<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=""><blockquote type="cite" class="">On Aug 25, 2016, at 11:48 PM, Xiaodi Wu <<a href="mailto:xiaodi.wu@gmail.com" class="">xiaodi.wu@gmail.com</a>> wrote:<br class=""></blockquote><div><blockquote type="cite" class=""><br class="Apple-interchange-newline"><div class=""><div dir="ltr" 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-stroke-width: 0px;" class=""><div class="gmail_extra"><div class="gmail_quote">On Thu, Aug 25, 2016 at 11:05 PM, Charles Srstka<span class="Apple-converted-space"> </span><span dir="ltr" class=""><<a href="mailto:cocoadev@charlessoft.com" target="_blank" class="">cocoadev@charlessoft.com</a>></span><span class="Apple-converted-space"> </span>wrote:<br class=""><blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-color: rgb(204, 204, 204); border-left-style: solid; padding-left: 1ex;"><div style="word-wrap: break-word;" class=""><span class=""><blockquote type="cite" class="">On Aug 25, 2016, at 10:24 PM, Xiaodi Wu <<a href="mailto:xiaodi.wu@gmail.com" target="_blank" class="">xiaodi.wu@gmail.com</a>> wrote:<br class=""></blockquote><div class=""><blockquote type="cite" class=""><br class=""><div class=""><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; float: none; display: inline !important;" class="">Yes, and it's covered in those previous threads. In brief, not all retroactive modeling involves extending a type:</span><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px;" class=""><br class=""></div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px;" class="">* A vendor supplies you with a closed-source library with a struct S that conforms to standard library protocol P.</div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px;" class=""><br class=""></div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px;" class="">* Protocol P requires a method named foo(), so struct S has its own implementation of foo().</div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px;" class=""><br class=""></div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px;" class="">* You extend protocol P by adding a new default implementation of foo().</div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px;" class=""><br class=""></div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px;" class="">This cannot be done if a keyword is required for overriding a default implementation. There is also nowhere for you to append any sort of "retro" keyword anywhere, because no part of your own code extends S in any way.</div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px;" class=""><br class=""></div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px;" class="">Please, please take the time to study the previous threads; we should not be re-playing existing discussions four or five times on this list.</div></div></blockquote></div><br class=""></span><div class="">How is this a problem, though? The implementation of foo() in S will not break, because it’s already been compiled in. The change will only affect code that is subsequently compiled with your extension of P in place. And it’ll be doing its job; making sure that your code to implement the protocol is there deliberately, and preventing you from accidentally mistyping the method name and thus silently failing to override the default implementation.</div></div></blockquote><div class=""><br class=""></div><div class="">So you're saying that the compiler should let you compile your extension of P, even though it knows that the default implementation of `foo()` is not explicitly overridden by S. Fine.</div><div class="">Now suppose the library is not closed-source; you're including it in your project and compiling it along with your own code. Are you supposed to fork the library?</div></div></div></div></div></blockquote></div><br class=""><div class="">Even if the library is open-source, you'll presumably be building in as its own target, which will be including only library code and not your extension, and linking it into the application in a later stage of the build process.</div><div class=""><br class=""></div><div class="">Charles</div><div class=""><br class=""></div></body></html>