Alternative to PartialFunctions for Extractors - avoids double unapply calls

gistfile1.scala

// pretend this extractor is performance critical
object EvenExtractor {
  def unapply(x: Int): Option[Int] = {
    println("Testing for EVEN: " + x)
    if (x % 2 == 0) Option(x) else None
  }
}

// when you use it from a PartialFunction, it gets executed TWICE for all applicable values
Seq(1, 2, 3, 4) collect {
  case EvenExtractor(x) => x
}

// as an alternative, extract values manually to an Option and defer collection
Seq(1, 2, 3, 4) map {
  x => EvenExtractor.unapply(x)
} collect {
  case Some(x) => x
}

// but what if you have multiple extractors?
object OddExtractor {
  def unapply(x: Int): Option[Int] = {
    println("Testing for ODD: " + x)
    if (x % 2 == 0) None else Option(x)
  }
}

Seq(1, 2, 3, 4) collect {
  case EvenExtractor(x) => x
  case OddExtractor(x) => x
}

// chain Options with orElse!
Seq(1, 2, 3, 4) map { 
  x => EvenExtractor.unapply(x) orElse OddExtractor.unapply(x)
} collect {
  case Some(x) => x
}