I think I speak for the entire Swift community when I say that Swift's enums, together with the ability to switch over them exhaustively and having compile-time guarantees, is one of the best features in Swift. I'm regularly annoyed by this when writing other languages like Java and JS (default: throw new IllegalArgumentException();)
Now, that's not to say I don't understand why this proposal is necessary. I totally get it, and the existing decisions make a lot of sense to me. But I'd like us to introduce this while maintaining the ability to guarantee exhaustive switch statements, no matter how the enum was defined. Example: imagine a theoretical SampleKit defines: public enum E { case A case B } It's implicitly non-exhaustive, possibly because the author was not aware of the default (which would likely happen often), or possibly because they made a conscious decision, as they expect to expand the cases in the future. In my app, I use SampleKit and decide that I want to make sure I handle all cases: switch e { case A: break case B: break default: break // This becomes necessary } As the proposal stands right now, I'm forced to handle any future cases. That's fine. What's not fine in my opinion, is that in doing so I lose the ability to keep this exhaustiveness moving forward. If I update SampleKit to v2.0, I want to know at compile-time if there are new cases I need to be aware of (instead of going to some generic handling path). Instead, I’m left in the same place I would in other languages like Java or JS: // No error :( switch e { case A: break case B: break default: break } Proposed Solution What I’m proposing is that we introduce a new keyword, unknown (or a better name), that serves as a way to handle cases that aren’t yet known, but not those that are. // Error: missing case C switch e { case A: break case B: break unknown: break // Would handle future cases } With this, you shouldn’t be able to use default AND unknown at the same time, as default implicitly includes unknown. Thanks for reading, and I hope you can consider this change (or some variation of it). Nacho Soto
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