free-monad.scala

trait Monad[F[_]] {
  def point[A](a: A): F[A]
  def bind[A, B](fa: F[A])(f: A => F[B]): F[B]

  def map[A, B](fa: F[A])(f: A => B): F[B] =
    bind(fa)(a => point(f(a))
  def join[A](ffa: F[F[A]): F[A] =
    bind(ffa)(fa => fa)
}
                     
sealed trait Free[F[_], A] {
  def point[F[_]](a: A): Free[F, A] = Point(a)
  def flatMap[B](f: A => Free[F, B])(implicit functor: Functor[F]): Free[F, B] =
    this match {
      case Point(a)  => f(a)
      case Join(ffa) => Join(ffa.map(fa => fa.flatMap(f)))
    }
  def map[B](f: A => B)(implicit functor: Functor[F]): Free[F, B] =
    flatMap(a => Point(f(a)))
}

case class Point[F[_], A](a: A) extends Free[F, A]
case class Join[F[_], A](ff: F[Free[F, A]]) extends Free[F, A]