service2.scala

import zio.{Task, ZIO, ZLayer}

object UserRegistration {
  // 1. service 
  class Service(notifier: UserNotifier.Service, userModel: UserModel.Service) {
    def register(u: User): Task[User] = {
      for {
        _ <- userModel.insert(u)
        _ <- notifier.notify(u, "Welcome!")
      } yield u
    }
  }

  // 2. layer
  val live: ZLayer[UserNotifier with UserModel, Nothing, UserRegistration] =
    ZLayer.fromServices[UserNotifier.Service, 
                        UserModel.Service, 
                        UserRegistration.Service](
      new Service(_, _)
    )

  // 3. accessor
  def register(u: User): ZIO[UserRegistration, Throwable, User] = 
    ZIO.accessM(_.get.register(u))
}

// ---

// 4. type alias in package.scala
type UserRegistration = Has[UserRegistration.Service]