Swift Design Pattern: Switch with 'when ... is' clauses

I like the enums in Swift, and I like the switch statement.

However, they not always work perfectly together. Take the following case:

enum Test {

    case First(data: Any)

    case Second

}

let t = Test.First(data: 3)

switch t {

case let .First(number) where number is Int: print("First(\(number)")

case .Second: print("Second")

}

In this case we get the error message:

error: switch must be exhaustive, consider adding a default clause

Which can be easily done as follows:

switch t {

case let .First(number) where number is Int: print("First(\(number)")

case .Second: print("Second")

default: print("Oeps")

}

But that does not sit right with me. I like a warning when I update an enum and forget to update an associated switch statement. Or as in de above case, suddenly start using the Test enum as .First(1.0) Using a default clause makes that impossible.

Unless... well the solution is obvious:

switch t {

case let .First(number) where number is Int: print("First(\(number)")

case .Second: print("Second")

default:

    switch t {

    case .First: print("Forgot to add a case with when..is clause")

    default: break

    }

}

A 'staggered' switch. It does not look all that nice, but it will catch any errors I might make. Either when adding new Enum cases, or when using the existing case with the wrong type for associated parameter.

Happy coding...

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