[Surfacing Hidden Design: A Better Alternative To Interrelated Mutable Fields](http://wp.me/p3ui6n-1q) - 1) Explicit and defect-preventing expression of the states and transitions

EpidemySimulator_Explicit.scala

def randomBelow = ... 

/** When should the health state change and what to */
class HealthChange(val time: Int, val newHealth: Health) {
  def immediate = (time == 0)
}
/** The root of the health hierarchy; the health (i.e. infection) evolves in stages */
sealed abstract class Health(val infectious: Boolean, val visiblyInfectious: Boolean) {
  def evolve(): Option[HealthChange] // returns None if no further change possibly/expected
}
/** In some stages the person is infected but it isn't visible*/
sealed abstract class CovertlyInfected extends Health(infectious = true, visiblyInfectious = false) {}
/** In other stages the person is infected and it is clearly visible*/
sealed abstract class VisiblyInfected extends Health(infectious = true, visiblyInfectious = true) {}

case object HEALTHY extends Health(infectious = false, visiblyInfectious = false) {
  def evolve() = // 40% chance of getting infected, triggered on meeting an infected person
    if (randomBelow(101) <= 40/*%*/)
      Some(new HealthChange(0, INCUBATIOUS))
    else
      None // no change, stays healthy
}
case object INCUBATIOUS extends CovertlyInfected {
  def evolve() = // After 6 days if infection without visible effects becomes sick
    Some(new HealthChange(6, SICK))
}
case object SICK extends VisiblyInfected {
  def evolve() = // die with 25% on day 14 or stays sick for 2 more days, then immune
    if (randomBelow(101) <= 25/*%*/)
        Some(new HealthChange(14 - 6, DEAD))
      else
        Some(new HealthChange(16 - 6, IMMUNE))
}
case object DEAD extends VisiblyInfected {
  def evolve() = None // Dead people stay dead
}
/** The symptoms have disappeared but the person can still infect others for 2 more days */
case object IMMUNE extends CovertlyInfected {
  def evolve() =
    Some(new HealthChange(18 - 16, HEALTHY))
}

class Person (val id: Int, var health: Health = HEALTHY) {
  def tryInfect() { // upon meeting an infected person
    if (health != HEALTHY) throw new IllegalStateException("Only healthy can be infected")
    health.evolve().foreach(
      healthChng => setHealth(healthChng.newHealth) // infection happens immediately
    )
  }
  /** Set the new health stage and schedule the next health change, if any */
  def setHealth(h: Health) {
    this.health = h
    h.evolve().foreach(hNext => {
      if (hNext.immediate)
        setHealth(hNext.newHealth)
      else
        afterDelay(hNext.time) {setHealth(hNext.newHealth)}
    })
  }
}