<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 Feb 20, 2017, at 12:40 AM, Niels Andriesse 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="">I'd like to discuss the possibility of treating the cases of a given enum as if they are subtypes of that enum. This seems like a natural thing to do because enum cases (especially when they have associated values) effectively define a closed set of subtypes.</div><div class=""><br class=""></div><div class="">Doing so would allow for constructions such as the following:</div><div class=""><br class=""></div><div class=""><span style="background-color:rgb(255,255,255)" class=""><font face="monospace, monospace" class="">enum Foo {</font></span></div><div class=""><span style="background-color:rgb(255,255,255)" class=""><font face="monospace, monospace" class=""><span style="white-space:pre" class=""> </span>case a(name: String)</font></span></div><div class=""><span style="background-color:rgb(255,255,255)" class=""><font face="monospace, monospace" class="">}</font></span></div><div class=""><span style="background-color:rgb(255,255,255)" class=""><font face="monospace, monospace" class=""><br class=""></font></span></div><div class=""><span style="background-color:rgb(255,255,255)" class=""><font face="monospace, monospace" class="">func isA(foo: Foo) -> Bool {</font></span></div><div class=""><span style="background-color:rgb(255,255,255)" class=""><font face="monospace, monospace" class=""> // The old way:</font></span></div><div class=""><span style="background-color:rgb(255,255,255)" class=""><font face="monospace, monospace" class=""> if case .a = foo { return true }</font></span></div><div class=""><span style="background-color:rgb(255,255,255)" class=""><font face="monospace, monospace" class=""> return false</font></span></div><div class=""><span style="background-color:rgb(255,255,255)" class=""><font face="monospace, monospace" class=""> // The new way:</font></span></div><div class=""><span style="background-color:rgb(255,255,255)" class=""><font face="monospace, monospace" class=""> return foo is .a</font></span></div><div class=""><span style="background-color:rgb(255,255,255)" class=""><font face="monospace, monospace" class="">}</font></span></div><div class=""><span style="background-color:rgb(255,255,255)" class=""><font face="monospace, monospace" class=""><br class=""></font></span></div><div class=""><span style="background-color:rgb(255,255,255)" class=""><font face="monospace, monospace" class="">func printNameIfFooIsA(foo: Foo) -> Bool {</font></span></div><div class=""><span style="background-color:rgb(255,255,255)" class=""><font face="monospace, monospace" class=""> // The old way:</font></span></div><div class=""><span style="background-color:rgb(255,255,255)" class=""><font face="monospace, monospace" class=""> if case let .a(name) = foo {</font></span></div><div class=""><span style="background-color:rgb(255,255,255)" class=""><font face="monospace, monospace" class=""> print(name)</font></span></div><div class=""><span style="background-color:rgb(255,255,255)" class=""><font face="monospace, monospace" class=""> }</font></span></div><div class=""><span style="background-color:rgb(255,255,255)" class=""><font face="monospace, monospace" class=""> // The new way (1):</font></span></div><div class=""><span style="background-color:rgb(255,255,255)" class=""><font face="monospace, monospace" class=""> if let a = foo as? .a {</font></span></div><div class=""><span style="background-color:rgb(255,255,255)" class=""><font face="monospace, monospace" class=""> print(<a href="http://a.name/" class="">a.name</a>)</font></span></div><div class=""><span style="background-color:rgb(255,255,255)" class=""><font face="monospace, monospace" class=""> }</font></span></div><div class=""><span style="background-color:rgb(255,255,255)" class=""><font face="monospace, monospace" class=""> // The new way (2):</font></span></div><div class=""><span style="background-color:rgb(255,255,255)" class=""><font face="monospace, monospace" class=""> if let name = (foo as? .a)?.name {</font></span></div><div class=""><span style="background-color:rgb(255,255,255)" class=""><font face="monospace, monospace" class=""> print(name)</font></span></div><div class=""><span style="background-color:rgb(255,255,255)" class=""><font face="monospace, monospace" class=""> }</font></span></div><div class=""><span style="background-color:rgb(255,255,255)" class=""><font face="monospace, monospace" class="">}</font></span></div><div class=""><br class=""></div><div class="">Treating an enum's cases as its subtypes would make enums easier to work with because handling them would be syntactically the same as handling other types.</div><div class=""><br class=""></div><div class="">The pattern matching capabilities of enums wouldn't be affected by this proposal.</div><div class=""><br class=""></div><div class="">Multiple other proposals have already attempted to simplify enum handling (they have particularly focused on getting rid of "if case" and adding the ability to treat enum case tests as expressions), but none of the solutions presented in those proposals have worked out so far.</div><div class=""><br class=""></div><div class="">I believe that this could be the right solution to multiple enum-related problems that have been brought up repeatedly.</div></div></div></blockquote><div><br class=""></div><div>I would like to see enum cases treated as subtypes of the enum type. This is an interesting way to refer to the type of a case. Unfortunately I don’t think it will work if we accept the proposal to give cases a compound name. If we do that the name of this case becomes `a(name:)` which is not a valid type name.</div><div><br class=""></div><div>My value subtyping manifesto discusses enum case types in detail. You may find it very interesting: <a href="https://gist.github.com/anandabits/5b7f8e3836387e893e3a1197a4bf144d" class="">https://gist.github.com/anandabits/5b7f8e3836387e893e3a1197a4bf144d</a></div><div><br class=""></div><div>In the manifesto I show how the author of an enum can give the type a name distinct from the case name. I also discuss “anonymous case types”. The are necessarily anonymous because we have no valid type identifier to use for them (but we could have access to them dynamically when we have a value of the case type).</div><div><br class=""></div><blockquote type="cite" class=""><div class=""><div dir="ltr" class=""><div class=""><br class=""></div></div>
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