Is there a way to "trickle down" implicits from top level applications to other imported modules?

Question

I am trying to refactor some code for a program which uses an ActorSystem as the backbone for Http calls.

My specific goal is to make my code more modular so I can write libraries of functions which make http calls using an ActorSystem where the ActorSystem is later expected to be provided by the application.

This is a general question though as I tend to run into this problem a reasonable amount.

I have two goals:

1. Minimize the number of ActorSystems I create to simplify tracking of them (one per top level application is the goal)
2. Avoid explicitly passing around the ActorSystem and context everywhere it's needed.

Conceptually - the code below illustrates how I'm thinking about it (of course this code would not compile).

import akka.actor.ActorSystem
import intermediateModule._

import scala.concurrent.ExecutionContextExecutor

object MyApp extends App {
  // Create the actorsystem and place into scope
  implicit val system = ActorSystem()
  implicit val context = system.dispatcher

  intermediateFunc1(300)

}

// Elsewhere in the intermediate module
object intermediateModule {
  import expectsActorSystemModule._

  def intermediateFunc1(x: Int) = {

    // Relies on ActorSystem and Execution context, 
    // but won't compile because, of course the application ActorSystem and
    // ec is not in scope 
    usesActorSystem(x)
  }

}


// In this modiule, usesActorSystem needs an ActorSystem
object expectsActorSystemModule {

  def usesActorSystem(x: Int)
                     (implicit system: ActorSystem, context: ExecutionContextExecutor) = ???
  //... does some stuff like sending http requests with ActorSystem
}

Is there a way to "trickle down" implicits through the sub-modules to achieve the goal of the top level application providing the needed implicits?

Can this be done in a way such that the "depth" of module imports doesn't matter (e.g. if I added a few more intermediate libraries in between the top level app and the module which requires the ActorSystem)?

Answer 1

The answer here is dependency injection. Every object that has dependencies on other objects should receive them as constructor parameters. The important thing here is that higher layers only get their own dependencies, and not their dependencies' dependencies.

In your example, IntermediateModule doesn't use the ActorSystem itself; it only needs it to pass it on to ExpectsActorSystemModule. This is bad, because if the latter changes and requires another dependency, you will need to change the former as well – that is too much coupling. You can refactor it like so:

import akka.actor.ActorSystem
import scala.concurrent.ExecutionContextExecutor

object MyApp extends App {
  // Create the actorsystem and place into scope
  // wire everything together  
  implicit val system = ActorSystem()
  implicit val context = system.dispatcher
  val expectsActorSystemModule = new ExpectsActorSystemModule
  val intermediateModule = new IntermediateModule(expectsActorSystemModule)

  // run stuff
  intermediateModule.intermediateFunc1(300)

}

// Elsewhere in the intermediate module
class IntermediateModule(expectsActorSystemModule: ExpectsActorSystemModule) {
  def intermediateFunc1(x: Int) = {
    // Note: no ActorSystem or ExecutionContext is needed, because they were
    // injected into expectsActorSystemModule
    expectsActorSystemModule.usesActorSystem(x)
  }
}


// In this module, usesActorSystem needs an ActorSystem
class ExpectsActorSystemModule(
  implicit system: ActorSystem,
  context: ExecutionContextExecutor) {

  def usesActorSystem(x: Int) = ???
  //... does some stuff like sending http requests with ActorSystem
}

Note that IntermediateModule no longer needs an ActorSystem or an ExecutionContext, because those were provided directly to ExpectsActorSystemModule.

The slightly annoying part is that at some point you have to instantiate all these objects in your application and wire them all together. In the above example it's only 4 lines in MyApp, but it will get significantly longer for more substantial programs.

There are libraries like MacWire or Guice to help with this, but I would recommend against using them. They make it much less transparent what is going on, and they don't save all that much code either – in my opinion, it's a bad tradeoff. And these two specifically have more downsides. Guice comes from the Java world and gives you basically no compile-time guarantees, meaning that your code might compile just fine and then fail to start because Guice. MacWire is better in that regard (everything is done at compile time), but it's not future-proof because it's implemented as a Scala 2 macro – it will not work on Scala 3 in its current form.

Another approach that is popular among the purely functional programming community is to use ZIO's ZLayer. But since you're working on an existing codebase that is based on the Lightbend tech stack, this is unlikely to be the means of choice in this particular case.