<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=""><br class=""></div><div class=""><br class=""></div><br class=""><div><blockquote type="cite" class=""><div class="">On Jan 4, 2017, at 4:50 PM, Douglas Gregor via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:</div></blockquote><div><br class=""></div><div><br class=""></div></div><div><br class=""><blockquote type="cite" class=""><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=""><ul style="box-sizing: border-box; padding-left: 2em; margin-top: 0px; margin-bottom: 16px; color: rgb(51, 51, 51); font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, 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; margin-top: 0.25em;" class=""><p style="box-sizing: border-box; margin-top: 16px; margin-bottom: 16px;" class="">A Swift 3-to-4 migrator is the hardest part of the story. Ideally, the migrator to only add <code style="box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 13.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">@objc</code> in places where it is needed, so that we see some of the expected benefits of code-size reduction. However, there are two problems with doing so:</p><ol style="box-sizing: border-box; padding-left: 2em; margin-top: 0px; margin-bottom: 0px; list-style-type: lower-roman;" class=""><li style="box-sizing: border-box;" class=""><p style="box-sizing: border-box; margin-top: 16px; margin-bottom: 16px;" class="">Some of the uses that imply the need to add <code style="box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 13.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">@objc</code> come from Objective-C code, so a Swift 3-to-4 migrator would also need to compile the Objective-C code (possibly with a modified version of the Objective-C compiler) and match up the "deprecated" warnings mentioned in the Swift 3 compatibility mode bullet with Swift declarations.</p></li><li style="box-sizing: border-box; margin-top: 0.25em;" class=""><p style="box-sizing: border-box; margin-top: 16px; margin-bottom: 16px;" class="">The migrator can't reason about dynamically-constructed selectors or the behavior of other tools that might directly use the Objective-C runtime, so failing to add a <code style="box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 13.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">@objc</code> will lead to migrated programs that compile but fail to execute correctly.</p></li></ol></li></ul></div></div></div></blockquote><div>This seems fairly worrisome, but I’m sure how much so. I personally would probably prefer a guaranteed non-breaking migrator (since Swift 4 isn’t supposed to break stuff…) that adds the @objc where it was previously inferred. Sure, this won’t make existing code smaller/faster, but more importantly it won’t break it. The developer than then opt to remove the unneeded annotations over time (allowing for incremental testing and later bisection if a problem crops up).</div><div><br class=""></div></div><div><blockquote type="cite" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class=""><h3 style="box-sizing: border-box; margin-top: 24px; margin-bottom: 16px; font-size: 1.25em; line-height: 1.25; color: rgb(51, 51, 51); font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; background-color: rgb(255, 255, 255);" class="">Proposal add-on: <code class="" style="box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: inherit; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;">@objc</code> annotations on class definitions and extensions</h3><p style="box-sizing: border-box; margin-top: 0px; margin-bottom: 16px; color: rgb(51, 51, 51); font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);" class="">If the annotation burden of <code style="box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 13.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">@objc</code> introduced by this proposal is too high, we could make <code style="box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 13.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">@objc</code> on a class definition or extension thereof imply <code style="box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 13.600000381469727px; padding: 0.2em 0px; margin: 0px; background-color: rgba(0, 0, 0, 0.0392157); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">@objc</code> on those members that can be exposed to Objective-C. For example:</p><div class="highlight highlight-source-swift" style="box-sizing: border-box; margin-bottom: 16px; color: rgb(51, 51, 51); font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol'; font-size: 16px; background-color: rgb(255, 255, 255);"><pre style="box-sizing: border-box; font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 13.600000381469727px; margin-top: 0px; margin-bottom: 0px; line-height: 1.45; word-wrap: normal; padding: 16px; overflow: auto; background-color: rgb(247, 247, 247); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; word-break: normal;" class=""><span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">@objc</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">extension</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">MyClass</span> {
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"><span class="pl-c" style="box-sizing: border-box;">//</span> implicitly @objc</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"></span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">func</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">f</span>() { }
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"><span class="pl-c" style="box-sizing: border-box;">//</span> Cannot be exposed to Objective-C because tuples aren't representable in Objective-C</span>
<span class="pl-c" style="box-sizing: border-box; color: rgb(150, 152, 150);"></span> <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">func</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163);">g</span>() <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">-></span> (<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Int</span>, <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">Int</span>) { <span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93);">return</span> (<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">1</span>, <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179);">2</span>) }
}</pre></div></div></div></blockquote><div>This seems like it goes counter to cleaning up the confusion by re-introducting silent non-exposure due to use of Swift-only types. It also seems odd that @objc would cascade to members when `public` and other access levels don’t (though it could do the same inference based on the access level of the types involved.</div><div><br class=""></div><div>Overall this looks pretty appealing!</div><div><br class=""></div><div>-tim</div><div><br class=""></div></div></body></html>