// =======================
  // service interfaces
  trait OnOffDevice {
    def on: Unit
    def off: Unit
  }
  trait SensorDevice {
    def isCoffeePresent: Boolean
  }

  // =======================
  // service implementations
  class Heater extends OnOffDevice {
    def on = println("heater.on")
    def off = println("heater.off")
  }
  class PotSensor extends SensorDevice {
    def isCoffeePresent = true
  }

  // =======================
  // service declaring two dependencies that it wants injected,
  // is using value injection to declare its dependencies
  trait Warmer {
    val potSensor: SensorDevice
    val heater: OnOffDevice

    def trigger = {
      if (potSensor.isCoffeePresent) heater.on
      else heater.off
    }
  }

  // =======================
  // instantiate the services in a configuration module
  object Context {
    // this is where injection happens
    val warmer = new Warmer {
      val potSensor = new PotSensor
      val heater = new Heater
    }
  }

  Context.warmer.trigger