find5_6.scala

  def find2[A](x:A, xs:List[A]):Option[A] = {
    if(xs.nonEmpty){
      if(xs.head == x) Some(xs.head)
      else find2(x, xs.tail)
    } else {
      None
    }
  }
  def find3[A](x:A, xs:List[A]):Option[A] = 
    xs.headOption match {
      case Some(y) if y == x => Some(y)
      case Some(y) => find3(x, xs.tail)
      case None => None
    }

  def find4[A](x:A, xs:List[A]):Option[A] = 
    xs.headOption flatMap { y => if(y == x) Some(y) else find4(x, xs.tail) }

// filter...
  def find5[A](x: A, xs: List[A]): Option[A] = xs.view.filter{x.==}.headOption

  // in scala 2.9 or later, collectFirst is available. :)
  def find6[A](x: A, xs: List[A]): Option[A] = xs collectFirst {case `x` => x}