trait Functor[F[_]]
  def [A,B](a : F[A]) map(func : A => B) : F[B]

trait Monad[F[_] : Functor]
  def [A](a : A) identity : F[A]
  def [A](mma : F[F[A]]) flatten : F[A]
  def [A,B](ma : F[A]) flatMap(func : A => F[B]) : F[B] =
    ma map func flatten

enum Maybe[T]
  case Some(t : T)
  case Empty()

object Maybe

  given Functor[Maybe]

    def [A,B](ma : Maybe[A]) map(func : A => B) : Maybe[B] =
      ma match
        case Maybe.Empty() => Maybe.Empty()
        case Maybe.Some(a) => Maybe.Some(func(a))

  given Monad[Maybe]

    def [A](a : A) identity : Maybe[A] = Maybe.Some(a)

    def [A] (mma : Maybe[Maybe[A]]) flatten : Maybe[A] =
      mma match
        case Maybe.Empty()  => Maybe.Empty()
        case Maybe.Some(ma) => ma


@main def m = println(Maybe.Some("a").flatMap(_ => Maybe.Some("b")))