为什么这种模式匹配在Scala中没有像预期的那样工作？

Question

I'm trying to replicate the powerful pattern matching example that Joshua Suereth presented in his Devoxx 2013 talk titled "How to wield Scala in the trenches". Unfortunately I cannot achieve what he described and I cannot understand what is wrong. Can someone give me a hint at what I'm missing? (My Scala version is 2.10.3)

Please see the self contained code below:

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 =
    Seq(Person("Emre", Seq(Residence("Antwerp", "BE"))),
      Person("Ergin", Seq(Residence("Istanbul", "TR"))))

  val Istanbul = new StringSeqContains("Istanbul")

  // #1 does not work as expected, WHY?
  println(
    people collect {
      case person @ LivesIn(Istanbul) => person
    }
  )

  // #2 works as expected
  println(
    people collect {
      case person @ LivesIn(cities) if cities.contains("Istanbul") => person
    }
  )

  // #3 works as expected
  println(
    people collect {
      case person @ Person(_, res) if res.contains(Residence("Istanbul", "TR")) => person
    }
  )

}

When I compile and run it I get:

List()
List(Person(Ergin,List(Residence(Istanbul,TR))))
List(Person(Ergin,List(Residence(Istanbul,TR))))

As denoted in the source code, I fail to grasp why the first pattern does not produce the same result as the remaining two pattern matches. Any ideas why?

Answer 1

Your LivesIn extractor requires a Seq for an argument.

The following variation does what you expect:

println(
  people collect {
    case person @ LivesIn(List("Istanbul")) => person
  }
)

Answer 2

After some thinking and Googling, I realized that one should add () to the inner extractor (thanks to The Neophyte's Guide to Scala Part 1: Extractors ).

In other words, the following works as expected:

people collect {
  case person @ LivesIn(Istanbul()) => person
}

whereas the following code silently, without any complaints, returns List() :

people collect {
  case person @ LivesIn(Istanbul) => person
}

Unless I'm mistaken in another way (eg there is way to make it work without parantheses), I think technical presenters should be more careful with the code snippets / pseudo-code snippets (so that some of the curious audience will not lose sleepless hours ;-)