<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 Feb 15, 2017, at 8:35 AM, Rien 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=""><blockquote type="cite" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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-size-adjust: auto; -webkit-text-stroke-width: 0px;" class=""><br class="Apple-interchange-newline">On 15 Feb 2017, at 17:02, Matthew Johnson <<a href="mailto:matthew@anandabits.com" class="">matthew@anandabits.com</a>> wrote:<br class=""><br class=""><blockquote type="cite" class=""><br class="">On Feb 15, 2017, at 9:59 AM, Rien <<a href="mailto:Rien@Balancingrock.nl" class="">Rien@Balancingrock.nl</a>> wrote:<br class=""><br class=""><blockquote type="cite" class=""><br class="">On 15 Feb 2017, at 16:45, Matthew Johnson <<a href="mailto:matthew@anandabits.com" class="">matthew@anandabits.com</a>> wrote:<br class=""><br class=""><blockquote type="cite" class=""><br class="">On Feb 15, 2017, at 9:35 AM, Rien <<a href="mailto:Rien@Balancingrock.nl" class="">Rien@Balancingrock.nl</a>> wrote:<br class=""><br class=""><br class=""><blockquote type="cite" class="">On 15 Feb 2017, at 16:11, Matthew Johnson via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:<br class=""><br class=""><br class=""><blockquote type="cite" class="">On Feb 15, 2017, at 5:59 AM, Jeremy Pereira via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:<br class=""><br class=""><br class=""><blockquote type="cite" class="">On 15 Feb 2017, at 11:11, Brent Royal-Gordon via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:<br class=""><br class=""><br class="">Our philosophy in general, however, is to default to the behavior which preserves the most flexibility for the library designer.<br class=""></blockquote><br class="">Actually, I thought the philosophy was to preserver type safety. When did that change?<br class=""><br class="">Also, when was the library designer prioritised ahead of the application developer?<br class=""><br class=""><br class=""><blockquote type="cite" class="">Both open and non-open classes are common, but we chose to give non-open classes the `public` keyword because that's the flexibility-preserving option.<br class=""></blockquote><br class="">No it isn’t, it’s the flexibility restricting option. The consumer of an open class can subclass it. The consumer of a public class cannot subclass it. How is the second more flexible than the first?<br class=""></blockquote><br class="">It reduces complexity for the library author by allowing them to opt-out of the complexity involved in supporting unknown, user-defined subclasses. It is important to allow libraries to have this flexibility. They are free to declare a class `open` if they want to allow subclassing. It’s even possibly for a library to declare all classes `open` if it wishes to do so. But *requiring* that would reduce the design space libraries are allowed to explore and / or introduce fragility by moving the subclass restriction to a comment.<br class=""><br class=""></blockquote><br class="">Why would a library author want to prohibit subclasses?<br class="">A library user can always wrap the class and subclass the wrapper.<br class=""></blockquote><br class="">This is composition, not inheritance. The most important difference is that a wrapper cannot override methods, it can only wrap and / or forward them. This means that when the superclass calls a method on `self` that method *always* invokes its version of that method rather than a subclass override. This is a very important difference.<br class=""><br class=""></blockquote><br class="">Agreed, however that does not answer the question why would a library developer want to disallow subclassing?<br class="">I do not see a use case for that. I.e. a feature that cannot be implemented without it. (without “open”)<br class=""></blockquote><br class="">The feature it enables is more robust libraries and the ability for library authors to better reason about their code. You may not find this benefit enough to be worth a language feature, but many of us do.<br class=""></blockquote><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">You start of with a claim “more robust libraries”.</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">I would really like to know the “how” of that. How does it make a library more robust?</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">I do write libraries myself, and if there is something I am missing, I very much would like to know.</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">Regards,</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">Rien.</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""></div></blockquote><div><br class=""></div><div><div>The usual thing I've run into in practice follows this general pattern:</div><div><br class=""></div><div>• A non-threadsafe class is introduced</div><div>• Years or decades pass</div><div>• Someone (me) tries to make the class threadsafe by adding locking</div><div>• In the intervening years, subclasses have been introduced that override methods now called while holding a lock, and reentrantly call back into other methods that take the lock</div><div><br class=""></div><div>I've worked around this using recursive mutexes in some cases, but in more extreme cases the subclass synchronously called out to another thread, and that thread reentered the lock.</div><div><br class=""></div><div>A defensive rule I've adopted in some new code is "public methods are not allowed to call other public methods", which avoids reentrancy issues, but also makes subclassing somewhat useless since the work is being done by private non-overridable methods.</div><div><br class=""></div><div><span class="Apple-tab-span" style="white-space: pre;"> </span>David</div><div class=""><br class=""></div></div><br class=""><blockquote type="cite" class=""><div class=""><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><blockquote type="cite" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: 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-size-adjust: auto; -webkit-text-stroke-width: 0px;" class=""><br class=""><blockquote type="cite" class=""><br class="">Rien.<br class=""><br class=""><blockquote type="cite" class=""><blockquote type="cite" class=""><br class="">There are cases where subclassing does not make sense. And thus preventing subclasses adds information for those users that don’t RTFM. But that imo is not worth the impact extra complexity places on all other users.<br class=""><br class="">Rien.<br class=""><br class=""><blockquote type="cite" class=""><blockquote type="cite" class=""><br class=""><br class="">_______________________________________________<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=""></blockquote><br class="">_______________________________________________<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=""></blockquote></blockquote></blockquote></blockquote></blockquote><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">_______________________________________________</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">swift-evolution mailing list</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class=""><a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a></span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class=""><a href="https://lists.swift.org/mailman/listinfo/swift-evolution" class="">https://lists.swift.org/mailman/listinfo/swift-evolution</a></span></div></blockquote></div><br class=""></body></html>