Can a swift switch be exhaustive for type Double without a default case?

Question

Can a swift switch be exhaustive for type Double without a default case?

This switch (without a default case) gives the error: switch must be exhaustive:

    var minY = 1.0

    switch minY {
    case -(Double.infinity)..<0.9:
        yAxisMinimum = 0.0
    case (0.9..<0.99):
        yAxisMinimum = 0.9
    case (0.99..<0.999):
        yAxisMinimum = 0.99
    case (0.999..<0.9999):
        yAxisMinimum = 0.999
    case (0.9999...Double.infinity):
        yAxisMinimum = 0.9999
    }

But this switch, with the (useless) default case, works:

    var minY = 1.0

    switch minY {
    case -(Double.infinity)..<0.9:
        yAxisMinimum = 0.0
    case (0.9..<0.99):
        yAxisMinimum = 0.9
    case (0.99..<0.999):
        yAxisMinimum = 0.99
    case (0.999..<0.9999):
        yAxisMinimum = 0.999
    case (0.9999...Double.infinity):
        yAxisMinimum = 0.9999
    default:
        yAxisMinimum = 0.0
    }

I try to avoid default cases with my switches, but don't know if that's possible with a Double.

Answer 1

No because only enum types can be exhaustively checked.

But in this case, the problem is even deeper. Even if Integers could be exhaustively checked, you still couldn't exhaustively check Double without a where clause. One of the options is .nan ("not a number"), which you're not considering. So you might think to just add that case:

case .nan:
    yAxisMinimum = .nan

Not only won't this make it exhaustive, it won't even work the way you'd expect.

var minY = Double.nan

switch minY {
case -(Double.infinity)..<0.9:
    yAxisMinimum = 0.0
// ...
case .nan:
    yAxisMinimum = .nan
default:
    yAxisMinimum = 0
}

yAxisMinimum // 0

Why? Because of this:

var minY = Double.nan
minY == .nan   // false

NaN is unequal to everything, including NaN. So there's no way to include it directly in a switch statement. You have to use a where clause:

case _ where minY.isNaN:
    yAxisMinimum = .nan

And that's definitely beyond the compiler's ability to validate.