Proposal: create an Extractor for PartialFunction

Sometimes, when I create a method, I need an extractor parameter for 
pattern matching, while there is not a generic extractor trait in the Scala 
Standard Library. So we have to use a PartialFunction as the parameter 
instead.


When I use the PartialFunction, I had to create pattern matching expression 
like this.


input match {

  case x if partialFunction.isDefined(x) =>

    val data = partialFunction.apply(x)

    doSomethingWith(data)

}


This approach is not perfect because:


   1. partialFunction.isDefined and partialFunction.apply checked x twice, 
   which is slow if isDefined has complex logical, checking whether a file 
   exists for example.
   2. The if condition around temporary variable x is too trivial.

I think the better approach is converting the PartialFunction to an 
extractor and matching with that extractor


implicit final class PartialFunctionAsExtractor[A, B](pf: 
PartialFunction[A, B]) {

  object Extractor {

    def unapply(a: A) = PartialFunction.condOpt(a)(pf)

  }

}


input match {
  case partialFunction.Extractor(data) =>
    doSomethingWith(data)
}

Since this is a common approach, I hope adding the 
PartialFunctionAsExtractor to scala.Predef .

I would like to create a pull request if people like this idea.

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