A Builder example in Scala using Phantom Types

builder-phantom-types.scala

case class Food(ingredients: Seq[String])

class Chef[Pizza <: Chef.Pizza] protected (ingredients: Seq[String]) {
  import Chef.Pizza._

  def addCheese(cheeseType: String): Chef[Pizza with Cheese] = Chef(ingredients :+ cheeseType)

  def addTopping(toppingType: String): Chef[Pizza with Topping] = Chef(ingredients :+ toppingType)

  def addDough: Chef[Pizza with Dough] = Chef(ingredients :+ "dough")

  def build(implicit ev: Pizza =:= FullPizza): Food = Food(ingredients)
}

object Chef {

  sealed trait Pizza
  object Pizza {
    sealed trait EmptyPizza extends Pizza
    sealed trait Cheese extends Pizza
    sealed trait Topping extends Pizza
    sealed trait Dough extends Pizza

    type FullPizza = EmptyPizza with Cheese with Topping with Dough
  }

  def apply[T <: Pizza](ingredients: Seq[String]): Chef[T] = new Chef[T](ingredients)

  def apply(): Chef[Pizza.EmptyPizza] = apply[Pizza.EmptyPizza](Seq())

}

Thanks for the great article on this pattern! It looks like this code is a bit different than what is in the article though. Particularly, the apply methods and the protected class constructor don't seem to be in the article. It might be worth mentioning these in the article since the builder is safer because it prevents someone from doing new Chef[Chef.Pizza.FullPizza]().build, which the original code from the article allows.