new in swift 5.2

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struct User {
    let email: String
    let isAdmin: Bool
}

let users = [User(email: "1", isAdmin: true), User(email: "11", isAdmin: false), User(email: "12", isAdmin: true)]
users.map { $0.email }.forEach({ print($0) })
users.compactMap({ $0.email }).forEach({ print($0) })
users.filter { $0.isAdmin }.forEach({ print($0) })
users.map(\.email)
users.filter(\.isAdmin)
users.map { $0[keyPath: \User.email] }

let f1: (User) -> String = \User.email
users.map(f1)

let f2: (User) -> String = \.email
users.map(f2)


let f4 = \.email as (User) -> String
users.map(f4)

// Multi-segment key paths
users.map(\.email.count)

prefix operator ^

prefix func ^ <Root, Value>(keyPath: KeyPath<Root, Value>) -> (Root) -> Value {
  return { root in root[keyPath: keyPath] }
}

users.map(^\.email)

users.filter(^\.isAdmin)

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enum E { case one, two }

func check(e: E) {
  if e != .three {
    print("okay")
  }
}
//Swift 5.1 中，可能会对这个错误提示比较困惑：

error: binary operator '!=' cannot be applied to operands of type 'E' and '_'
  if e != .three {
     ~ ^  ~~~~~~
//在 Swift 5.2 中，将可以立马找到问题原因：

error: type 'E' has no member 'three'
  if e != .three {
          ~^~~~~

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struct ghjk {
    enum ConfigKey {
        case A, B
    }

    var flag: Bool

    var configDictionary: ConfigKey {
        return flag ? .A : .B
    }
}
let t = ghjk.init(flag: false)
let dict = t.configDictionary