Demo of phantom types. Inspired by this article https://www.swiftbysundell.com/articles/phantom-types-in-swift/

PhantomTypes.swift

// MARK: Phantom types

protocol Person {
  var name: String { get set }
  var position: String { get set }

  func formatTitle() -> String
}

struct BasePerson<Character>: Person {
  var name: String
  var position: String

  func formatTitle() -> String {
    return "\(name) [\(position)]"
  }
}

enum PersonCharacter {
  enum Policeman {}
  enum Doctor {}
  enum Fireman {}
}

typealias Policeman = PersonCharacter.Policeman
typealias Doctor = PersonCharacter.Doctor
typealias Fireman = PersonCharacter.Fireman

let policeman = BasePerson<Policeman>(name: "Nik", position: "Officer")
let doctor = BasePerson<Doctor>(name: "Bob", position: "Surgeon")
let fireman = BasePerson<Fireman>(name: "Tom", position: "Operator fire engine")

extension BasePerson where Character == Policeman {
  var policeDepartment: String {
    "second devision"
  }

  var typeOfGun: String {
    "pistol"
  }
}

extension BasePerson where Character == Doctor {
  var hospitalType: String {
    "state hospital"
  }

  var doctorExperience: Int {
    10
  }
}

extension BasePerson where Character == Fireman {
  var countOfFireEngines: Int {
    10
  }

  var devision: [String] {
    ["main", "secondary", "backup"]
  }
}

policeman.policeDepartment
policeman.formatTitle()
policeman.typeOfGun

doctor.doctorExperience
doctor.formatTitle()
doctor.hospitalType

fireman.countOfFireEngines
fireman.formatTitle()
fireman.devision

// MARK: End of phantom section