swift4StringExtension.swift

import Foundation

extension String {
    subscript(value: PartialRangeUpTo<Int>) -> Substring {
        get {
            return self[..<index(startIndex, offsetBy: value.upperBound)]
        }
    }
    
    subscript(value: PartialRangeThrough<Int>) -> Substring {
        get {
            return self[...index(startIndex, offsetBy: value.upperBound)]
        }
    }
    
    subscript(value: PartialRangeFrom<Int>) -> Substring {
        get {
            return self[index(startIndex, offsetBy: value.lowerBound)...]
        }
    }

    subscript(value: CountableRange<Int>) -> Substring {
        get {
            return self[index(startIndex, offsetBy: value.lowerBound)..<index(startIndex, offsetBy: value.upperBound)]
        }
    }

    subscript(value: CountableClosedRange<Int>) -> Substring {
        get {
            return self[index(startIndex, offsetBy: value.lowerBound)...index(startIndex, offsetBy: value.upperBound)]
        }
    }

    subscript(value: Int) -> Substring {
        get {
            return self[index(startIndex, offsetBy: value)..<index(startIndex, offsetBy: value+1)]
        }
    }
}

// let text = "Hello world"
// text[...4]      // "Hello"
// text[5]         // " "
// text[6...]      // "world"
// text[3...]      // "lo world"
// text[3..<7]     // "lo w"
// text[3...7]     // "lo wo"