Can Swift enums be inferred and restricted generically?

Question

I can't find the syntax, but I want to do something like this:

class MyClass {
    let stringValue: String // filled in later
    let integerValue: Int   // filled in later

    init(stringValue: String) {
        self.stringValue = stringValue
        self.integerValue = stringValue.hashValue
    }

    init(integerValue: Int) {
        self.integerValue = integerValue
        self.stringValue = String(integerValue)
    }
}

extension MyClass {
    //                     This is invalid syntax, but I think you can understand
    //           vvvvvvvvv I'm trying to give back an enum whose type is inferred
    var enumValue<T: enum>: T? {
        get {
            // This is also invalid; I want to check the type of the enum's raw value
            if T is String {
                return T(rawValue: self.stringValue)
            } else if T is Int {
                return T(rawValue: self.integerValue)
            } else {
                return nil
            }
        }
    }
}

The usage would be like:

enum MyEnum: String {
    case foo
    case bar
}

func baz(_ some: MyClass) {
    if let myEnum: MyEnum = some.enumValue {
        print(myEnum)
    }
}

let some = MyClass(stringValue: "foo")
baz(some) // prints "foo"

Is this possible in Swift? That is, to have a generically-typed field or function whose type is constricted to enums and inferred based on usage, then use that to instantiate an enum value?

Martin R · Accepted Answer · 2016-10-17T14:24:39.457

A possible solution would be a generic overloaded function:

extension MyClass {
    func enumValue<T: RawRepresentable>() -> T? where T.RawValue == String {
            return T(rawValue: stringValue)
    }
    func enumValue<T: RawRepresentable>() -> T? where T.RawValue == Int {
            return T(rawValue: integerValue)
    }
}

which is then called as

func baz(_ some: MyClass) {
    if let myEnum: MyEnum = some.enumValue() {
        print(myEnum)
    }
}

Alternatively, pass the enum type as an argument:

extension MyClass {
    func enumValue<T: RawRepresentable>(_ type: T.Type) -> T? where T.RawValue == String {
            return T(rawValue: stringValue)
    }
    func enumValue<T: RawRepresentable>(_ type: T.Type) -> T? where T.RawValue == Int {
            return T(rawValue: integerValue)
    }
}

and call it like

func baz(_ some: MyClass) {
    if let myEnum = some.enumValue(MyEnum.self) {
        print(myEnum)
    }
}

Can Swift enums be inferred and restricted generically?

1 Answers1