<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 12 Mar 2017, at 08:50, Xiaodi Wu <<a href="mailto:xiaodi.wu@gmail.com" class="">xiaodi.wu@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class="">On Sun, Mar 12, 2017 at 1:39 AM, Karl Wagner <span dir="ltr" class=""><<a href="mailto:razielim@gmail.com" target="_blank" class="">razielim@gmail.com</a>></span> wrote:<br class=""><div class="gmail_extra"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex"><div style="word-wrap:break-word" class=""><span class="gmail-"><br class=""><div class=""><blockquote type="cite" class=""><div class="">On 12 Mar 2017, at 08:21, Xiaodi Wu <<a href="mailto:xiaodi.wu@gmail.com" target="_blank" class="">xiaodi.wu@gmail.com</a>> wrote:</div><br class="gmail-m_-5255648766755841190Apple-interchange-newline"><div class=""><div dir="ltr" class="">Sorry, I'm confused. The following works:<div class=""><br class=""></div><div class="">```</div><div class=""><div style="margin:0px;font-size:11px;line-height:normal;font-family:menlo" class=""><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)" class="">protocol</span><span style="font-variant-ligatures:no-common-ligatures" class=""> Promise {</span></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(186,45,162)" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""> </span><span style="font-variant-ligatures:no-common-ligatures" class="">associatedtype</span><span style="font-variant-ligatures:no-common-ligatures" class=""> Result</span></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:menlo" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">}</span></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;min-height:13px" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""></span><br class=""></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:menlo" class=""><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)" class="">protocol</span><span style="font-variant-ligatures:no-common-ligatures" class=""> Scanner {</span></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:menlo" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)" class="">associatedtype</span><span style="font-variant-ligatures:no-common-ligatures" class=""> ScannerPromise : Promise</span></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:menlo" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)" class="">func</span><span style="font-variant-ligatures:no-common-ligatures" class=""> frobnicate<T>(</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)" class="">_</span><span style="font-variant-ligatures:no-common-ligatures" class="">: </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)" class="">T</span><span style="font-variant-ligatures:no-common-ligatures" class="">) -> ScannerPromise</span></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:menlo" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)" class="">where</span><span style="font-variant-ligatures:no-common-ligatures" class=""> ScannerPromise.Result == </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)" class="">T</span></div><div style="margin:0px;font-size:11px;line-height:normal;font-family:menlo" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">}</span></div></div><div class="">```</div><div class=""><br class=""></div><div class="">Why does it matter if `ScannerPromise` is generic or not?</div><div class="gmail_extra"><br class=""></div></div></div></blockquote></div><br class=""></span><div class="">That’s some pretty strange syntax. I admit I didn’t even try that. ScannerPromise would necessarily have to be generic in this context, because I want to bind one of its associated types to a generic parameter from a function. There is no way a non-generic type could do that.</div><div class=""><br class=""></div><div class="">That said, even though the compiler accepts your code, it doesn’t seem to actually work:</div><div class=""><br class=""></div><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px" class=""><div class=""><div class=""><font face="Courier" class="">protocol Promise {</font></div></div><div class=""><div class=""><font face="Courier" class=""> associatedtype Result</font></div></div><div class=""><div class=""><font face="Courier" class=""> func await() -> Result</font></div></div><span class="gmail-"><div class=""><div class=""><font face="Courier" class="">}</font></div></div><div class=""><div class=""><font face="Courier" class=""><br class=""></font></div></div><div class=""><div class=""><font face="Courier" class="">protocol Scanner {</font></div></div><div class=""><div class=""><font face="Courier" class=""> associatedtype ScannerPromise : Promise</font></div></div><div class=""><div class=""><font face="Courier" class=""> func frobnicate<T>(_: T) -> ScannerPromise</font></div></div><div class=""><div class=""><font face="Courier" class=""> where ScannerPromise.Result == T</font></div></div><div class=""><div class=""><font face="Courier" class="">}</font></div></div><div class=""><div class=""><font face="Courier" class=""><br class=""></font></div></div></span><div class=""><div class=""><font face="Courier" class="">func use<S: Scanner, T>(_ s: S, _ t: T) -> T {</font></div></div><div class=""><div class=""><font face="Courier" class=""><span class="gmail-m_-5255648766755841190Apple-tab-span" style="white-space:pre-wrap"> </span>return s.frobnicate(t).await()</font></div></div><div class=""><div class=""><font face="Courier" class="">}</font></div></div></blockquote><div class=""><div class=""><span class="gmail-m_-5255648766755841190Apple-tab-span" style="white-space:pre-wrap"> </span> </div></div><div class=""><br class=""></div><div class="">3.0.2: Segfault</div><div class=""><br class=""></div><div class="">3.1:</div><div class=""><br class=""></div><div class="">error: repl.swift:13:14: error: cannot invoke 'frobnicate' with an argument list of type '(T)'<br class=""> return s.frobnicate(t).await()<br class=""> ^<br class=""><br class="">repl.swift:13:14: note: expected an argument list of type '(T)'<br class=""> return s.frobnicate(t).await()</div></div></blockquote><div class=""><br class=""></div><div class="">That's because your `T` in `use` has no relationship with your `T` in `protocol Scanner`; you just happen to have chosen the same letter of the alphabet. This becomes clear if you rename:</div><div class=""><br class=""></div><div class="">```</div><div class=""><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: menlo;" class=""><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)" class="">func</span><span style="font-variant-ligatures:no-common-ligatures" class=""> use<S: </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)" class="">Scanner</span><span style="font-variant-ligatures:no-common-ligatures" class="">, U>(</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)" class="">_</span><span style="font-variant-ligatures:no-common-ligatures" class=""> s: </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)" class="">S</span><span style="font-variant-ligatures:no-common-ligatures" class="">, </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)" class="">_</span><span style="font-variant-ligatures:no-common-ligatures" class=""> t: </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)" class="">U</span><span style="font-variant-ligatures:no-common-ligatures" class="">) -> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)" class="">U</span><span style="font-variant-ligatures:no-common-ligatures" class=""> {</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: menlo;" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)" class="">return</span><span style="font-variant-ligatures:no-common-ligatures" class=""> s.frobnicate(t).await()</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: menlo;" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">}</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: menlo;" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""><br class=""></span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: menlo;" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""><b class="">// cannot invoke 'frobnicate' with an argument list of type '(U)'</b></span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: menlo;" class=""><b class="">// expected an argument list of type '(T)'</b><br class=""></div></div><div class="">```</div><div class=""><br class=""></div><div class="">However, this works:</div><div class=""><br class=""></div><div class="">```</div><div class=""><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: menlo;" class=""><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)" class="">func</span><span style="font-variant-ligatures:no-common-ligatures" class=""> use<S: </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)" class="">Scanner</span><span style="font-variant-ligatures:no-common-ligatures" class="">, T>(</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)" class="">_</span><span style="font-variant-ligatures:no-common-ligatures" class=""> s: </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)" class="">S</span><span style="font-variant-ligatures:no-common-ligatures" class="">, </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)" class="">_</span><span style="font-variant-ligatures:no-common-ligatures" class=""> t: </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)" class="">T</span><span style="font-variant-ligatures:no-common-ligatures" class="">) -> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)" class="">T</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: menlo;" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)" class="">where</span><span style="font-variant-ligatures:no-common-ligatures" class=""> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)" class="">S</span><span style="font-variant-ligatures:no-common-ligatures" class="">.ScannerPromise.Result == </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)" class="">T</span><span style="font-variant-ligatures:no-common-ligatures" class=""> {</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: menlo; color: rgb(49, 89, 93);" class=""><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)" class="">return</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> s.</span><span style="font-variant-ligatures:no-common-ligatures" class="">frobnicate</span><span style="font-variant-ligatures: no-common-ligatures;" class="">(t).</span><span style="font-variant-ligatures:no-common-ligatures" class="">await</span><span style="font-variant-ligatures: no-common-ligatures;" class="">()</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: menlo;" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">}</span></div></div><div class="">```</div><div class=""><br class=""></div><div class="">...or just this:</div><div class=""><br class=""></div><div class="">```</div><div class=""><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: menlo;" class=""><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)" class="">func</span><span style="font-variant-ligatures:no-common-ligatures" class=""> use<S: </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)" class="">Scanner</span><span style="font-variant-ligatures:no-common-ligatures" class="">>(</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: menlo;" class=""><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)" class=""> _</span><span style="font-variant-ligatures:no-common-ligatures" class=""> s: </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)" class="">S</span><span style="font-variant-ligatures:no-common-ligatures" class="">, </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)" class="">_</span><span style="font-variant-ligatures:no-common-ligatures" class=""> t: </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)" class="">S</span><span style="font-variant-ligatures:no-common-ligatures" class="">.ScannerPromise.Result</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: menlo;" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">) -> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)" class="">S</span><span style="font-variant-ligatures:no-common-ligatures" class="">.ScannerPromise.Result {</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: menlo; color: rgb(49, 89, 93);" class=""><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)" class="">return</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> s.</span><span style="font-variant-ligatures:no-common-ligatures" class="">frobnicate</span><span style="font-variant-ligatures: no-common-ligatures;" class="">(t).</span><span style="font-variant-ligatures:no-common-ligatures" class="">await</span><span style="font-variant-ligatures: no-common-ligatures;" class="">()</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: menlo;" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">}</span></div></div><div class="">```</div><div class=""><br class=""></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex"><div style="word-wrap:break-word" class=""><span class="gmail-HOEnZb"><font color="#888888" class=""><div class=""></div><div class="">- Karl</div></font></span></div></blockquote></div><br class=""></div></div>
</div></blockquote></div><br class=""><div class="">I’m pretty sure that the compiler is interpreting the constraint with “reverse emphasis”, so to speak. That is:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><div class="">func frobnicate<T>(_: T) -> ScannerPromise where ScannerPromise.Result == T</div></div></blockquote><div class=""><br class=""></div><div class="">becomes</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class="">func frobnicate<T>(_: T) -> ScannerPromise where T == ScannerPromise.Result</div></blockquote><div class=""><br class=""></div><div class="">becomes</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class="">func frobnicate(_: ScannerPromise.Result) -> ScannerPromise</div></blockquote><div class=""><br class=""></div><div class="">which is why you can’t invoke it with a generic ’T’</div><div class=""><br class=""></div><div class="">- Karl</div></body></html>