object UserEntityModule {
  case class UserEntityF[F[_], G[_]](id: G[Option[Long]] = None, username: F[String], password: F[String])

  type Id[A] = A
  type Forget[A] = Unit

  // You can also just use Option if you don't care
  // for the domain-specific type
  sealed trait Updatable[+A] {
    def foreach(f : A => Unit): Unit = this match {
      case Update(a) => f(a)
      case Keep => ()
    }
  }
  case class Update[A](a :A) extends Updatable[A]
  case object Keep extends Updatable[Nothing]


  type UserEntity = UserEntityF[Id, Id]
  type UserEntityUpdate = UserEntityF[Updatable, Forget]


  def doCreate(user : UserEntity): Unit = {
  	println(s"Created user $user")
  }

  def doUpdate(user : UserEntityUpdate): Unit = {
    user.username.foreach(u => println(s"Update username to be $u"))
    user.password.foreach(p => println(s"Update password to be $p"))
    println(s"I can't do anything with id since it's ${user.id}")
  }
}

/*
USAGE:

scala> doCreate(UserEntityF[Id, Id](Some(1), "hi", "lol"))
Created user UserEntityF(Some(1),hi,lol)

scala> doUpdate(UserEntityF[Updatable, Forget]((), Update("hi there"), Keep))
Update username to be hi there
I can't do anything with id since it's ()
*/