<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="">Currently, implementing an ‘<font face="SourceCodePro-Regular" class="">==</font>‘ function for a custom class can be somewhat cumbersome. In the simple hierarchy:<div class=""><br class=""></div><div class=""><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: 'Source Code Pro'; color: rgb(170, 35, 145);" class="">class<span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> A {</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: 'Source Code Pro';" class=""> <span style="font-variant-ligatures: no-common-ligatures; color: #aa2391" class="">var</span> a: <span style="font-variant-ligatures: no-common-ligatures; color: #5c2699" class="">Int</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: 'Source Code Pro';" class=""> <span style="font-variant-ligatures: no-common-ligatures; color: #aa2391" class="">init</span>(<span style="font-variant-ligatures: no-common-ligatures; color: #aa2391" class="">_</span> a: <span style="font-variant-ligatures: no-common-ligatures; color: #5c2699" class="">Int</span>) {</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: 'Source Code Pro';" class=""> <span style="font-variant-ligatures: no-common-ligatures; color: #aa2391" class="">self</span>.<span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">a</span> = a</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: 'Source Code Pro';" class=""> }</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: 'Source Code Pro';" class="">}</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: 'Source Code Pro'; min-height: 14px;" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: 'Source Code Pro';" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #aa2391" class="">class</span> B : <span style="font-variant-ligatures: no-common-ligatures; color: #406e74" class="">A</span> {</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: 'Source Code Pro';" class=""> <span style="font-variant-ligatures: no-common-ligatures; color: #aa2391" class="">var</span> b: <span style="font-variant-ligatures: no-common-ligatures; color: #5c2699" class="">Int</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: 'Source Code Pro';" class=""> <span style="font-variant-ligatures: no-common-ligatures; color: #aa2391" class="">init</span>(<span style="font-variant-ligatures: no-common-ligatures; color: #aa2391" class="">_</span> b: <span style="font-variant-ligatures: no-common-ligatures; color: #5c2699" class="">Int</span>, <span style="font-variant-ligatures: no-common-ligatures; color: #aa2391" class="">_</span> a: <span style="font-variant-ligatures: no-common-ligatures; color: #5c2699" class="">Int</span>) {</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: 'Source Code Pro';" class=""> <span style="font-variant-ligatures: no-common-ligatures; color: #aa2391" class="">self</span>.<span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">b</span> = b</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: 'Source Code Pro';" class=""> <span style="font-variant-ligatures: no-common-ligatures; color: #aa2391" class="">super</span>.<span style="font-variant-ligatures: no-common-ligatures; color: #aa2391" class="">init</span>(a)</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: 'Source Code Pro';" class=""> }</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: 'Source Code Pro';" class="">}</div></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: 'Source Code Pro';" class=""><br class=""></div><div style="margin: 0px; line-height: normal;" class="">A reasonable implementation of <font face="SourceCodePro-Regular" class="">==</font> would seem to be</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: 'Source Code Pro';" class=""><br class=""></div><div style="margin: 0px; line-height: normal;" class=""><div style="font-size: 11px; font-family: 'Source Code Pro'; margin: 0px; line-height: normal;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #aa2391" class="">func</span> ==(lhs: <span style="font-variant-ligatures: no-common-ligatures; color: #406e74" class="">A</span>, rhs: <span style="font-variant-ligatures: no-common-ligatures; color: #406e74" class="">A</span>) -> <span style="font-variant-ligatures: no-common-ligatures; color: #5c2699" class="">Bool</span> {</div><div style="font-size: 11px; font-family: 'Source Code Pro'; margin: 0px; line-height: normal;" class=""> <span style="font-variant-ligatures: no-common-ligatures; color: #aa2391" class="">return</span> lhs.<span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">a</span> == rhs.<span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">a</span></div><div style="font-size: 11px; font-family: 'Source Code Pro'; margin: 0px; line-height: normal;" class="">}</div><div style="font-size: 11px; font-family: 'Source Code Pro'; margin: 0px; line-height: normal; min-height: 14px;" class=""><br class=""></div><div style="font-size: 11px; font-family: 'Source Code Pro'; margin: 0px; line-height: normal;" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #aa2391" class="">func</span> ==(lhs: <span style="font-variant-ligatures: no-common-ligatures; color: #406e74" class="">B</span>, rhs: <span style="font-variant-ligatures: no-common-ligatures; color: #406e74" class="">B</span>) -> <span style="font-variant-ligatures: no-common-ligatures; color: #5c2699" class="">Bool</span> {</div><div style="font-size: 11px; font-family: 'Source Code Pro'; margin: 0px; line-height: normal;" class=""> <span style="font-variant-ligatures: no-common-ligatures; color: #aa2391" class="">return</span> lhs.<span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">a</span> == rhs.<span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">a</span> &&</div><div style="font-size: 11px; font-family: 'Source Code Pro'; margin: 0px; line-height: normal;" class=""> lhs.<span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">b</span> == rhs.<span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">b</span></div><div style="font-size: 11px; font-family: 'Source Code Pro'; margin: 0px; line-height: normal;" class="">}</div><div style="font-size: 11px; font-family: 'Source Code Pro'; margin: 0px; line-height: normal;" class=""><br class=""></div><div style="margin: 0px; line-height: normal;" class="">However, this fails in the case that the static type of compared variables differs from their dynamic type. E.g:</div><div style="margin: 0px; line-height: normal;" class=""><br class=""></div><div style="margin: 0px; line-height: normal;" class=""><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: 'Source Code Pro';" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #aa2391" class="">let</span> x = <span style="font-variant-ligatures: no-common-ligatures; color: #406e74" class="">A</span>(<span style="font-variant-ligatures: no-common-ligatures; color: #1c27cf" class="">3</span>)</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: 'Source Code Pro';" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #aa2391" class="">let</span> y: <span style="font-variant-ligatures: no-common-ligatures; color: #406e74" class="">A</span> = <span style="font-variant-ligatures: no-common-ligatures; color: #406e74" class="">B</span>(<span style="font-variant-ligatures: no-common-ligatures; color: #1c27cf" class="">3</span>, <span style="font-variant-ligatures: no-common-ligatures; color: #1c27cf" class="">4</span>)</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: 'Source Code Pro'; min-height: 14px;" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: 'Source Code Pro'; color: rgb(49, 89, 93);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">x</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> </span>==<span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">y </span><span style="color: rgb(0, 116, 0);" class="">// true</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: 'Source Code Pro'; color: rgb(49, 89, 93);" class=""><span style="color: rgb(0, 116, 0);" class=""><br class=""></span></div><div style="margin: 0px; line-height: normal;" class="">The immediately obvious solution is to add a check to every <font face="SourceCodePro-Regular" class="">==</font> implementation that may need to be implemented for a subtype:</div></div><div style="margin: 0px; line-height: normal;" class=""><br class=""></div><div style="margin: 0px; line-height: normal;" class=""><div style="font-size: 11px; font-family: 'Source Code Pro'; margin: 0px; line-height: normal;" class=""><span style="color: rgb(170, 35, 145);" class="">func</span> ==(lhs: <span style="color: rgb(64, 110, 116);" class="">A</span>, rhs: <span style="color: rgb(64, 110, 116);" class="">A</span>) -> <span style="color: rgb(92, 38, 153);" class="">Bool</span> {</div><div style="font-size: 11px; font-family: 'Source Code Pro'; margin: 0px; line-height: normal;" class=""> <span style="color: rgb(170, 35, 145);" class="">if</span> (lhs.<span style="color: rgb(170, 35, 145);" class="">dynamicType</span> != rhs.<span style="color: rgb(170, 35, 145);" class="">dynamicType</span>) {</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: 'Source Code Pro'; color: rgb(170, 35, 145);" class=""><span style="color: rgb(0, 0, 0);" class=""> </span>return<span style="color: rgb(0, 0, 0);" class=""> </span>false</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: 'Source Code Pro';" class=""> }</div><div style="font-size: 11px; font-family: 'Source Code Pro'; margin: 0px; line-height: normal;" class=""> <span style="color: rgb(170, 35, 145);" class="">return</span> lhs.<span style="color: rgb(79, 129, 135);" class="">a</span> == rhs.<span style="color: rgb(79, 129, 135);" class="">a</span></div><div style="font-size: 11px; font-family: 'Source Code Pro'; margin: 0px; line-height: normal;" class="">}</div></div><div style="margin: 0px; line-height: normal;" class=""><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: 'Source Code Pro';" class=""><br class=""></div><div style="margin: 0px; line-height: normal;" class="">But this results in annoying boilerplate for what should be a simple computation, and furthermore fails to solve every case:</div><div style="margin: 0px; line-height: normal;" class=""><br class=""></div><div style="margin: 0px; line-height: normal;" class=""><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: 'Source Code Pro';" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #aa2391" class="">let</span> w: <span style="font-variant-ligatures: no-common-ligatures; color: #406e74" class="">A</span> = <span style="font-variant-ligatures: no-common-ligatures; color: #406e74" class="">B</span>(<span style="font-variant-ligatures: no-common-ligatures; color: #1c27cf" class="">1</span>, <span style="font-variant-ligatures: no-common-ligatures; color: #1c27cf" class="">2</span>)</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: 'Source Code Pro';" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #aa2391" class="">var</span> z: <span style="font-variant-ligatures: no-common-ligatures; color: #406e74" class="">A</span> = <span style="font-variant-ligatures: no-common-ligatures; color: #406e74" class="">B</span>(<span style="font-variant-ligatures: no-common-ligatures; color: #1c27cf" class="">1</span>, <span style="font-variant-ligatures: no-common-ligatures; color: #1c27cf" class="">3</span>)</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: 'Source Code Pro'; min-height: 14px;" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: 'Source Code Pro'; color: rgb(0, 116, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">w</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #31595d" class="">==</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #4f8187" class="">z</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> </span>// still true</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: 'Source Code Pro'; color: rgb(0, 116, 0);" class=""><br class=""></div><div style="margin: 0px; line-height: normal;" class="">I’d be interested to know if there is any interest taken in this problem and whether possible solutions have been discussed. If <font face="SourceCodePro-Regular" class="">==</font> were instead behaved as if it were a method of its first argument (as in a <font face="SourceCodePro-Regular" class="">.equals(other)</font> method) then the solution above is sufficient to avoid returning the wrong result, but being forced to use <span style="font-family: 'Source Code Pro';" class="">.</span><span style="font-family: 'Source Code Pro'; color: rgb(170, 35, 145);" class="">dynamicType</span><span style="font-family: 'Source Code Pro';" class=""> </span>for something as basic as equality checking seems cumbersome to me.</div><div style="margin: 0px; line-height: normal;" class=""><br class=""></div><div style="margin: 0px; line-height: normal;" class="">FKL</div></div></div></div></body></html>