Scala: Pattern to add missing methods to already existing classes.

add_methods_to_exising_classes.scala

// Here we add the 'userInput' method to the Future class
implicit class FutureCompanionOps(f: Future.type) extends AnyVal {
  def userInput(message: String): Future[String] = Future {
    readLine(message)
  }
}

// Without the sintactic sugar:
class FutureCompanionOps(f: Future.type) extends AnyVal {
  def userInput(message: String): Future[String] = Future {
    readLine(message)
  }
}
implicit def f2ops(f: Future.type) = new FutureCompanionOps(f)

// This implicit conversion will be called every time you call
// a non-existing method on the Future companion object
// – Future.userInput thus automatically becomes f2ops(Future).userInput.

// The 'extends AnyVal' part is just an optimization telling the compiler
// to avoid instantiating the FutureCompanionOps object where possible
// and call its methods directly.

// whenever you want to add missing methods to an already existing
// class implementation, you should use this pattern.