return _customerRepository.GetById(id)
    .ToResult("Customer with such Id is not found: " + id)
    .Ensure(customer => customer.CanBePromoted(), "The customer has the highest status possible")
    .OnSuccess(customer => customer.Promote())
    .OnSuccess(customer => _emailGateway.SendPromotionNotification(customer.PrimaryEmail, customer.Status))
    .OnBoth(result => result.IsSuccess ? Ok() : Error(result.Error));