List#contains as a type class: https://groups.google.com/forum/#!topic/scala-functional/UKoFkCiABtc

console.txt

scala> List("a", "b", "c").has(42)
<console>:10: error: type mismatch;
 found   : Int(42)
 required: String
              List("a", "b", "c").has(42)
                                      ^

scala> List("a", "b", "c").has("b")
res1: Boolean = true

Container.scala

trait Container[A] {
  def has(a: A): Boolean
}

implicit def listContainer[A](xs: List[A]): Container[A] =
  new Container[A] {
    override def has(a: A): Boolean = xs.contains(a)
  }