gistfile1.scala

case class ElectricCar(b: Battery) { def batteryLevel = b.filledPercentage }

case class GasolineCar(g: GasTank) { def gasLevel = g.filledPercentage }

case class Battery(filledPercentage: Int) { def fill: Battery = Battery(100) }

case class GasTank(filledPercentage: Int) { def fill: GasTank = GasTank(100) }

trait Fills[C] {
  def fill(car: C): C
}

trait Implicits {
  implicit object ElectricCarFills extends Fills[ElectricCar] {
    def fill(car: ElectricCar) = 
      if (car.batteryLevel < 100) ElectricCar(car.b.fill) else car
  }

  implicit object GasolineCarFills extends Fills[GasolineCar] {
    def fill(car: GasolineCar) = 
      if (car.gasLevel < 100) GasolineCar(car.g.fill) else car
  }
}

object Implicits extends Implicits

object Fill {
  def fill[C](car: C)(implicit f: Fills[C]) = f.fill(car)
}

import Implicits._
import Fill._
val eCar = ElectricCar(Battery(25))
fill(eCar).batteryLevel == 100