cake_pattern.scala

// from : http://jonasboner.com/real-world-scala-dependency-injection-di/

case class User(name: String)

trait UserRepositoryComponent {
  val userRepository: UserRepository
  trait UserRepository {
    def authenticate(username: String, password: String): User
    def create(user: User): Unit
    def delete(user: User): Unit
  }
}

trait UserServiceComponent { this: UserRepositoryComponent =>
  val userService: UserService
  class UserService {
    def authenticate(username: String, password: String): User = userRepository.authenticate(username, password)
    def create(username: String) = userRepository.create(new User(username))
    def delete(user: User) = userRepository.delete(user)
  }
}

object ComponentRegistry extends UserServiceComponent with UserRepositoryComponent {
  val userRepository = new UserRepository { // this is where dependency injection happens
    def authenticate(username: String, password: String): User = {
      println("authenticating user: " + username)
      User(username)
    }
    def create(user: User) = println("creating user: " + user)
    def delete(user: User) = println("deleting user: " + user)
  }
  val userService = new UserService
}

val userService = ComponentRegistry.userService
userService.authenticate("me", "xxxx")