[swift-evolution] Union instead of Optional

[Leonardo Pessoa]

I can understand this proposal nite and I don't really think they're related to enums at all. You can today achieve the very same behaviour using an empty protocol.

protocol P { }

class A : P { }
class B : P { }
struct C : P { }

func test(value : P) { }

IMO, the proposed syntax will only create a shortcut for this. And we'd still need to test type and cast the to get anything useful. My question now is: is this such a common practice that justifies the shortcut?

-----Original Message-----
From: "Haravikk via swift-evolution" <swift-evolution at swift.org>
Sent: ‎16/‎05/‎2016 07:35 AM
To: "Austin Zheng" <austinzheng at gmail.com>
Cc: "Adrian Zubarev via swift-evolution" <swift-evolution at swift.org>; "Cao Jiannan" <frogcjn at 163.com>
Subject: Re: [swift-evolution] Union instead of Optional


> 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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