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

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))