Implicits example

implicitSort.scala

def sort[A](items: List[A])(implicit ordA: Ordering[A]): List[A] = ...

scala-repl-session.txt

scala> loudThenQuiet(new TV())
Turning the TV volume up
Turning the TV volume down

scala> loudThenQuiet(new OperaSinger())
Asking the diva to belt it out
Begging the diva to save our ears

scala> loudThenQuiet(new Baby())
<console>:41: error: Could not work out how to adjust the volume for a Baby.
              loudThenQuiet(new Baby())
                           ^

VolumeAdjuster.scala

/**
 * Adjust the volume on something that produces noise.
 */
@annotation.implicitNotFound("Could not work out how to adjust the volume for a ${A}.")
trait VolumeAdjuster[A] {
  def makeLouder(a: A): Unit
  def makeQuieter(a: A): Unit
}

case class TV()
case class OperaSinger()
case class Baby()

def loudThenQuiet[A](a: A)(implicit va: VolumeAdjuster[A]): Unit = {
  va.makeLouder(a)
  java.lang.Thread.sleep(2000)
  va.makeQuieter(a)
}

implicit object TvRemote extends VolumeAdjuster[TV] {
  def makeLouder(tv: TV) { println("Turning the TV volume up using the TV Remote control") }
  def makeQuieter(tv: TV) { println("Turning the TV volume down using the TV Remote control") }
}

implicit object Conductor extends VolumeAdjuster[OperaSinger] {
  def makeLouder(os: OperaSinger) { println("Conductor is asking the diva to belt it out"); }
  def makeQuieter(os: OperaSinger) { println("Conductor is begging the diva to save our ears"); }
}