service4.scala

import zio.Task

trait DB {
  def execute(sql: String): Task[Unit]
}

class RelationalDB(cp: ConnectionPool) extends DB {
  override def execute(sql: String): Task[Unit] =
    Task {
      println(s"Running: $sql, on: $cp")
    }
}

// ---

trait UserModel {
  def insert(u: User): Task[Unit]
}

// service implementation
class DefaultUserModel(db: DB) extends UserModel {
  override def insert(u: User): Task[Unit] = 
    db.execute(s"INSERT INTO user VALUES ('${u.name}')")
}

// ---

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