A concrete demonstration of the technique presented by Joshua Suereth in Devoxx 2013.

PatternPower.scala

case class Person(name: String, residence: Seq[Residence])
case class Residence(city: String, country: String)

object LivesIn {
  def unapply(p: Person): Option[Seq[String]] =
    Some(
      for(r <- p.residence)
      yield r.city
    )
}

class StringSeqContains(value: String) {
  def unapply(in: Seq[String]): Boolean =
    in contains value
}

object PatternPower extends App {

  val people =
    List(Person("Emre",  List(Residence("Antwerp", "BE"))),
         Person("Berk",  List(Residence("Antwerp", "BE"))),
         Person("Ergin", List(Residence("Istanbul", "TR"),
                              Residence("Ankara", "TR"))),
         Person("Ahmet", List(Residence("Istanbul", "TR"))))

  val Istanbul = new StringSeqContains("Istanbul")

  val peopleInIstanbul = people collect {
    case person @ LivesIn(Istanbul()) => person.name
  }

  println(peopleInIstanbul)

}