right map of sequence of ValidationNel

case class PermutingCheckout(skuRules: Map[String, SKUPricer]) extends TotalCalculator {
  // as above but runs the rules in all permuted orders to find the best order to minimise the price
  val calculateTotal = (items: List[String]) => 
    items.foldMap(x => Map(x -> 1)) // create a map of SKU -> count of 'scanned' items
        .map { case (sku, skuCount) =>
        // get the rules for the SKU (if they exist) and map them to get the results for each SKU group
        skuRules.get(sku).map(rule => (rule.getPrice(skuCount)).success)
          .getOrElse(s"'$sku' rule not found".failureNel)
      }.reduce(_ |+| _) // sum the results for each SKU to get the final total
    }

    // we now have a list of ValidationNel[String, Int] for each permutation
    // so sequence to transform List[ValidationNel[String, Int]] to ValidationNel[String, List[Int]]
    // and pick the minimum price by mapping the right hand side
    totalsList.sequenceU.:->(_.min)
  }
}