> On Aug 18, 2017, at 6:36 AM, Jonathan Hull via swift-evolution
> <swift-evolution@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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