Decorator pattern (Design Patterns in Scala)

Decorator.scala

// http://en.wikipedia.org/wiki/Decorator_pattern

trait Coffee {
  def cost:Double
  def ingredients: String
}

class SimpleCoffee extends Coffee {
  override def cost = 1
  override def ingredients = "Coffee"
}


abstract class CoffeeDecorator(decoratedCoffee: Coffee) extends Coffee {
  val sep = ", "

  override def cost = decoratedCoffee.cost
  override def ingredients = decoratedCoffee.ingredients
}

class Milk(decoratedCoffee: Coffee) extends CoffeeDecorator(decoratedCoffee) {
  override def cost = super.cost + 0.5
  override def ingredients = super.ingredients + sep + "Milk"
}

class Whip(decoratedCoffee: Coffee) extends CoffeeDecorator(decoratedCoffee) {
  override def cost = super.cost + 0.7
  override def ingredients = super.ingredients + sep + "Whip"
}

class Sprinkles(decoratedCoffee: Coffee) extends CoffeeDecorator(decoratedCoffee) {
  override def cost = super.cost + 0.2
  override def ingredients = super.ingredients + sep + "Sprinkles"
}

object DecoratorSample {
  def main(args: Array[String]) = {
    var c:Coffee = new SimpleCoffee
    printf("Cost: %f Ingredients %s\n", c.cost, c.ingredients)
    c = new Milk(c)
    printf("Cost: %f Ingredients %s\n", c.cost, c.ingredients)
    c = new Sprinkles(c)
    printf("Cost: %f Ingredients %s\n", c.cost, c.ingredients)
    c = new Whip(c)
    printf("Cost: %f Ingredients %s\n", c.cost, c.ingredients)
    c = new Sprinkles(c)
    printf("Cost: %f Ingredients %s\n", c.cost, c.ingredients)
  }
}

DecoratorSample.main(Array())

Sometimes you want to implement decorator in this way:

class Coffee {
  val sep = ", "
  def cost:Double  = 1
  def ingredients: String = "Coffee"
}

trait Milk extends Coffee {
  abstract override def cost = super.cost + 0.5
  abstract override def ingredients = super.ingredients + sep + "Milk"
}

trait Whip extends Coffee {
  abstract override def cost = super.cost + 0.7
  abstract override def ingredients = super.ingredients + sep + "Whip"
}

trait Sprinkles extends Coffee {
  abstract override def cost = super.cost + 0.2
  abstract override def ingredients = super.ingredients + sep + "Sprinkles"
}

object DecoratorSample {
  def main(args: Array[String]) {
    var c: Coffee = new Coffee with Sprinkles
    printf("Cost: %f Ingredients %s\n", c.cost, c.ingredients)
    c = new Coffee with Sprinkles with Milk
    printf("Cost: %f Ingredients %s\n", c.cost, c.ingredients)
    c = new Coffee with Sprinkles with Milk with Whip
    printf("Cost: %f Ingredients %s\n", c.cost, c.ingredients)
  }
}

DecoratorSample.main(Array())

@EddieJamsession If the decorated coffee can't be modified at runtime it doesn't really fit the decorator pattern.

@EddieJamsession, You should remove this comment, as it is misleading. You are clearly violating ISP.

Can I buy a coffee with milk and sprinkle in the main()?

@Ehsan1997 Can't see your point about violating ISP. ISP says that no implementation should depend on unecessary functionality this ist clearly not the case by using mixin compositions in this scenario.