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

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

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.