Abc.scala

enum Maybe[+A]:
  case Just(a:A)
  case None

given Functor[Maybe]:
  def [A,B](fa: Maybe[A]).map(f: A ⇒ B): Maybe[B] =
    import Maybe._
    fa match
      case Just(a) ⇒ Just(f(a))
      case None ⇒ None

given Monad[Maybe]:
  import Maybe._
  val functor = summon[Functor[Maybe]]
  def pure[A](a: A) = Just(a)
  def [A](ffa: Maybe[Maybe[A]]).join = 
    ffa match
      case Just(ma) ⇒ ma
      case None ⇒ None