Scala: Mocking and the Cake Pattern

Question

I\'ve been trying to adopt the Cake Pattern but I\'m having difficulties adapting to this programming styles, especially where unit tests are concerned.

Lets assume that

Answer 1

I started using the cake pattern after I read this blog post: https://github.com/precog/staticsite/blob/master/contents/blog/Existential-Types-FTW/index.md The approach is different from most Cake Pattern posts in that existential-types are used instead of self-types.

I have been using this pattern for a few months and it seems to work out well as I can specify a mock when I want to. It does have more a dependency injection feel to it, but it has all the benefits you get of having your code in traits.

My bastardized version of your problem using existential-types would be something like this:

case class Pet(val name: String)
trait ConfigComponent {
  type Config
  def config: Config
}

trait Vet {
  def vaccinate(pet: Pet) = {println ("Vaccinate:" + pet)}
}

trait PetStoreConfig {
  val vet: Vet
}
trait PetStore extends ConfigComponent {

    type Config <: PetStoreConfig

    def sell(pet: Pet) {
      config.vet.vaccinate(pet)
      // do some other stuff
    }
}

You can put it all together in your app

class MyApp extends PetStore with PetStoreConfig {

  type Config = MyApp
  def config = this  

  val vet = new Vet{}
  sell(new Pet("Fido"))

}

scala> new MyApp
Vaccinate:Pet(Fido)
res0: MyApp = MyApp@668dd96c

And you can test the components individually by creating an instance of VetLike and also creating a mock of VetLike an using it your PetStore test.

//Test VetLike Behavior
scala> val vet = new Vet{}
scala> vet.vaccinate(new Pet("Fido"))
Vaccinate:Pet(Fido)


//Test Petstore Behavior

class VetMock extends Vet {
   override def vaccinate(pet: Pet) = println("MOCKED")
}

class PetStoreTest extends PetStore with PetStoreConfig {
   type Config = PetStoreTest
   def config = this

   val vet = new VetMock
   val fido = new Pet("Fido")
   sell(fido)
}

scala> new PetStoreTest
MOCKED