【Swift】Substringを簡単にしたい

/// String extension for convenient of substring
extension String {
    
    /// Index with using position of Int type
    func index(at position: Int) -> String.Index {
        return index((position.signum() >= 0 ? startIndex : endIndex), offsetBy: position)
    }
    
    /// Subscript for using like a "string[i]"
    subscript (position: Int) -> String {
        let i = index(at: position)
        return String(self[i])
    }

    /// Subscript for using like a "string[start..<end]"
    subscript (bounds: CountableRange<Int>) -> String {
        let start = index(at: bounds.lowerBound)
        let end = index(at: bounds.upperBound)
        return String(self[start..<end])
    }

    /// Subscript for using like a "string[start...end]"
    subscript (bounds: CountableClosedRange<Int>) -> String {
        let start = index(at: bounds.lowerBound)
        let end = index(at: bounds.upperBound)
        return String(self[start...end])
    }
    
    /// Subscript for using like a "string[..<end]"
    subscript (bounds: PartialRangeUpTo<Int>) -> String {
        let i = index(at: bounds.upperBound)
        return String(prefix(upTo: i))
    }

    /// Subscript for using like a "string[...end]"
    subscript (bounds: PartialRangeThrough<Int>) -> String {
        let i = index(at: bounds.upperBound)
        return String(prefix(through: i))
    }

    /// Subscript for using like a "string[start...]"
    subscript (bounds: PartialRangeFrom<Int>) -> String {
        let i = index(at: bounds.lowerBound)
        return String(suffix(from: i))
    }
}

let string = "0123456789"

print(string[3]) // -> "3"
print(string[-3]) // -> "7"
print(string[1...5]) // -> "12345"
print(string[1..<5]) // -> "1234"
print(string[...5])  // -> "012345"
print(string[...(-3)])  // -> "01234567"
print(string[..<5])  // -> "01234"
print(string[..<(-3)])  // -> "0123456"
print(string[5...])  // -> "56789"
print(string[(-3)...])  // -> "789"