MaybeMonadLaws.scala

Person.persons foreach { p =>

  // left unit                                                                                                                                        
  assert((Just(p) flatMap { _.mother }) == p.mother)
}

val maybes = MaybeNot +: (Person.persons map { Just(_) })
maybes foreach { m =>

  // right unit                                                                                                   
  assert((m flatMap { Just(_) }) == m)

  // associativity
  assert(
    (m flatMap { _.mother } flatMap { _.father }) ==
    (m flatMap { _.mother flatMap { _.father } }))
}