Using temperatures Celcius, Fahrenheit and Kelvin as an AnyVal example

temps-anyval.scala

/**
 * To be a valid value class, the following rules must be followed (taken from Programming Scala 2nd Ed.):
 * 1. The value class has one and only one public val argument (as of Scala 2.11, 
 *    the argument can also be nonpublic).
 * 2. The type of the argument must not be a value class itself.
 * 3. If the value class is parameterized, the @specialized annotation can’t be used.
 * 4. The value class doesn’t define secondary constructors.
 * 5. The value class defines only methods, but no other vals and no vars.
 * 6. However, the value class can’t override equals and hashCode.
 * 7. The value class defines no nested traits, classes, or objects.
 * 8. The value class cannot be subclassed.
 * 9. The value class can only inherit from universal traits. An universal trait extends from Any.
 * 10. The value class must be a top-level type or a member of an object that can be referenced
 */
 
trait Temperature extends Any {
  def toKelvin: Kelvin
  def toCelcius: Celcius
  def toFahrenheit: Fahrenheit
}

class Kelvin(val t: Double) extends AnyVal with Temperature {
  override def toKelvin = new Kelvin(t)
  override def toCelcius = new Celcius(t - 273.15d)
  override def toFahrenheit = new Fahrenheit((t - 273.15d) * 1.8 + 32.0)
}

class Fahrenheit(val t: Double) extends AnyVal with Temperature {
  override def toKelvin = new Kelvin((t + 459.67) * 5/9)
  override def toCelcius = new Celcius((t - 32) / 1.8)
  override def toFahrenheit = new Fahrenheit(t)
}

class Celcius(val t: Double) extends AnyVal with Temperature {
  override def toKelvin = new Kelvin(t + 273.15)
  override def toCelcius = new Celcius(t)
  override def toFahrenheit = new Fahrenheit(t * 1.8 + 32.0)
}

class TemperatureReporter(t: Temperature) {
  override def toString = "[" + t.toKelvin.t + "K, " + t.toFahrenheit.t + "F, " + 
    t.toCelcius.t + "C]"
}

Seq(new Celcius(100), new Fahrenheit(212), new Kelvin(373.15)).foreach(
  t => println(new TemperatureReporter(t)))
Seq(new Celcius(0), new Fahrenheit(32), new Kelvin(273.15)).foreach(
  t => println(new TemperatureReporter(t)))