Question

From time to time I see projects that has a boot.scala or Boot.scala file in. Though this does not appear to be a hard and fast Scala rule, it does appear to be folklore of sorts to include a Boot.scala file in some projects. This specific project uses Akka and Spray, which translates to actor pattern and REST services.

Will someone please explain what type of functionality can normally be expected in such a file and if this is a common pattern of sorts?

As an extension to this question (please answer the first bit first :-), I would be grateful to know how to read this code, which is in a project with multiple Boot.scala files.

Boot.scala in web package:

trait Web {
  this: Api with Core =>
....
}

Boot.scala in api package:

trait Api {
  this: Core =>
....
}

Boot.scala in core package:

trait Core {
  implicit def actorSystem: ActorSystem
  implicit val timeout = Timeout(30000)

  val application = actorSystem.actorOf(
    props = Props[ApplicationActor],
    name = "application"
  )

  Await.ready(application ? Start(), timeout.duration)

}

One can gather that the one package depends on the other, and that the Boot.scala files may be a common sight in actor based systems, but what how does one ‘read’ the relationships? For example, how would I read trait Web {this: Api with Core =>…} in english?

In the particular instance, the starting point of the application lies in a main file:

object Main extends App {
  implicit val system = ActorSystem("RESTService")

  class Application(val actorSystem: ActorSystem) extends Core with Api with Web {

  }

  new Application(system)

  sys.addShutdownHook {
    system.shutdown()
  }

}

I realize my questions may seem trivial to some, but I’m trying to get into the Scala tribe here, and the ‘secret password’ is not in any manual.

Answer 1

I don’t know whether Boot.scala is a common pattern, but it is used in Lift and contains the main configuration of the application. Since Lift is rather old (relative to other scala projects) it might have set a convention.

For the other question you should read up on the cake pattern and self types. For example

trait Web {
  this: Api with Core =>
....
}

defines a trait Web that can only be mixed into classes or traits that inherit from API and Core or have those as part of their self types. Thus

class Application(val actorSystem: ActorSystem) extends Core with Api with Web {

}

would not type check, if the Api trait wasn’t mixed in, because it is required by the Web trait.