Why use scala's cake pattern rather than abstract fields? - scala

Question

I have been reading about doing Dependency Injection in scala via the cake pattern. I think I understand it but I must have missed something because I still can't see the point in it! Why is it preferable to declare dependencies via self types rather than just abstract fields?
Given the example in Programming Scala TwitterClientComponent declares dependencies like this using the cake pattern:
//other trait declarations elided for clarity
...
trait TwitterClientComponent {
self: TwitterClientUIComponent with
TwitterLocalCacheComponent with
TwitterServiceComponent =>
val client: TwitterClient
class TwitterClient(val user: TwitterUserProfile) extends Tweeter {
def tweet(msg: String) = {
val twt = new Tweet(user, msg, new Date)
if (service.sendTweet(twt)) {
localCache.saveTweet(twt)
ui.showTweet(twt)
}
}
}
}
How is this better than declaring dependencies as abstract fields as below?
trait TwitterClient(val user: TwitterUserProfile) extends Tweeter {
//abstract fields instead of cake pattern self types
val service: TwitterService
val localCache: TwitterLocalCache
val ui: TwitterClientUI
def tweet(msg: String) = {
val twt = new Tweet(user, msg, new Date)
if (service.sendTweet(twt)) {
localCache.saveTweet(twt)
ui.showTweet(twt)
}
}
}
Looking at instantiation time, which is when DI actually happens (as I understand it), I am struggling to see the advantages of cake, especially when you consider the extra keyboard typing you need to do for the cake declarations (enclosing trait)
//Please note, I have stripped out some implementation details from the
//referenced example to clarify the injection of implemented dependencies
//Cake dependencies injected:
trait TextClient
extends TwitterClientComponent
with TwitterClientUIComponent
with TwitterLocalCacheComponent
with TwitterServiceComponent {
// Dependency from TwitterClientComponent:
val client = new TwitterClient
// Dependency from TwitterClientUIComponent:
val ui = new TwitterClientUI
// Dependency from TwitterLocalCacheComponent:
val localCache = new TwitterLocalCache
// Dependency from TwitterServiceComponent
val service = new TwitterService
}
Now again with abstract fields, more or less the same!:
trait TextClient {
//first of all no need to mixin the components
// Dependency on TwitterClient:
val client = new TwitterClient
// Dependency on TwitterClientUI:
val ui = new TwitterClientUI
// Dependency on TwitterLocalCache:
val localCache = new TwitterLocalCache
// Dependency on TwitterService
val service = new TwitterService
}
I'm sure I must be missing something about cake's superiority! However, at the moment I can't see what it offers over declaring dependencies in any other way (constructor, abstract fields).

Answer 1

Traits with self-type annotation is far more composable than old-fasioned beans with field injection, which you probably had in mind in your second snippet.
Let's look how you will instansiate this trait:
val productionTwitter = new TwitterClientComponent with TwitterUI with FSTwitterCache with TwitterConnection
If you need to test this trait you probably write:
val testTwitter = new TwitterClientComponent with TwitterUI with FSTwitterCache with MockConnection
Hmm, a little DRY violation. Let's improve.
trait TwitterSetup extends TwitterClientComponent with TwitterUI with FSTwitterCache
val productionTwitter = new TwitterSetup with TwitterConnection
val testTwitter = new TwitterSetup with MockConnection
Furthermore if you have a dependency between services in your component (say UI depends on TwitterService) they will be resolved automatically by the compiler.

Answer 2

Think about what happens if TwitterService uses TwitterLocalCache. It would be a lot easier if TwitterService self-typed to TwitterLocalCache because TwitterService has no access to the val localCache you've declared. The Cake pattern (and self-typing) allows for us to inject in a much more universal and flexible manner (among other things, of course).

Answer 3

I was unsure how the actual wiring would work, so I've adapted the simple example in the blog entry you linked to using abstract properties like you suggested.
// =======================
// service interfaces
trait OnOffDevice {
def on: Unit
def off: Unit
}
trait SensorDevice {
def isCoffeePresent: Boolean
}
// =======================
// service implementations
class Heater extends OnOffDevice {
def on = println("heater.on")
def off = println("heater.off")
}
class PotSensor extends SensorDevice {
def isCoffeePresent = true
}
// =======================
// service declaring two dependencies that it wants injected
// via abstract fields
abstract class Warmer() {
val sensor: SensorDevice
val onOff: OnOffDevice
def trigger = {
if (sensor.isCoffeePresent) onOff.on
else onOff.off
}
}
trait PotSensorMixin {
val sensor = new PotSensor
}
trait HeaterMixin {
val onOff = new Heater
}
val warmer = new Warmer with PotSensorMixin with HeaterMixin
warmer.trigger
in this simple case it does work (so the technique you suggest is indeed usable).
However, the same blog shows at least other three methods to achieve the same result; I think the choice is mostly about readability and personal preference. In the case of the technique you suggest IMHO the Warmer class communicates poorly its intent to have dependencies injected. Also to wire up the dependencies, I had to create two more traits (PotSensorMixin and HeaterMixin), but maybe you had a better way in mind to do it.

Answer 4

In this example I think there is no big difference. Self-types can potentially bring more clarity in cases when a trait declares several abstract values, like
trait ThreadPool {
val minThreads: Int
val maxThreads: Int
}
Then instead of depending on several abstract values you just declare dependency on a ThreadPool.
Self-types (as used in Cake pattern) for me are just a way to declare several abstract members at once, giving those a convenient name.