compactMap vs flatMap，Swift[重复]

Question

有人能描述一下flatMap与compactMap的这种行为吗？compactMap不是刚刚重命名为flatMap吗？因为我发现他们表现不同

struct Person {
  let cars: [String]?
  let name: String
  let surname: String
  let age: Int
}

let people = [
  Person(cars: nil, name: "Adam", surname: "Bayer", age: 19),
  Person(cars: ["Audi","BMW"], name: "Michael", surname: "Knight", age: 40),
  Person(cars: ["BMW", "Mercedes", "Audi"], name: "Freddy", surname: "Krueger", age: 62)
]

let flatCars = people.flatMap { $0.cars }.flatMap { $0 }
let flatCompactCars = people.flatMap { $0.cars }.compactMap { $0 }

let compactFlatCars = people.compactMap { $0.cars }.flatMap { $0 }
let compactCars = people.compactMap { $0.cars }.compactMap { $0 }

这是它打印的

flatMap flatMap:        ["Audi", "BMW", "BMW", "Mercedes", "Audi"]
flatMap compactMap:     [["Audi", "BMW"], ["BMW", "Mercedes", "Audi"]]
compactMap flatMap:     ["Audi", "BMW", "BMW", "Mercedes", "Audi"]
compactMap compactMap:  [["Audi", "BMW"], ["BMW", "Mercedes", "Audi"]]

有人能告诉我为什么当我使用compactMap嵌套数组不扁平吗？

Answer 1

对于序列，过去有两个不同的函数名为flatMap；一个用于过滤掉nil值，另一个用于连接序列。现在，其中一个（nil变体）已重命名为compactMap