monads.scala

object monads {
  // Needed for the `M[_]` notation.
  import scala.language.higherKinds

  // The typeclass represented using a trait.
  trait Monad[M[_]] {
    def unit[A](a: A): M[A]
    def bind[A,B](a: M[A])(f: A => M[B]): M[B]
  }

  object Monad {
    // Monad instance for Option.
    implicit val optionMonad: Monad[Option] = new Monad[Option] {
      def unit[A](a: A): Option[A] = Some(a)
      def bind[A,B](m: Option[A])(f: A => Option[B]): Option[B] = m.flatMap(f)
    }
  }

  // Sample usage.
  def main(args: Array[String]): Unit = {
    val M = implicitly[Monad[Option]]
    M.unit(1) // Some(1)
    M.bind(M.unit(1)) { a => M.unit(a * 2) } // Some(2)
  }
}

There doesn't seem to be a point in using this if you have to pass the M around.