<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=""><div class="">Way back in the macOS pre-X days, I remember some APIs had types like:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">struct Sample1 {</font></div></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">int a;</font></div></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">int b;</font></div></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">float c;</font></div></blockquote></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">};</font></div><div class=""><font face="Monaco" class=""><br class=""></font></div><div class=""><font face="Monaco" class="">struct Sample2 {</font></div></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">int a;</font></div></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">int b;</font></div></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">double d;</font></div></blockquote></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">};</font></div></blockquote><div class=""><br class=""></div><div class="">The types are supposed to be related, and it was supposed to be guaranteed that if you type-punned Sample1 to or from Sample2, the common members (“a” and “b” here) would have the same semantics. I guess it could be done like this:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">struct Sample {</font></div></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">int a;</font></div></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">int b;</font></div></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">union {</font></div></blockquote></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">float c;</font></div></blockquote></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">double d;</font></div></blockquote></blockquote></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">}</font></div></blockquote></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">};</font></div></blockquote><div class=""><br class=""></div><div class="">too.</div><div class=""><br class=""></div><div class="">I wonder if we need something similar in Swift:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">enum Sample {</font></div></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">@common case withFloat(a: Int, b: Int, c: Float)</font></div></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">@common case withDouble(a: Int, b: Int, d: Double)</font></div></blockquote></blockquote><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">}</font></div></blockquote><div class=""><br class=""></div><div class="">The attribute would ensure that the shared initial members would map to the same internal offset. And, if there were no cases that didn’t have the common members, you can use them as instance level properties:</div><div class=""><br class=""></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class=""><font face="Monaco" class="">var x: Sample = .withFloat(1, 2, 3.0)</font></div><div class=""><font face="Monaco" class="">//…</font></div><div class=""><font face="Monaco" class="">assert(x.a == 1) // Instead of “x.withFloat.a"</font></div></blockquote><div class=""><br class=""></div><div class="">Good idea? Does this already exist?</div><div class=""><br class=""></div><div class="">
<div style="color: rgb(0, 0, 0); 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; word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class="">— </div><div class="">Daryle Walker<br class="">Mac, Internet, and Video Game Junkie<br class="">darylew AT mac DOT com </div></div>
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