Enum with identical cases names with associated values of different types

Question

The following Swift code compiles:

enum GraphDataSource  {
    case array(data: [Double], start: Double?, step: Double?)
    case pairs(XYValues: [Double: Double])
    case pairs(dateValues: [Date: Double])
    case function((Double) -> Double?)

    func localizedName() -> String {
        // TODO: Create localizable strings
        return NSLocalizedString(Mirror(reflecting: self).children.first?.label ?? "", comment: "")
    }
}

It has two enum cases named pairs. But when I try to extract associated value, it turns out that I can't choose the one I want.

    var graphData = GraphDataSource.function(sin)

    switch graphData {
    case .pairs(dateValues: let vals):
        vals.keys.forEach({print($0)})
    case .pairs(XYValues: let xy): // without this case everyting compiles OK
        xy.keys.forEach({print($0)})
    default:
        break
    }

The error is: "Tuple pattern element label 'XYValues' must be 'dateValues'". Is this normal? Feels like compiler should either disallow cases of the same name or allow to switch on both.

Rob Napier · Accepted Answer · 2019-03-17 21:00:07Z

18

This is a Swift compiler bug. See SR-10077.

answered Mar 17, 2019 at 21:00

Rob Napier

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5 Comments

Paul B Over a year ago

What a relief to know that!

rmaddy Over a year ago

Can you clarify your answer about which part is the bug? Is it the mistaken ability to have two enums with the same name but different associated values or is it the inability to code the switch statement for the two cases?

Rob Napier Over a year ago

According to the bug, it is supposed to be legal; the compiler just can't actually do it. I'm not aware of anything in the Swift docs that says whether this should be legal or not.

itMaxence Over a year ago

For people coming by, this issue is still open at the day of comment. Feel free to register on the swift board to watch and up vote this bug. o/

Roman Over a year ago

And how many years does Apple need to fix this?

iUrii · Accepted Answer · 2022-02-17 22:51:23Z

There are two workaround to pass different types of associated values throw the one case:

Use Any type for an associated value and then dynamically type check/cast it:

enum DataType {
    case data(_ value: Any)
}

func process(_ type: DataType) {
    switch type {
    case .data(let value):
        if value is Int {
            print("Int value - \(value)")
        }
        else if value is String {
            print("String value - \(value)")
        }
    }
}

process(.data(10)) // Outputs: Int value - 10
process(.data("Text")) // Outputs: String value - Text

Use an additional enum to specify a needed type:

enum ValueType {
    case int(_ value: Int)
    case string(_ value: String)
}

enum DataType {
    case data(_ value: ValueType)
}

func process(_ type: DataType) {
    switch type {
    case .data(let value):
        switch value {
        case .int(let value):
            print("Int value - \(value)")
        case .string(let value):
            print("String value - \(value)")
        }
    }
}

process(.data(.int(10))) // Outputs: Int value - 10
process(.data(.string("Text"))) // Outputs: String value - Text

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