The fileprivate access control modifier was used in Swift 3 to make important data visible outside the class in which it was declared but within the same file. This is how it all works in the case of extensions:

class Person {
    fileprivate let name: String
    fileprivate let age: Int
    fileprivate let address: String
    init(name: String, age: Int, address: String) {
        self.name = name
        self.age = age
        self.address = address
    }
}

extension Person {
    func info() -> String {
    return "(self.name) (self.age) (self.address)"
    }
}

let bestFriend = Person(name: "Robert", age: 31)
bestFriend.info()

In the preceding code, we created an extension for the Person class and accessed its private properties in the info() method using String interpolation. Swift encourages the use of extensions to break code into logical groups. In Swift 4, you can now use the private access level instead of fileprivate in order to access class properties declared earlier, that is, in the extension:

class Person {
    private let name: String
    private let age: Int
    private let address: String
    init(name: String, age: Int, address: String) {
          self.name = name
          self.age = age
          self.address = address
         }
    }
    extension Person {
    func info() -> String {
     return "(self.name) (self.age) (self.address)"
     }
}
let bestFriend = Person(name: "Robert", age: 31)
bestFriend.info()

These were all the changes introduced in Swift 4. Now we will take a look at the new introductions to the language.