Swift 4.2,Xcode playground中的字符串firstIndex()函数错误

我正在读“ Swift编程语言
Swift 4.2“和”子串“部分下的”字符串和字符“一章中,以下代码为例：

let greeting = "Hello,world!"
let index = greeting.firstIndex(of: ",") ?? greeting.endindex
let beginning = greeting[..<index]
// beginning is "Hello"

// Convert the result to a String for long-term storage.
let newString = String(beginning)

我将这个块复制并粘贴到我的Xcode游乐场;但是,我收到以下错误：

Playground execution Failed:

error: MyPlayground.playground:6:13: error: value of type 'String' has 
no member 'firstIndex'
let index = greeting.firstIndex(of: ",") ?? greeting.endindex
            ^~~~~~~~ ~~~~~~~~~~

我检查了https://developer.apple.com/documentation/swift/string上的String类确实有一个名为firstIndex()的方法.

我有进口UIKit;在我操场的顶部.

你能让我知道为什么我会收到这个错误吗？

你可以尝试索引(

let index = greeting.index(of: ",") ?? greeting.endindex

as firstIndex存在于Xcode 10 beta Doc中