Cake Pattern: How to share an instance?

Question

I have a configuration component in my Scala project.

Obviously I don't want to have more than one instance of this component. I'm using the cake pattern, but I'm not sure how to tweak it to fit my requirements:

// Library
// =================================================
trait ConfigComp {

  trait Config {
    def get(k: String): String
  }

  def config: Config
}

trait QueueComp {
  self: ConfigComp =>

  class Queue {
    val key = config.get("some-key")
  }

  lazy val queue = new Queue
}

// Application
// =================================================

trait MyConfig extends ConfigComp {

  lazy val config = new Config {
    println("INITIALIZING CONFIG")

    def get(k: String) = "value"
  }
}

object Frontend extends QueueComp with MyConfig
object Backend  extends QueueComp with MyConfig

Frontend.queue.key
Backend.queue.key

prints:

INITIALIZING CONFIG
INITIALIZING CONFIG

How to make the cake pattern share the anonymous instance of Config?

Answer 1

Something like this ?

// Library
// =================================================
trait Config {
  def get(k: String): String
}

trait ConfigComp {
  def config: Config
}

trait QueueComp {
  self: ConfigComp =>
  class Queue {
    val key = config.get("some-key")
  }
  lazy val queue = new Queue
}

// Application
// =================================================

object SingleConfig extends ConfigComp {
  lazy val config = new Config {
    println("INITIALIZING CONFIG")
    def get(k: String) = "value"
  }
}

object Frontend extends QueueComp with ConfigComp {
  val config = SingleConfig.config
}
object Backend  extends QueueComp with ConfigComp {
  val config = SingleConfig.config
}

Frontend.queue.key
Backend.queue.key

If your Config trait is placed inside ConfigComp, I could not workaround these type errors:

error: type mismatch;
 found   : MyConfig.Config
 required: Frontend.Config
    (which expands to)  Frontend.Config
                override def config = MyConfig.config

error: type mismatch;
 found   : MyConfig.Config
 required: Backend.Config
    (which expands to)  Backend.Config
                override def config = MyConfig.config