[String] 的 Swift 扩展？

我正在尝试编写一个扩展方法[String].

看来你不能延长[String]直接（“类型‘元素’限制为非协议类型‘字符串’”），尽管我遇到了这个技巧：

protocol StringType { }
extension String: StringType { }

但我仍然不能让 Swift 类型系统对此感到满意：

extension Array where Element: StringType {
    // ["a","b","c","d","e"] -> "a, b, c, d, or e".
    func joinWithCommas() -> String {
        switch count {
        case 0, 1, 2:
            return joinWithSeparator(" or ")
        default:
            return dropLast(1).joinWithSeparator(", ") + ", or " + last!
        }
    }
}

The joinWithSeparator电话是“不明确的”。我已经尝试了我能想到的一切，比如使用(self as! [String])（以及一堆类似的变体），但似乎没有任何效果。

我怎样才能让 Swift 编译器满意呢？

编辑/更新

Swift 4或者稍后最好将集合元素限制为 StringProtocol，它也将覆盖子字符串。

extension BidirectionalCollection where Element: StringProtocol {
    var joinedWithCommas: String {
        guard let last = last else { return "" }
        return count > 2 ? dropLast().joined(separator: ", ") + ", or " + last : joined(separator: " or ")
    }
}

如果所有元素都是字符，我们可以简单地扩展 StringProtocol：

extension StringProtocol {
    func joined(with separator: String = ",", conector: String = "") -> String {
        guard let last = last else { return "" }
        if count > 2 {
            return dropLast().map(String.init).joined(separator: separator + " ") + separator + " " + conector + " " + String(last)
        }
        return map(String.init).joined(separator: " " + conector + " ")
    }
}

let elements = "abc"
let elementsJoined = elements.joined()                   // "a, b, c"
let elementsSeparated = elements.joined(conector: "or")  // "a, b, or c"
let elementsConected = elements.joined(conector: "and")  // "a, b, and c"

原答案

In 斯威夫特3.1（Xcode 8.3.2）您可以简单地将 Array 约束元素类型扩展为 String

extension Array where Element == String {
    var joinedWithCommas: String {
        guard let last = last else { return "" }
        return count > 2 ? dropLast().joined(separator: ", ") + ", or " + last : joined(separator: " or ")
    }
}

["a","b","c"].joinedWithCommas    // "a, b, or c"