Scala cake-pattern compile error with Precog config pattern

Question

Following from this question, I have now the following:

case class Pet(val name: String)

trait ConfigComponent {
  type Config

  def config: Config
}

trait VetModule extends ConfigComponent {
  type Config <: VetModuleConfig

  def vet: Vet

  trait Vet {
    def vaccinate(pet: Pet)
  }

  trait VetModuleConfig {
    def extra: String
  }

}

trait VetModuleImpl extends VetModule {
  override def vet: Vet = VetImpl

  object VetImpl extends Vet {
    def vaccinate(pet: Pet) = println("Vaccinate:" + pet + " " + config.extra)
  }

}

trait AnotherModule extends ConfigComponent {
  type Config <: AnotherConfig

  def getLastName(): String

  trait AnotherConfig {
    val lastName: String
  }

}

trait AnotherModuleImpl extends AnotherModule {
  override def getLastName(): String = config.lastName
}

trait PetStoreModule extends ConfigComponent {
  type Config <: PetStoreConfig

  def petStore: PetStore

  trait PetStore {
    def sell(pet: Pet): Unit
  }

  trait PetStoreConfig {
    val petStoreName: String
  }

}

trait PetStoreModuleImpl extends PetStoreModule {
  self: VetModule with AnotherModule =>
  override def petStore: PetStore = PetstoreImpl

  object PetstoreImpl extends PetStore {
    def sell(pet: Pet) {
      vet.vaccinate(pet)
      println(s"Sold $pet! [Store: ${config.petStoreName}, lastName: $getLastName]")
    }
  }
}

class MyApp extends PetStoreModuleImpl with VetModuleImpl with AnotherModuleImpl {

  type Config = PetStoreConfig with AnotherConfig

  override object config extends PetStoreConfig with AnotherConfig {
    val petStoreName = "MyPetStore"
    val lastName = "MyLastName"
  }

  petStore.sell(new Pet("Fido"))
}


object Main {
  def main(args: Array[String]) {
    new MyApp
  }
}

I get the following compile error:

value petStoreName is not a member of PetStoreModuleImpl.this.Config
     println(s"Sold $pet! [Store: ${config.petStoreName}, lastName: $getLastName]")
                                   ^

This is actually the error I have been struggling with. Can somebody explain why it occurs? Currently, as a work-around, I just explicitly cast the config object in each module implementation.

Answer 1

What you've written should work, but doesn't, because of this bug.

There are a number of workarounds you could use. Adding type Config = PetStoreConfig with AnotherConfig to your module implementation is probably a little less unpleasant than casting.

Update: As som-snytt notes in a comment and answer, adding with PetStoreModuleImpl (crucially not with PetStoreModule, as you might expect) to the end of the self-type is a better solution.

As a footnote: as discussed in the comments on SI-7255, the Dependent Object Types calculus (which is designed to be "a new foundation for Scala's type system") will address this "fundamental problem in Scala's type system".

Answer 2

You can wrangle your self-type to keep the abstract type member you want, since the last bound wins:

trait PetStoreModuleImpl extends PetStoreModule {
  self: VetModule with AnotherModule with PetStoreModuleImpl =>
  override def petStore: PetStore = PetstoreImpl

  object PetstoreImpl extends PetStore {
    def sell(pet: Pet) {
      vet.vaccinate(pet)
      println(s"Sold $pet! [Store: ${config.petStoreName}, lastName: $getLastName]")
    }
  }
}

Then it will tell you that the vet module isn't configured:

class MyApp extends PetStoreModuleImpl with VetModuleImpl with AnotherModuleImpl {

  override type Config = PetStoreConfig with VetModuleConfig with AnotherConfig

  override object config extends PetStoreConfig with VetModuleConfig with AnotherConfig {
    val petStoreName = "MyPetStore"
    val lastName = "MyLastName"
    val extra = "vet-info"
  }

  petStore.sell(new Pet("Fido"))
}