<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 22, 2016, at 5:41 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=""><span 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; float: none; display: inline !important;" class="">There's been agreement even from the core team that the quality of diagnostics when conforming to a protocol is sub-par.</span><br 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=""><br 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=""><span 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; float: none; display: inline !important;" class="">The modified rule you propose has also been suggested before. The reason it doesn't help is that (1) if a method signature is mismatched accidentally due to a typo, you get a compilation error already because your type doesn't conform to the protocol (it's the quality of the error message that needs improvement);</span></div></blockquote><div><br class=""></div><div>You don’t get any error at all if there’s a default value.</div><div><br class=""></div><div>protocol P {</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>func doSomething()</div><div>}</div><div><br class=""></div><div>extension P {</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>func doSomething() { print(“Do This Thing”) }</div><div>}</div><div><br class=""></div><div>struct S: P {</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>func doSomthing() { print(“Do This Instead”) } // Whoops, doesn’t get called. And we don’t find out until mysterious behavior occurs at runtime.</div><div>}</div><div><br class=""></div><div>If there were a way to tell the compiler that the function was meant to satisfy a protocol, we could prevent the mistake above from occurring.</div><br class=""><blockquote type="cite" class=""><div class=""><span 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; float: none; display: inline !important;" class=""> (2) otherwise, if your type fulfills all protocol requirements but also implements an additional method unnecessary for conformance, what is the harm that is being prevented by a compiler error?</span></div></blockquote></div><br class=""><div class="">The fact that implementing protocol requirements that have default values is, in effect, stringly typed.</div><div class=""><br class=""></div><div class="">Charles</div><div class=""><br class=""></div></body></html>