 119); border-left-width: 4px; border-left-style: solid; border-left-color: rgb(221, 221, 221); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class=""><div style="box-sizing: border-box; margin-top: 0px; margin-bottom: 0px;" class=""><a href="https://github.com/apple/swift-evolution/blob/master/process.md" style="box-sizing: border-box; background-color: transparent; color: rgb(64, 120, 192); text-decoration: none;" class="">https://github.com/apple/swift-evolution/blob/master/process.md</a></div></blockquote><p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(51, 51, 51); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class="">Thank you,</p><p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(51, 51, 51); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class="">Doug Gregor</p><p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(51, 51, 51); font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class="">Review Manager</p></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=""></div></div></div></div></body></html>