Insertion sort in Scala

object InsertionSort {
  def sort[T <% Ordered[T]](xs: List[T]): List[T] = xs match {
    case Nil => Nil
    case h :: t => insert(h, sort(t))
  }

  private def insert[T <% Ordered[T]](x: T, xs: List[T]): List[T] = xs match {
    case Nil => x :: Nil
    case h :: t if x <= h => x :: xs
    case h :: t => h :: insert(x, t)
  }
}