Exactly<br><div class="gmail_quote"><div dir="ltr">On Tue, 21 Feb 2017 at 19:50, Patrick Pijnappel via swift-evolution <<a href="mailto:swift-evolution@swift.org">swift-evolution@swift.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr" class="gmail_msg">Just to clarify, the proposal doesn't suggest to allow the associated value to be used as a subtype of the enum.<div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg"><font face="monospace, monospace" class="gmail_msg">enum Result<T> { case .success(T), .error(Error) }</font></div><div class="gmail_msg"><font face="monospace, monospace" class="gmail_msg"><br class="gmail_msg"></font></div><div class="gmail_msg"><font face="monospace, monospace" class="gmail_msg">func foo(_ x: Result<Int>) { /* ... */ }</font></div><div class="gmail_msg"><font face="monospace, monospace" class="gmail_msg">func bar(_ x: Result<Int>.success) { /* ... */ }<br class="gmail_msg"></font></div><div class="gmail_msg"><font face="monospace, monospace" class="gmail_msg"><br class="gmail_msg"></font></div><div class="gmail_msg"><font face="monospace, monospace" class="gmail_msg">// Not this:</font></div><div class="gmail_msg"><font face="monospace, monospace" class="gmail_msg">foo(5)</font></div><div class="gmail_msg"><font face="monospace, monospace" class="gmail_msg">bar(5)</font></div><div class="gmail_msg"><font face="monospace, monospace" class="gmail_msg">// But rather:</font></div><div class="gmail_msg"><font face="monospace, monospace" class="gmail_msg">foo(.success(5))</font></div><div class="gmail_msg"><font face="monospace, monospace" class="gmail_msg">bar(.success(5))</font></div><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">Effectively, Result<T>.success would behave like a struct that is a subtype of Result<T>.</div><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_extra gmail_msg"><br class="gmail_msg"><div class="gmail_quote gmail_msg"></div></div></div><div dir="ltr" class="gmail_msg"><div class="gmail_extra gmail_msg"><div class="gmail_quote gmail_msg">On Tue, Feb 21, 2017 at 12:50 PM, Joe Groff via swift-evolution <span dir="ltr" class="gmail_msg"><<a href="mailto:swift-evolution@swift.org" class="gmail_msg" target="_blank">swift-evolution@swift.org</a>></span> wrote:<br class="gmail_msg"></div></div></div><div dir="ltr" class="gmail_msg"><div class="gmail_extra gmail_msg"><div class="gmail_quote gmail_msg"><blockquote class="gmail_quote gmail_msg" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word" class="gmail_msg"><br class="gmail_msg"><div class="gmail_msg"><div class="gmail_msg"><div class="m_6673134308046907744m_-5758775723457976545h5 gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg">On Feb 20, 2017, at 1:53 PM, Matthew Johnson <<a href="mailto:matthew@anandabits.com" class="gmail_msg" target="_blank">matthew@anandabits.com</a>> wrote:</div><br class="m_6673134308046907744m_-5758775723457976545m_-7962314986248442653Apple-interchange-newline gmail_msg"><div class="gmail_msg"><blockquote type="cite" style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><div class="gmail_msg"><br class="m_6673134308046907744m_-5758775723457976545m_-7962314986248442653Apple-interchange-newline gmail_msg">On Feb 20, 2017, at 3:22 PM, Joe Groff <<a href="mailto:jgroff@apple.com" class="gmail_msg" target="_blank">jgroff@apple.com</a>> wrote:</div><br class="m_6673134308046907744m_-5758775723457976545m_-7962314986248442653Apple-interchange-newline gmail_msg"><div class="gmail_msg"><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg"><br class="m_6673134308046907744m_-5758775723457976545m_-7962314986248442653Apple-interchange-newline gmail_msg">On Feb 20, 2017, at 1:04 PM, Matthew Johnson <<a href="mailto:matthew@anandabits.com" class="gmail_msg" target="_blank">matthew@anandabits.com</a>> wrote:</div><br class="m_6673134308046907744m_-5758775723457976545m_-7962314986248442653Apple-interchange-newline gmail_msg"><div class="gmail_msg"><blockquote type="cite" style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><div class="gmail_msg"><br class="m_6673134308046907744m_-5758775723457976545m_-7962314986248442653Apple-interchange-newline gmail_msg">On Feb 20, 2017, at 2:38 PM, Joe Groff <<a href="mailto:jgroff@apple.com" class="gmail_msg" target="_blank">jgroff@apple.com</a>> wrote:</div><br class="m_6673134308046907744m_-5758775723457976545m_-7962314986248442653Apple-interchange-newline gmail_msg"><div class="gmail_msg"><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg"><br class="m_6673134308046907744m_-5758775723457976545m_-7962314986248442653Apple-interchange-newline gmail_msg">On Feb 20, 2017, at 7:32 AM, Matthew Johnson via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="gmail_msg" target="_blank">swift-evolution@swift.org</a>> wrote:</div><br class="m_6673134308046907744m_-5758775723457976545m_-7962314986248442653Apple-interchange-newline gmail_msg"><div class="gmail_msg"><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg"><br class="m_6673134308046907744m_-5758775723457976545m_-7962314986248442653Apple-interchange-newline gmail_msg">On Feb 20, 2017, at 12:40 AM, Niels Andriesse via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="gmail_msg" target="_blank">swift-evolution@swift.org</a>> wrote:</div><br class="m_6673134308046907744m_-5758775723457976545m_-7962314986248442653Apple-interchange-newline gmail_msg"><div class="gmail_msg"><div dir="ltr" class="gmail_msg"><div class="gmail_msg">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="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">Doing so would allow for constructions such as the following:</div><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg"><span style="background-color:rgb(255,255,255)" class="gmail_msg"><font face="monospace, monospace" class="gmail_msg">enum Foo {</font></span></div><div class="gmail_msg"><span style="background-color:rgb(255,255,255)" class="gmail_msg"><font face="monospace, monospace" class="gmail_msg"><span style="white-space:pre-wrap" class="gmail_msg"> </span>case a(name: String)</font></span></div><div class="gmail_msg"><span style="background-color:rgb(255,255,255)" class="gmail_msg"><font face="monospace, monospace" class="gmail_msg">}</font></span></div><div class="gmail_msg"><span style="background-color:rgb(255,255,255)" class="gmail_msg"><font face="monospace, monospace" class="gmail_msg"><br class="gmail_msg"></font></span></div><div class="gmail_msg"><span style="background-color:rgb(255,255,255)" class="gmail_msg"><font face="monospace, monospace" class="gmail_msg">func isA(foo: Foo) -> Bool {</font></span></div><div class="gmail_msg"><span style="background-color:rgb(255,255,255)" class="gmail_msg"><font face="monospace, monospace" class="gmail_msg"> // The old way:</font></span></div><div class="gmail_msg"><span style="background-color:rgb(255,255,255)" class="gmail_msg"><font face="monospace, monospace" class="gmail_msg"> if case .a = foo { return true }</font></span></div><div class="gmail_msg"><span style="background-color:rgb(255,255,255)" class="gmail_msg"><font face="monospace, monospace" class="gmail_msg"> return false</font></span></div><div class="gmail_msg"><span style="background-color:rgb(255,255,255)" class="gmail_msg"><font face="monospace, monospace" class="gmail_msg"> // The new way:</font></span></div><div class="gmail_msg"><span style="background-color:rgb(255,255,255)" class="gmail_msg"><font face="monospace, monospace" class="gmail_msg"> return foo is .a</font></span></div><div class="gmail_msg"><span style="background-color:rgb(255,255,255)" class="gmail_msg"><font face="monospace, monospace" class="gmail_msg">}</font></span></div><div class="gmail_msg"><span style="background-color:rgb(255,255,255)" class="gmail_msg"><font face="monospace, monospace" class="gmail_msg"><br class="gmail_msg"></font></span></div><div class="gmail_msg"><span style="background-color:rgb(255,255,255)" class="gmail_msg"><font face="monospace, monospace" class="gmail_msg">func printNameIfFooIsA(foo: Foo) -> Bool {</font></span></div><div class="gmail_msg"><span style="background-color:rgb(255,255,255)" class="gmail_msg"><font face="monospace, monospace" class="gmail_msg"> // The old way:</font></span></div><div class="gmail_msg"><span style="background-color:rgb(255,255,255)" class="gmail_msg"><font face="monospace, monospace" class="gmail_msg"> if case let .a(name) = foo {</font></span></div><div class="gmail_msg"><span style="background-color:rgb(255,255,255)" class="gmail_msg"><font face="monospace, monospace" class="gmail_msg"> print(name)</font></span></div><div class="gmail_msg"><span style="background-color:rgb(255,255,255)" class="gmail_msg"><font face="monospace, monospace" class="gmail_msg"> }</font></span></div><div class="gmail_msg"><span style="background-color:rgb(255,255,255)" class="gmail_msg"><font face="monospace, monospace" class="gmail_msg"> // The new way (1):</font></span></div><div class="gmail_msg"><span style="background-color:rgb(255,255,255)" class="gmail_msg"><font face="monospace, monospace" class="gmail_msg"> if let a = foo as? .a {</font></span></div><div class="gmail_msg"><span style="background-color:rgb(255,255,255)" class="gmail_msg"><font face="monospace, monospace" class="gmail_msg"> print(<a href="http://a.name/" class="gmail_msg" target="_blank">a.name</a>)</font></span></div><div class="gmail_msg"><span style="background-color:rgb(255,255,255)" class="gmail_msg"><font face="monospace, monospace" class="gmail_msg"> }</font></span></div><div class="gmail_msg"><span style="background-color:rgb(255,255,255)" class="gmail_msg"><font face="monospace, monospace" class="gmail_msg"> // The new way (2):</font></span></div><div class="gmail_msg"><span style="background-color:rgb(255,255,255)" class="gmail_msg"><font face="monospace, monospace" class="gmail_msg"> if let name = (foo as? .a)?.name {</font></span></div><div class="gmail_msg"><span style="background-color:rgb(255,255,255)" class="gmail_msg"><font face="monospace, monospace" class="gmail_msg"> print(name)</font></span></div><div class="gmail_msg"><span style="background-color:rgb(255,255,255)" class="gmail_msg"><font face="monospace, monospace" class="gmail_msg"> }</font></span></div><div class="gmail_msg"><span style="background-color:rgb(255,255,255)" class="gmail_msg"><font face="monospace, monospace" class="gmail_msg">}</font></span></div><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">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="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">The pattern matching capabilities of enums wouldn't be affected by this proposal.</div><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">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="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">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 class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">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></div></blockquote><br class="gmail_msg"></div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg">I think there are definitely places where having cases be a subtype of an enum make sense, but I don't think it makes sense for *all* cases to be subtypes. For example, with "biased" containers like Optional and Result, it makes sense for the "right" side to be a subtype and the "wrong" side to be explicitly constructed, IMO. If the types of cases overlap, it would also be *ambiguous* which case ought to be constructed when the payload is converted to the enum type</div></div></blockquote><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><br class="gmail_msg"></div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg">Identical case types would definitely be a problem but I don’t think overlapping case types are always a problem. I imagine this conversion working the same as any other ordinary overload resolution for ad-hoc overloads.</div></div></blockquote><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">Conversions happen at runtime too. `0 as Any as? Either<Int, Int>` wouldn't have any way to tell what `Either` to form if both arms of the Either were subtype candidates. An Either<T, U> in <T, U> context can end up being bound to Either<Int, Int> at runtime and interacting with runtime casts that way.</div></div></div></blockquote><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><br class="gmail_msg"></div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg">Hmm. This is unfortunate.</div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><br class="gmail_msg"></div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg">In cases where T and U overlap and form a linear hierarchy but are not identical couldn’t the runtime determine the most direct path and choose that?</div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><br class="gmail_msg"></div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg">If the compiler prohibited cases with exactly the same types like `Either<Int, Int>` from being expressed statically how do these types end up getting formed dynamically? Is there any way those operations could be failable?</div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><br class="gmail_msg"></div><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><blockquote type="cite" style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><div class="gmail_msg"><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><br class="gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg"><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><br class="gmail_msg"></div><blockquote type="cite" style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><div class="gmail_msg"><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg">—remember that enums are sums, not unions, and that's important for composability and uniform behavior with generics.<span class="m_6673134308046907744m_-5758775723457976545m_-7962314986248442653Apple-converted-space gmail_msg"> </span></div></div></blockquote><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><br class="gmail_msg"></div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg">I’ve always thought of enums as nominal discriminated unions. Maybe I’m using the wrong terminology. Can you elaborate on the difference between sums and unions? When you say union are you talking about the kind of thing some people have brought up in the past where any members in common are automatically made available on the union type?</div></div></blockquote><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">Sums maintain structure whereas unions collapse it. As a sum, Optional<T> maintains its shape even when T = Optional<U>. If it were a union, T u Nil u Nil would collapse to T u Nil, losing the distinction between the inner and outer nil and leading to problems in APIs that use the outer nil to communicate meaning about some outer structure, such as asking for the `first` element of a collection of Optionals.</div></div></div></blockquote><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><br class="gmail_msg"></div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg">Got it. This is certainly a problem for `Optional`.</div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><br class="gmail_msg"></div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg">But sometimes this behavior of collapsing the syntactic specification to a canonical sum type would be very useful. What is the reason we can’t have something syntactic type expressions like `Int | String`, `Int | String | String, `String | Int | String | Int`, etc all collapse to the same canonical structural sum type:</div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><br class="gmail_msg"></div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg">enum {</div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"> sub case int(Int), string(String)</div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg">}</div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><br class="gmail_msg"></div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg">This is how I’ve been thinking about those syntactic types. We already allow existential types to be formed using syntax that collapses to a canonical type:</div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><br class="gmail_msg"></div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg">typealias Existential1 = Protocol1 & Protocol2</div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg">typealias Existential2 = Protocol2 & Existential1 & Protocol 3 & Protocol1</div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg">typealias Existential3 = Existential1 & Protocol3</div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><br class="gmail_msg"></div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg">In this example Existential1 and Existential3 are different names for the same type.</div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><br class="gmail_msg"></div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg">Is there a reason we can’t have similar syntax that collapses to a similarly canonical sum type? If we’re going to allow case subtypes this feels to me like a very natural and useful direction. </div></div></blockquote><div class="gmail_msg"><br class="gmail_msg"></div></div></div><div class="gmail_msg">A couple reasons that come to mind:</div><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">- Most directly, we don't allow abstraction over generic constraints. `ExistentialN<T, U> = T & U` isn't allowed. As soon as you have abstraction over either unions or intersections, type checking becomes an unbounded search problem in the worst case, since every T binding is potentially equivalent to a T1 & T2 or T1 | T2 with T1 == T2 == T.</div><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">- Sums and unions both imply a matching branch structure in the code somewhere to handle both possibilities. If the number of actual possibilities is different in different situations, that's a source of bugs, such as the overloading of `nil` I mentioned previously. Even if you did allow generic T & T types, the worst result of someone seeing that as T1 & T2 is that the operations enabled through conforming to T1 and T2 map to the same conformance.</div><div class="gmail_msg"><br class="gmail_msg"></div><div class="gmail_msg">-Joe</div><span class="gmail_msg"><br class="gmail_msg"><blockquote type="cite" class="gmail_msg"><div class="gmail_msg"><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><br class="gmail_msg"></div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg">If we don’t allow it there are two problems: people have to invent a largely meaningless name for the enum and it is incompatible with any other similarly structured enum. Neither is a significant problem but they do add (seemingly) unnecessary friction to the language.</div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><br class="gmail_msg"></div><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg">I wouldn’t expect these to be widely used - they would play a similar role as tuples - but they would be very appreciated where they are used.</div><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><blockquote type="cite" style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><div class="gmail_msg"><div style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px" class="gmail_msg"><br class="gmail_msg"></div><span style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px;float:none;display:inline!important" class="gmail_msg">-Joe</span></div></blockquote></div></blockquote></span></div><br class="gmail_msg"></div><br class="gmail_msg"></blockquote></div></div></div><div dir="ltr" class="gmail_msg"><div class="gmail_extra gmail_msg"><div class="gmail_quote gmail_msg"><blockquote class="gmail_quote gmail_msg" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">_______________________________________________<br class="gmail_msg">
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