Sorting algorithms on top of OrderedList monoid

OrderedListMonoid.scala

object OrderedListMonoid {                                                                                                                                                        
  def zero: List[Int] = List[Int]()
  def plus(left: List[Int], right: List[Int]): List[Int] =
  (left, right) match {
    case (Nil, r) => r
    case (l, Nil) => l
    case (lhead :: ltail, rhead :: rtail) =>
      if (lhead <= rhead) lhead :: plus(ltail, right)
      else rhead :: plus(left, rtail)
  }
}

def insertionSort(list: List[Int]): List[Int] = { 
  var sorted = OrderedListMonoid.zero
  list.foreach { v => sorted = OrderedListMonoid.plus(sorted, List(v)) }
  sorted
}

def mergeSort(list: List[Int]): List[Int] = { 
  if (list.length <= 1) {
    list
  } else {
    val (left, right) = list.splitAt(list.length / 2)
    OrderedListMonoid.plus(mergeSort(left), mergeSort(right))
  }
}