[swift-evolution] Fast enums (was: typed throws)

[John McCall]

> On Aug 18, 2017, at 6:36 AM, Jonathan Hull via swift-evolution <swift-evolution at swift.org> wrote:
> The typed throws discussion brought me back to an old thought.
> 
> I would really like to see a new structural type, similar to tuples, which act as an anonymous enum.  These would actually be a distinct type from enums (not sure what to call them), in the same way that structs and tuples are different.  They would have a very similar syntax to enums though, so they would be easy to learn.

This is the commonly-rejected "Disjunctions in type constraints" feature.

John.

> 
> There would be two major difference from enums:
> 
> 1) Because they are structural, they can’t have associated functions or extensions
> 
> 2) They can concatenate with one another freely 
> 
> For example:
> 
> 	func foo( speed: .slow | .med | .fast ){
> 		bar(speed: speed)
> 	}
> 
> 	func bar(speed: .slow | .med | .fast | .ludicrous) {
> 		//but we couldn't call foo here because it doesn’t take .ludicrous
> 	}
> 
> Each case is it’s own mini-type in a way.  One ‘.slow’ is equivalent to any ‘.slow’ (even one from a regular enum). Essentially, it is a loosely bound group of cases, and type checking just means seeing if the list/value being passed is a subset of the list of possible cases.
> 
> I’d also like to see sugar for quick conversion from normal Swift enums:
> 
> 	enum Speed {
> 		case slow
> 		case med
> 		case fast
> 	}
> 
> 	func foo(speed: Speed | .ludicrous) {
> 		//we can’t call any functions/extensions of Speed, just like we can’t call a func from int on (Int, Int) 
> 	}
> 
> In the above case, Speed gets converted via sugar to “.speed(Speed)” and then gets concatenated with .ludicrous. Ideally, it would have the added ability to truly convert to ".slow | .med | .fast | .ludicrous” when passed to something that doesn’t know about Speed:
> 
> 	func foo(speed: Speed | .ludicrous) {
> 		switch speed {
> 		case .speed(let s): //Do something with the Speed value
> 		case .ludicrous: //Do something ludicrous
> 		} 
> 		bar(speed: speed) //This can convert to pass by unwrapping Speed to a bag of cases
> 	}
> 
> 	func bar(speed: .slow | .med | .fast | .ludicrous) {
> 		switch speed {
> 		case .slow: //
> 		case .med: //
> 		case .fast: //
> 		case .ludicrous: //
> 		}
> 		//We can’t reference Speed above because we just passed a bag of potential cases
> 	}
> 	
> 
> The end result here is that in addition to building one-off enums quickly, it lets us concatenate and extend enums for use in a limited scope.  I don’t know about you, but I run into the situation of “I want exactly this enum, but with one extra case” all the time.
> 
> I don’t know if we want typed throws, but this type of quick concatability would be very useful for adding/merging potential errors.  With the same sugar used on Speed above, it would also allow something similar to Union types, but without the most of the implementation headache that would cause.  You can take in multiple types, and you get back something you can switch on to recover the type which was passed:
> 
> 	func myFakeUnion(_ intOrStr: Int | String){
> 		switch intOrStr {
> 		case .int(let i): //Do something with int
> 		case .string(let s): //Do something with string
> 		}
> 	} 
> 
> 	myFakeUnion(12) //Sugar!
> 	myFakeUnion(.string(“Hey”)) //This works too
> 
> 
> Finally, I would love to see the switch equivalent of ‘a ? b : c’ in Swift.  I am not sure what the best syntax would be, but it would essentially work a bit like like a dictionary:
> 
> 	let mph = speed ? [.slow:10, .med:35, .fast:75]
> 
> 
> Thanks,
> Jon
> 
> 
> 
> 
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