Check if passed argument value is nil or empty

In Swift you can write,

func right(value: String, length: Int) -> String {
    if value.count <= length {
        return value
    } else {
        let index = value.index(value.startIndex, offsetBy: value.count-length)
        return String(value[..<index])
    }
}

There is no need to check for empty string. It will be covered in the else condition itself.

Example:

right(value: "abcdefgh", length: 3) //abcde