[swift-evolution] [Review] SE-0155: Normalize Enum Case Representation

[Daniel Duan]

I prefer patterns and matching cases have a many-to-one relation.

Keep in mind, our goal is to make match site more readable/teachable. I’d venture to say that given the definition of “Color”, this pattern is really unreadable. So is this kind of attempt to disambiguate, if we allow mixture of long/short forms:

case .color(hue: _, _, let alpha): // yuck


> On Apr 2, 2017, at 11:46 PM, Brent Royal-Gordon <brent at architechies.com> wrote:
> 
>> On Apr 2, 2017, at 11:17 PM, Daniel Duan via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>>> On Apr 2, 2017, at 11:10 PM, Xiaodi Wu <xiaodi.wu at gmail.com <mailto:xiaodi.wu at gmail.com>> wrote:
>>> 
>>> This rule cannot hold. You cannot have the shorthand syntax you propose, disallow mixing of shorthand syntax and the longer form, *and* allow `_` to substitute for any pattern without a label.
>>> 
>>> ```
>>> enum E {
>>>   case foo(a: Int, b: Int, c: Int)
>>>   case foo(a: String, c: String, e: String)
>>> }
>>> 
>>> let e = /* instance of E */
>>> 
>>> switch e {
>>> case foo(let a, _, _):
>>>   // what type is `a` here?
>>>   break
>>> default:
>>>   fatalError()
>>> }
>>> ```
> 
>> 
> 
>> That seems like straight up ambiguity with or without restriction on the label though? This kind of usability problem should and is discouraged by the proposed solution.
> 
> Let's make it a little more reasonable:
> 
> 	enum Color {
> 		case color(red: CGFloat, green: CGFloat, blue: CGFloat, alpha: CGFloat)
> 		case color(hue: CGFloat, saturation: CGFloat, value: CGFloat, alpha: CGFloat)
> 	}
> 
> 	let c: Color = …
> 	
> 	switch c {
> 	case .color(_, _, _, let alpha):
> 		…
> 	}
> 
> Interestingly, in this example it would probably be appropriate to match both cases. I wonder if that's true in the general case.
> 
> -- 
> Brent Royal-Gordon
> Architechies
> 

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