airfram_example.scala

case class User(id: Int, name: String, item: Seq[Item])

case class Item(id: Int, name: String)

trait UserRepository[F[_]] {
  def find(id: Int): OptionT[F, User]
  def store(user: User): F[Unit]
}

trait ItemRepository[F[_]] {
  def find(id: Int): OptionT[F, Item]
  def store(item: Item): F[Unit]
}

trait UserService[F[_]] {
  val ur = bind[UserRepository[F]]
  val ir = bind[ItemRepository[F]]

  def addItemToUser(userId: Int, itemId: Int): OptionT[F, Unit] = {
    for {
      user <- ur.find(userId)
      item <- ir.find(itemId)
      newUser = user.copy(item = user.item ++ Seq(item))
      _ <- ur.store(newUser)
    } yield ()
  }
}