How to write filter on Either, if such a thing must exist

EitherFilter.scala

trait Semigroup[A] {
  def op(a1: A, a2: A): A
}

trait Monoid[A] extends Semigroup[A] {
  def id: A
}

object EitherFilter {
  def filterR[A, B](e: Either[A, B])(p: B => Boolean)(implicit M: Monoid[B]): Either[A, B] =
    e match {
      case Left(a) => Left(a)
      case Right(b) => Right(if(p(b)) M.id else b)
    }
  def filterL[A, B](e: Either[A, B])(p: A => Boolean)(implicit M: Monoid[A]): Either[A, B] =
    e match {
      case Left(a) => Left(if(p(a)) M.id else a)
      case Right(b) => Right(b)
    }
}