An attempt to use higher kinded types to "extend" the List and Option classes.

hkt.scala

// Enter :paste mode

trait Length[M[_]] {
  def length[A](xs: M[A]): Int
}

object Length {
  implicit val LengthList: Length[List] = new Length[List] {
    def length[A](xs: List[A]): Int = {
      xs.length
    }
  }

  implicit val LengthOption: Length[Option] = new Length[Option] {
    def length[A](xs: Option[A]): Int = {
      if(xs == None) 0
      else 1
    }
  }
}

// Exit :paste mode

def len[M[_]: Length, A](xs: M[A]): Int = {
  val l = implicitly[Length[M]]
  l.length(xs)
}

val list = List(1,2,3,4)
val option: Option[Int] = Some(1)

len(list)
len(option)


// With "enrich my library" pattern

trait LengthOp[M[_], A] {
  val l: Length[M]
  val value: M[A]
  def len: Int = l.length(value)
}

implicit def toLengthOp[M[_]: Length, A](xs: M[A]) = new LengthOp[M, A] {  
  val l = implicitly[Length[M]]
  val value = xs
}

object LengthOp[A] {
  val l = implicitly[Length[A]]
  l.length()
}

list.len
option.len