Enums with Static Subscripts Explained in Swift

This post assumes that you understand Swift subscripts and the difference between instance and static methods. If not, I suggest reading the following articles before continuing.

Swift Methods Explained

Subscripts Explained in Swift

Static Subscript in an Enum

enum Days: CaseIterable {
    
    case monday
    case tuesday
    case wednesday
    case thursday
    case friday
    case saturday
    case sunday
    
    static subscript(index: Int) -> Days {
        
        Days.allCases[index]
    }
}

let day = Days[3]
print(day) // thursday        

In this code, the Days enum conforms to the CaseIterable protocol. Inside Days, there are seven cases representing the seven days of the week.

Then a static subscript is declared with a parameter named index, that is of type Int.

By marking it as static, this subscript operates on Days itself — not on an instance of Days.

It maps an integer index to a specific Days case.

The implementation uses Days.allCases[index] to access the case at the given index in the allCases array.

Lastly, the static subscript is called with an index of 3.

Inside the subscript, Days.allCases[3] retrieves the fourth element (0-based indexing) from the allCases array.

Since the cases were defined in order, Days.allCases[3] corresponds to .thursday.

The day variable is assigned the .thursday case and then printed.

Note: By default a subscript is mutating, so there’s no need to explicitly mark it as such.