[swift-evolution] [Pitch] Use "where" in combination with "??" to provide Ternary-like functionality

[Dany St-Amant]

Why reinvent the wheel, when the old trusty (but a bit cryptic according to some) tri-op can do the trick…

> Le 23 mai 2016 à 04:29, Charles Constant via swift-evolution <swift-evolution at swift.org> a écrit :
> 
> Here's a few examples of what this change would allow. 
> 
> I just plucked the first instances of other people's switch statements that I found on GitHub. 
> 
> If there were an easy way to search GitHub for chained ternary expressions, I would have added some examples of those too, since they could all be improved with this where clause + ??. 
> 
> 
> 	mutating func toggle() {
> 		switch self{
> 		case Off:
> 			self = On
> 		case On:
> 			self = Off	
> 		}	
> 	}
> 
> 
> 
> 	mutating func toggle() {
> 		self = .On where (self == .Off) ?? .Off
> 	}
> 

mutating func toggle() { self = self == .Off ? .On : .Off }

> 
> 	switch switchNumberThree {
> 		case 10, 11, 12:
> 			println("It is \(switchNumberThree)")
> 		default:
> 			("It is none of them!")
> 	}
> 
> 
> 	println(
> 		"It is \(switchNumberThree)" where 10...12 ~= switchNumberThree
> 		?? "It is none of them!"
> 	)
> 

print( 10...12 ~= switchNumberThree ? "It is \(switchNumberThree)"
       : "It's none of them" )

> 
> 	switch x {
> 	case 1:
> 		j++
> 	case 2:
> 		j++
> 	case 3:
> 		j++
> 	case 4:
> 		j++
> 		fallthrough
> 	case 5:
> 		j++
> 		fallthrough
> 	default:
> 		j++
> 	}
> 
> 
> 	j = j+1 where (4...5 ~= x) ?? j+2
> 

Broken conversion:
j += 4...5 ~= x ? 1 : 2

Proper conversion:
j += 4 ~= x ? 3 : 5 ~= x ? 2 : 1

Earlier e-mail example:
>     let foo = 
>         "positive" where ( bar > 0 )  ?? 
>         "negative" where ( bar < 0 ) ?? 
>         "zero"

let foo = bar > 0 ? "positive" :
          bar < 0 ? "negative" :
          "zero"

Dany
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