Extension to `String` to allow numeric indexes for ranges

String+IntegerIndex.swift

extension String {
    public subscript(range: Range<String.IndexDistance>) -> Substring {
        let lowerBound = index(startIndex, offsetBy: range.lowerBound)
        let upperBound = index(startIndex, offsetBy: range.upperBound)
        return self[lowerBound..<upperBound]
    }

    public subscript(range: ClosedRange<String.IndexDistance>) -> Substring {
        let lowerBound = index(startIndex, offsetBy: range.lowerBound)
        let upperBound = index(startIndex, offsetBy: range.upperBound)
        return self[lowerBound...upperBound]
    }

    public subscript(range: PartialRangeFrom<String.IndexDistance>) -> Substring {
        let lowerBound = index(startIndex, offsetBy: range.lowerBound)
        return self[lowerBound...]
    }

    public subscript(range: PartialRangeThrough<String.IndexDistance>) -> Substring {
        let upperBound = index(startIndex, offsetBy: range.upperBound)
        return self[...upperBound]
    }

    public subscript(range: PartialRangeUpTo<String.IndexDistance>) -> Substring {
        let upperBound = index(startIndex, offsetBy: range.upperBound)
        return self[..<upperBound]
    }
}

Thus:

let string = "Hello, World"
let result = string[3...4]
print(result)

Resulting in

lo

Obviously, result is still Substring, so you you really needed a String, you’d create one:

func foo(_ string: String) { ... }

foo(String(result))

Or modify your function to accept any StringProtocol:

func foo<S: StringProtocol>(_ string: S) { ... }

foo(result)