public struct Person {
  public var firstName: String
  public var middleName: String?
  public var lastName: String
  public var ageInYears: Int

  public var fullName: String {
    return [firstName, middleName, lastName].flatMap{ $0 }.joinWithSeparator(" ")
  }
}

var john = Person(firstName: "John", middleName: nil, lastName: "Doe", ageInYears: 30)

/* Getting */

let firstNameState = john[.firstName] // Person.PropertyState.firstName("John")
let ageInYearsState = john[.ageInYears] // Person.PropertyState.ageInYears(30)
let fullNameState = john[.fullName] // Person.PropertyState.fullName("John Doe")

// Mutate ageInYears
john.ageInYears = 80
let ageInYearsState2 = john[.ageInYears] // Person.PropertyState.ageInYears(80)

/* Initializing keys from strings */

if let validKey = Person.Property(rawValue: "lastName") { // Person.Property.lastName
  // Get value
  let validKeyState = john[validKey] // Person.PropertyState.lastName("Doe")
}

let invalidKey = Person.Property(rawValue: "nonExistant") // nil

/* Creating from arbitrary values */

if let
  validatedFirstName = Person.PropertyState(validating: "Bob", property: .firstName),
  validatedAgeInYears = Person.PropertyState(validating: 50, property: .ageInYears)
{
  var bob = john
  // Make alterations
  validatedFirstName.set(&bob)
  validatedAgeInYears.set(&bob)
  print(bob) // Person(firstName: "Bob", middleName: nil, lastName: "Doe", ageInYears: 50)
}

// Invalid values
let failedFirstName = Person.PropertyState(validating: ["Not going to work"], property: .firstName) // nil
let failedAgeInYears = Person.PropertyState(validating: "Expects an int", property: .ageInYears) // nil

/* Transfer from one to another */

var alice = Person(firstName: "Alice", middleName: nil, lastName: "Jones", ageInYears: 40)
var susan = Person(firstName: "Susan", middleName: nil, lastName: "Smith", ageInYears: 20)

// Transfer lastName "Jones" from alice to susan
alice[.lastName].set(&susan)
print(susan) // Person(firstName: "Susan", middleName: nil, lastName: "Jones", ageInYears: 20)

// Attempt to transfer get-only fullName
alice[.fullName].set(&susan) // Fatal error (not settable)