Monad is a principle. Cheating on the principle is alleged to be helpful. It is pro-actively unhelpful.

filter.scala

trait Monad[F[_]] {
  def flatMap[A, B](f: A => F[B]): F[A] => F[B]
  def point[A]: A => F[A]
  def filter[A](p: A => Boolean): F[A] => F[A] = 
    error("I will give you lots of money if you achieve this")
}