<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 Nov 16, 2016, at 7:35, David Sweeris via swift-users <<a href="mailto:swift-users@swift.org" class="">swift-users@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><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=""><br class=""><div class=""><blockquote type="cite" class=""><div class="">On Nov 15, 2016, at 11:55 PM, Howard Lovatt <<a href="mailto:howard.lovatt@gmail.com" class="">howard.lovatt@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class="">@Dave,<div class=""><br class=""></div><div class="">How do I write that though.</div><div class=""><br class=""></div><div class="">I can't write:</div><div class=""><br class=""></div><div class=""> extension Array: Equatable {<br class=""> static func ==(lhs: Array, rhs: Array) -> Bool {<br class=""> let size = lhs.count<br class=""> precondition(rhs.count == size, "The arrays must be the same length")<br class=""> for i in 0 ..< size {<br class=""> if (lhs[i] as! Equatable) != (rhs[i] as! Equatable) {<br class=""> return false<br class=""> }<br class=""> }<br class=""> return true<br class=""> }<br class=""> }</div><div class=""><br class=""></div><div class="">Because I can't cast to an Equatable, because Equatable uses Self.</div><div class=""><br class=""></div><div class="">Am I missing something?</div><div class=""><br class=""></div><div class=""> -- Howard.</div></div><div class="gmail_extra"><br clear="all" class=""><div class=""><div class="gmail_signature" data-smartmail="gmail_signature"> -- Howard.<br class=""></div></div>
<br class=""><div class="gmail_quote">On 16 November 2016 at 16:35, David Sweeris <span dir="ltr" class=""><<a href="mailto:davesweeris@mac.com" target="_blank" class="">davesweeris@mac.com</a>></span> wrote:<br class=""><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="HOEnZb"><div class="h5"><br class="">
> On Nov 15, 2016, at 21:39, Howard Lovatt via swift-users <<a href="mailto:swift-users@swift.org" class="">swift-users@swift.org</a>> wrote:<br class="">
><br class="">
> Hi All,<br class="">
><br class="">
> Does anyone have a good workaround for generics not currently supporting conditional conformance to a protocol. As stated in the Generics Manifesto something like this would be nice:<br class="">
><br class="">
> extension Array: Equatable where Element: Equatable {<br class="">
> static func ==(lhs: Array, rhs: Array) -> Bool { ... }<br class="">
> }<br class="">
><br class="">
> But I would currently write a wrapper, something like:<br class="">
><br class="">
> struct ArrayE<T: Equatable> {<br class="">
> var elements: [T]<br class="">
> }<br class="">
> extension ArrayE: Equatable {<br class="">
> static func ==(lhs: ArrayE, rhs: ArrayE) -> Bool { ... }<br class="">
> }<br class="">
><br class="">
> This can get unwieldy when there are a lot of conditional protocol extensions required, i.e. wrappers round wrappers.<br class="">
><br class="">
> Is there a better way?<br class="">
><br class="">
> Thanks for any tips,<br class="">
><br class="">
> -- Howard.<br class="">
</div></div>> ______________________________<wbr class="">_________________<br class="">
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<br class="">
Can you make Array conform to Equatable for any T and then in the == function, if T conforms to Equatable loop the Arrays to check if they're equal, and if it doesn't conform just return false?<br class="">
<br class="">
I mean, it's still "wrong", but at least you won't get any false positives.<br class="">
<br class="">
- Dave Sweeris</blockquote></div><br class=""></div>
</div></blockquote><br class=""></div><div class="">You are correct. The work-around is to use two extensions and overload the == operator:</div><div class=""><br class=""></div><div class=""><div style="margin: 0px; line-height: normal; font-family: Menlo; color: rgb(112, 61, 170);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">extension</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures" class="">Array</span><span style="font-variant-ligatures: no-common-ligatures;" class="">: </span><span style="font-variant-ligatures: no-common-ligatures" class="">Equatable</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> {</span></div><div style="margin: 0px; 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: #bb2ca2" class="">public</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">static</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">func</span><span style="font-variant-ligatures: no-common-ligatures" class=""> == (lhs: </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">Array</span><span style="font-variant-ligatures: no-common-ligatures" class="">, rhs: </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">Array</span><span style="font-variant-ligatures: no-common-ligatures" class="">) -> </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">Bool</span><span style="font-variant-ligatures: no-common-ligatures" class=""> {</span></div><div style="margin: 0px; line-height: normal; font-family: Menlo; color: rgb(187, 44, 162);" class=""><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures" class="">return</span><span style="font-variant-ligatures: no-common-ligatures;" class=""> </span><span style="font-variant-ligatures: no-common-ligatures" class="">false</span></div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> }</span></div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">}</span></div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">extension</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">Array</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">where</span><span style="font-variant-ligatures: no-common-ligatures" class=""> Element : Equatable {</span></div><div style="margin: 0px; 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: #bb2ca2" class="">public</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">static</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">func</span><span style="font-variant-ligatures: no-common-ligatures" class=""> == (lhs: </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">Array</span><span style="font-variant-ligatures: no-common-ligatures" class="">, rhs: </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">Array</span><span style="font-variant-ligatures: no-common-ligatures" class="">) -> </span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">Bool</span><span style="font-variant-ligatures: no-common-ligatures" class=""> {</span></div><div style="margin: 0px; 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: #bb2ca2" class="">return</span><span style="font-variant-ligatures: no-common-ligatures" class=""> lhs.</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">count</span><span style="font-variant-ligatures: no-common-ligatures" class=""> == rhs.</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">count</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #3d1d81" class="">&&</span><span style="font-variant-ligatures: no-common-ligatures" class=""> {</span></div><div style="margin: 0px; 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: #bb2ca2" class="">for</span><span style="font-variant-ligatures: no-common-ligatures" class=""> i </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">in</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #272ad8" class="">0</span><span style="font-variant-ligatures: no-common-ligatures" class="">..<lhs.</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">count</span><span style="font-variant-ligatures: no-common-ligatures" class=""> {</span></div><div style="margin: 0px; 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: #bb2ca2" class="">if</span><span style="font-variant-ligatures: no-common-ligatures" class=""> lhs[i] != rhs[i] {</span></div><div style="margin: 0px; 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: #bb2ca2" class="">return</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">false</span></div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> }</span></div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> }</span></div><div style="margin: 0px; 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: #bb2ca2" class="">return</span><span style="font-variant-ligatures: no-common-ligatures" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">true</span></div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> }()</span></div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> }</span></div><div style="margin: 0px; line-height: normal; font-family: Menlo;" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">}</span></div><div class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><br class=""></span></div></div>It works in playgrounds (Xcode 8.1 (8B62)), but I haven’t tested it outside a few trivial cases.</div></div></blockquote><br class=""></div><div>This does not work. The == dispatch for Arrays is static in this case, not dynamic. You can test this by writing something generic on Equatable.</div><div><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div>func same<T: Equatable>(_ x: T, _ y: T) -> Bool { return x == y }</div><div>print(same([1], [2]))</div></blockquote><div class=""><br class=""></div>Rule of thumb: overloads are resolved statically, protocol requirements are invoked dynamically. You cannot get dynamic behavior out of just overloads, ever.<div class=""><br class=""></div><div class="">I don't think there's a way to get this behavior today, unfortunately.</div><div class=""><br class=""><div class="">Jordan</div></div></body></html>