@usVexMownCzar

苹果关于 ArraySlice 的文档（ https://developer.apple.com/documentation/swift/arrayslice ）原话是这么说的“Sharing indices between collections and their subsequences is an important part of the design of Swift’s collection algorithms.”

除了 Array 和 ArraySlice ，甚至 String 和 Substring ，Data 和 Data.SubSequence 都是这样子的。

感觉最直观的好处就是可以在原对象和切片之间需要频繁调整 index 的步骤简化掉这一步骤。

比如 Apple 的示例：

```
let absences = [0, 2, 0, 4, 0, 3, 1, 0]

if let i = absences.firstIndex(where: { $0 > 0 }) { // 1
let absencesAfterFirst = absences[(i + 1)...] // 2
if let j = absencesAfterFirst.firstIndex(where: { $0 > 0 }) { // 3
print("The first day with absences had \(absences[i]).") // 4
print("The second day with absences had \(absences[j]).")
}
}

// Prints "The first day with absences had 2."
// Prints "The second day with absences had 4."
```