trait AccountService {

  type Valid[A] = OptionT[Future, A]
  type AccountOperation[A] = Kleisli[Valid, AccountRepository, A]

  def debit(no: String, amount: Amount): AccountOperation[Account] = ???
  def credit(no: String, amount: Amount): AccountOperation[Account] = ???
  def transfer(from: String, to: String, amount: Amount) : AccountOperation[(Account,Account)]  = for {
    debitAcc <-  debit(from, amount)
    creditAcc <- credit(to, amount)
  } yield (debitAcc, creditAcc) // You need OptionT transformer inside Kleisli/ReaderT to avoid extra mapping to extract account from stacked Future[Option] monads.
}

object AccountService extends AccountService

object AccountRepository extends AccountRepositoryInMemory