How to avoid dependency injection in Scala?

Question

I read Dependency Injection Without the Gymnastics PDF which indicates there's no need for any fancy DI framework, but it's beyond my grasp (at least without concrete examples). I'll try watching Dependency Injection Without the Gymnastics and Dead Simple Dependency Injection when I have a chance.

Using Guice in Java, if A depends on both B and C and both B and C depend on D, one would have something like:

public class A {
    @Inject
    public A(B b, C c) {
        this.b = b;
        this.c = c;
    }
}

public class B {
    @Inject
    public B(D d) {
        this.d = d;
    }
}

public class C {
    @Inject
    public C(D d) {
        this.d = d;
    }
}

public class D { /* ... */ }

and a module that says which implementation of D to use, then one would just ask for an instance of A from the injector:

A a = injector.createInstance(A.class);

Given what's presented in the URLs above, how would the Scala-equivalent of the above code look?

FWIW, I'm also investigating https://github.com/dickwall/subcut/blob/master/GettingStarted.md and am simply trying to understand the anti-DI solution.

Answer 1

Implicit parameters are completely sufficient for the use case you're describing.

case class A(implicit b: B, c: C)
case class B(implicit d: D)
case class C(implicit d: D)
class D { /* ... */ }

implicit val theD = new D
implicit val theB = B()
implicit val theC = C()

Now you can ask for an A just by:

val a = A()

Answer 2

You may solve it with self-types.

A depends on both B and C and both B and C depend on D

so one could write this like that:

class A {
  self: B with C => 
}

trait B { 
  self: D => 
}

trait C {
  self: D => 
}

trait D {}

and then on a call side:

val x = new A with BImpl with CImpl with DImpl

but code below won't compile, because dependencies on B,C,D classes not resolved:

val x = new A

Answer 3

It's tricky to provide that type of dependency injection. Most of the above examples require you to create the implicits near where the classes are instantiated.

Closest I could come up with is:

class A(implicit b:B, c:C)
class B(implicit d:D)
class C(implicit d:D)
trait D { //the interface 
  def x:Unit
}

object Implicits {
  implicit def aFactory:A = new A
  implicit lazy val bInstance:B = new B
  implicit def cFactory:C = new C
  implicit def dFactory:D = new D {
     def x:Unit = {/* some code */}
  }
}

And then in your code you use it like this:

import Implicits._

object MyApplication {
   def main(args: Array[String]):Unit = {
      val a = new A
   }
}

If you need to be able to specify different versions when you (for example) are testing, you could do something like this:

import Implicits._

object MyApplication {

  // Define the actual implicits
  Implicits.module = new Module {
    import Implicits._

    def a = new A
    lazy val b = new B
    def c = new C
    def d = new D {
      def x = println("x")
    }
  }

  def main(args: Array[String]):Unit = {
    val a = new A // or val a = implicitly[A] 
  }

}

// The contract (all elements that you need)
trait Module {
  def a: A
  def b: B
  def c: C
  def d: D
}

// Making the contract available as implicits
object Implicits {
  var module: Module = _

  implicit def aFactory:A = module.a
  implicit def bFactory:B = module.b
  implicit def cFactory:C = module.c
  implicit def dFactory:D = module.d
}

This would allow you to simply import Implicits._ in any file and would provide a similar workflow as the one in the original question.

In most cases however I would not use this tactic. I would simply make the implicit available in classes that create instances:

object MyApplication {

  implicit def a: A = new A
  implicit lazy val b: B = new B
  implicit def c: C = new C
  implicit def d: D = new D {
    def x: Unit = println("x")
  }

  def main(args: Array[String]): Unit = {
    val a = implicitly[A]
    val e = new E
  }

}

class E(implicit d:D) {
    new C
}

Here E is defined in another file and creates an instance of C. We require D to be passed to E and with that document that E depends on D (via C).