Fltn.scala

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scala> :paste
// Entering paste mode (ctrl-D to finish)

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

implicit class MonadOps[F[_], A](fa: F[A])(implicit F: Monad[F]) {
  def fltn[B](implicit ev: A =:= F[B]): F[B] = F.bind(fa)(x => x)
  def fltn2[B](implicit ev: A <:< F[B]): F[B] = F.bind(fa)(x => x)
  def fltn3[B](implicit ev: A <:< F[B], ev2: F[B] <:< A): F[B] = F.bind(fa)(x => x)
}

implicit val mo = new Monad[Option] {
  def pure[A](a: A) = Some(a)
  def bind[A, B](fa: Option[A])(f: A => Option[B]) = fa.flatMap(f)
}

case class Invariant[A](a: A)
implicit val mi = new Monad[Invariant] {
  def pure[A](a: A) = Invariant(a)
  def bind[A, B](fa: Invariant[A])(f: A => Invariant[B]) = f(fa.a)
}

// Exiting paste mode, now interpreting.

warning: there were two feature warnings; re-run with -feature for details
defined trait Monad
defined class MonadOps
mo: Monad[Option]{def pure[A](a: A): Some[A]} = $anon$1@16267862
defined class Invariant
mi: Monad[Invariant] = $anon$2@453da22c

scala> Option(Option(1)).fltn
<console>:17: error: Cannot prove that Option[Int] =:= Option[B].
       Option(Option(1)).fltn
                         ^

scala> Option(Option(1)).fltn2
res1: Option[Int] = Some(1)

scala> Option(Option(1)).fltn3
res2: Option[Int] = Some(1)

scala> Invariant(Invariant(1)).fltn
<console>:17: error: Cannot prove that Invariant[Int] =:= Invariant[B].
       Invariant(Invariant(1)).fltn
                               ^

scala> Invariant(Invariant(1)).fltn2
res4: Invariant[Int] = Invariant(1)

scala> Invariant(Invariant(1)).fltn3
res5: Invariant[Int] = Invariant(1)