[swift-evolution] Union instead of Optional

Austin Zheng austinzheng at gmail.com
Mon May 16 05:39:02 CDT 2016

I like the justification of unions as lightweight restricted ad-hoc enums. Definitely agreed with you in that the compiler will have to do more work, and should be able to handle the implicit conversion without going bonkers.



> On May 16, 2016, at 3:35 AM, Haravikk <swift-evolution at haravikk.me> wrote:
>> On 16 May 2016, at 11:17, Austin Zheng via swift-evolution <swift-evolution at swift.org> wrote:
>> If A, B, and C are not related via protocol or class inheritance, then there is almost nothing you can do with value. Otherwise you still need to test against the concrete type using a case statement or a if-else ladder.
> I think that a case statement or similar syntax will still be needed, and the case names would just be the types themselves. This would work best with support for type-narrowing, for example:
> 	func someMethod(value:(A|B|C)) {
> 		switch (value) {
> 			case .A:
> 				value.someMethodForTypeA()
> 			case .B:
> 				value.someMethodForTypeB()
> 			case .C:
> 				value.someMethodForTypeC()
> 		}
> 	}
> A union should really just be though of as a lightweight, restricted form of enum that can be declared in a quick ad-hoc fashion, similar to how tuples are a simpler form of struct.
> I’m generally a +1 for the feature, but I’d be interested to hear about how well equipped the compiler is for optimising something like this. In most cases an Optional covers what I need, and in more complex cases I’d probably declare overloads for each type (i.e- someMethod(value:A), someMethod(value:B) etc.); unions could make the latter case simpler, but will the compiler produce the same code behind the scenes, i.e- by isolating what’s unique to each type?

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