Phantom types in Scala: leverage type safety over conditional branching

demo.scala

package scalaInAction.chapter8ScalableExtensibleComponents

import org.scalatest.{FlatSpec, Matchers}

class FoodReadyToBeEaten
sealed trait Pizza

object Pizza {
  sealed trait EmptyPizza extends Pizza
  sealed trait Cheese extends Pizza
  sealed trait Topping extends Pizza
  type FullPizza = EmptyPizza with Cheese with Topping
}

class Chef[CurrentPizza <: Pizza] {
  import Pizza._

  def addCheese() = new Chef[CurrentPizza with Cheese]
  def addTopping() = new Chef[CurrentPizza with Topping]
  def build(implicit pizza: CurrentPizza =:= FullPizza) = new FoodReadyToBeEaten
}


/**
  * A phantom type is a manifestation of abstract type that has no effect on the runtime. These are useful to prove
  * static properties of the code using type evidences. As they have no effect on the runtime they can be erased
  * from the resulting code by the compiler once it has shown the constraints hold.
  *
  * For example, they can be useful for checking states.
  *
  * @see https://medium.com/@maximilianofelice/builder-pattern-in-scala-with-phantom-types-3e29a167e863
  *
  * The operator <i>=:=</i> used in the code below is called <b>generalized type constraints</b> and allows the
  * developers to add constraints on a parametric type in a given function. Their purpose compared to type bound is
  * that they can be used only on one method/function.
  *
  * @see https://stackoverflow.com/q/3427345/2780334
  */
class PhantomTypes extends FlatSpec with Matchers {
  import Pizza._

  "Phantom types" can "be used as a builder pattern to guarantee the state and the right order of calls" in {
    val pizzaReadyToBeEaten = new Chef[EmptyPizza]
      .addCheese()
      .addTopping()
      .build

    pizzaReadyToBeEaten shouldBe a [FoodReadyToBeEaten]

    // The code below doesn't compile because build can be called without a FullPizza!
    "new Chef[EmptyPizza].addCheese.build" shouldNot compile
    "new Chef[EmptyPizza].addTopping.build" shouldNot compile
  }
}