Pimp my library with monoid operations on list

MonoidPML.scala

package io.datastrophic

trait Monoid[A]{
  def append(a1: A, a2: A): A
  def unit: A
}

object Monoid{
  implicit val IntMonoid = new Monoid[Int]{
    def append(a1: Int, a2: Int): Int = a1 + a2
    def unit: Int = 0
  }

  implicit val StringMonoid = new Monoid[String]{
    override def append(a1: String, a2: String): String = a1 concat a2
    override def unit: String = ""
  }
}

class ExtendedList[A: Monoid](xs: List[A]){
  def specialSum(implicit m: Monoid[A]): A = {
    xs.foldLeft(m.unit)(m.append)
  }
}

object ListConversions{
  implicit def expandList[A: Monoid](xs: List[A]): ExtendedList[A] = new ExtendedList(xs)
}

object ImplicitConversionExample {
  import ListConversions._
  import Monoid._
  
  def main(args: Array[String]): Unit ={
    println(List(1,2,3,4,5).specialSum)
    println(List("a", "b", "c", "d").specialSum)
    
    //custom operation monoid
    val multiMonoid: Monoid[Int] = new Monoid[Int] {
      override def append(a1: Int, a2: Int): Int = a1 * a2
      override def unit: Int = 1
    }

    println(List(1,2,3,4,5).specialSum(multiMonoid))
  }
}