Using Reader Monad for Dependency Injection

Question

I recently saw the talks Dead-Simple Dependency Injection and Dependency Injection Without the Gymnastics about DI with Monads and was impressed. I tried to apply it on a simple problem, but failed as soon as it got non-trivial. I really would like to see a running version of dependency injection where

• a class that depends on more than one value that has to be injected
• a class that depends on a class that depends on something to be injected

as in the following example

trait FlyBehaviour { def fly() }
trait QuackBehaviour { def quack() }
trait Animal { def makeSound() }
// needs two behaviours injected
class Duck(val flyBehaviour: FlyBehaviour, val quackBehaviour: QuackBehaviour) extends Animal 
{
 def quack() = quackBehaviour.quack()
 def fly() = flyBehaviour.fly()
 def makeSound() = quack()
}
// needs an Animal injected (e.g. a Duck)
class Zoo(val animal: Animal)
// Spring for example would be able to provide a Zoo instance
// assuming a Zoo in configured to get a Duck injected and
// a Duck is configured to get impl. of FlyBehaviour and QuackBehaviour injected
val zoo: Zoo = InjectionFramework.get("Zoo")
zoo.animal.makeSound()

It would be really helpful to see a sample implementation using the reader Monad since I just feel that I am missing a push in the right direction.

Thanks!

Answer 1

The "reader monad" is just Function1, so all you need to do is accept an argument containing all the things you need. For example:

trait Config {
 def fly: FlyBehaviour
 def quack: QuackBehaviour
}
type Env[A] = Config => A

Now if you want to construct a Duck based on this environment:

val a: Env[Animal] = c => new Duck(c.fly, c.quack)

And then constructing a Zoo based on that is easy:

val z: Env[Zoo] = a andThen (new Zoo(_))

Using Scalaz (or with a bit of work on your own) you can make use of some syntax niceties to "ask" for the config c:

val z: Env[Zoo] = for {
 c <- ask
} yield new Zoo(Duck(c.fly, c.quack))

• Manuel Schmidt: Thanks for your example. I am curious, are you (or are you aware of any bigger projects) using this in place of spring or goolge guice for DI? Is it preferable according to your experience?
• Apocalisp: Yes, we are using this extensively and it is much preferable. If I ever see Spring or Guice again it will be too soon.
• Noel Yap: @Apocalisp, could you help me apply this solution to stackoverflow.com/questions/12341867/…?
• Nicolas Rinaudo: I'm a bit confused by this last example. ask returns a Reader[Config, Config], but then new Duck... returns a Duck, doesn't it? How does it magically turn into a, I suppose, Reader[Config, Duck]? Or maybe it was meant to read a = new Duck... ?