A pedagogical free(er) monad in 23 lines of Scala

FreeMonad.scala

sealed trait Free[F[_], A] { self =>
  final def map[B](ab: A => B): Free[F, B] = Free.flatMap(self, ab andThen (Free.point[F, B](_)))
  final def flatMap[B](afb: A => Free[F, B]): Free[F, B] = Free.flatMap(self, afb)
  final def interpret[G[_]: Monad](fg: F ~> G): G[A] = self match {
    case Free.Point(a0) => a0().point[G]
    case Free.Effect(fa) => fg(fa)
    case fm : Free.FlatMap[F, A] =>
    	val ga0 = fm.fa.interpret[G](fg)
    	ga0.flatMap(a0 => fm.afb(a0).interpret[G](fg))
  }
}
object Free {
  def point[F[_], A](a: => A): Free[F, A] = Point[F, A](() => a)
  def liftF[F[_], A](fa: F[A]): Free[F, A] = Effect[F, A](fa)
  private final case class Point[F[_], A](a0: () => A) extends Free[F, A]
  private final case class Effect[F[_], A](fa: F[A]) extends Free[F, A]
  private sealed trait FlatMap[F[_], B] extends Free[F, B] {
    type A; def fa: Free[F, A]; def afb: A => Free[F, B]
  }
  private def flatMap[F[_], A0, B](fa0: Free[F, A0], afb0: A0 => Free[F, B]): FlatMap[F, B] = new FlatMap[F, B] {
    type A = A0; def fa = fa0; def afb = afb0
  }
}