<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="">This could be useful in categorizing and grouping within large enums ErrorType enums come to mind. Would there be any problem with making the subset more explicit? e.g.</div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><br class=""></div><div class="">``` swift<br class="">enum Colors {<br class=""><span class="" style="white-space: pre;"> </span>case red, orange, yellow, green, blue, indigo, violet<br class=""><span class="" style="white-space: pre;"> </span>…<br class="">}<br class=""><font color="#5856d6" class=""><br class=""></font>extension Colors {<br class=""><span class="" style="white-space: pre;"> </span>subset LCDColors red, green, blue</div><div class="">}<br class=""><div dir="ltr" class=""><div class="">```</div></div></div><div class=""><div dir="ltr" class=""><div class=""><br class=""></div></div></div><div><blockquote type="cite" class=""><div class="">On Jun 3, 2016, at 7:22 AM, T.J. Usiyan via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class=""><div class="">This is loosely related to but not meant to 'compete' with the ad hoc enum proposal.</div><div class=""><br class=""></div><div class="">## Introduction</div><div class=""><br class=""></div><div class="">This proposal adds/creates syntax to allow ad hoc creation of enums whose members are strict subsets of explicitly defined enums.</div><div class=""><br class=""></div><div class="">Swift-evolution thread: [Discussion thread topic for that proposal](<a href="http://news.gmane.org/gmane.comp.lang.swift.evolution" class="">http://news.gmane.org/gmane.comp.lang.swift.evolution</a>)</div><div class=""><br class=""></div><div class="">## Motivation</div><div class="">Consider a situation where we have an enum `Colors` which represents the entire set of colors relevant to your application with many salient methods and operations. We have also declared an enum `LCDColorModel` with only three colors, `red, blue, green` .</div><div class=""><br class=""></div><div class="">``` swift</div><div class="">enum Colors {</div><div class=""><span class="" style="white-space:pre"> </span>case red, orange, yellow, green, blue, indigo, violet</div><div class=""><span class="" style="white-space:pre"> </span>…</div><div class="">}</div><div class=""><br class=""></div><div class="">enum LCDColors {</div><div class=""><span class="" style="white-space:pre"> </span>case red, green, blue</div><div class="">}</div><div class="">```</div><div class=""><br class=""></div><div class="">The cases in `LCDColors` in our scenario do not require different behavior from their similarly named cases in `Colors`. We would like, simply stated, to explicitly restrict the cases allowed within a specific portion of our software. There are, currently, a few approaches </div><div class=""><span class="" style="white-space:pre"> </span>1. Duplicate functionality in `LCDColors` </div><div class=""><span class="" style="white-space:pre"> </span>- Completely manually</div><div class=""><span class="" style="white-space:pre"> </span>- Protocols with 'minimal' manual duplication</div><div class=""><span class="" style="white-space:pre"> </span>2. Avoid duplication by allowing conversion to `Colors`. </div><div class=""><br class=""></div><div class="">Neither of these solutions make the subset relationship between `Colors` and `LCDColors` clear or strict.</div></div></div></blockquote><div><br class=""></div><blockquote type="cite" class=""><div class=""><div dir="ltr" class=""><div class="">## Proposed solution</div><div class=""><br class=""></div><div class="">Add syntax to describe a restricted set of cases from an enum. </div><div class=""><br class=""></div><div class="">```swift</div><div class="">typealias LCDColors = Colors.(red|green|blue)</div><div class="">```</div><div class=""><br class=""></div><div class="">`LCDColors ` has all of the type and instance methods of `Colors`. Cases must appear in the same order as their original declaration. </div><div class=""><br class=""></div><div class=""><br class=""></div><div class="">## Detailed design</div><div class=""><br class=""></div><div class="">While I am unsure of the entirety of the design, I propose that name mangling be used which, along with the declaration order restriction should mean that all possible subsets have a stable and predictable name which contains all of the information necessary to infer cases. </div><div class=""><br class=""></div><div class="">## Impact on existing code</div><div class=""><br class=""></div><div class="">This is an additive change which should have no breaking change to existing code.</div><div class=""><br class=""></div><div class=""><br class=""></div></div>
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