cake-pattern.scala

// 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
// =================================================

object MyConfig extends ConfigComp {
  val config = new Config {
    println("INITIALIZING CONFIG")

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

trait MyConfig extends ConfigComp {

  lazy val config = MyConfig.config
}

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

Frontend.queue.key
Backend.queue.key

output

[error] overriding method config in trait ConfigComp of type => MyConfig.this.Config;
[error]  lazy value config has incompatible type
[error]   lazy val config = MyConfig.config
[error]            ^
[error] one error found

I don't know exactly why nested trait doesn't work, but this one should.

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
}

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

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

trait MyConfig extends ConfigComp {

  lazy val config = SharedConfig.config
}

Not sure if this helps a lot.

that seems to work! great, thanks for your help!

if you nest Config in ConfigComp, it should work by doing this:

trait ConfigComp {

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

  def config: ConfigComp#Config
}

search for path-dependent types for more info.