Binding a variable with a sequence of expressions

Questions
5

This class generates payment details:

generatePaymentCalculationPeriods : (Fetch f) => CalculationPeriodDates -> Optional ResetDates -> Optional OtherDates -> Optional PaymentDates -> f [PaymentCalculationPeriod]
generatePaymentCalculationPeriods cpds rds ods (fmap (checkPaymentDates cpds) -> pds) = do
  -- Roll out calculation periods
  cps <-
    case (rds, ods) of
      (Some r, None) -> generateResetPeriods cpds rds
      (None, Some o) -> generateOtherPeriods cpds ads
      (Some r, Some o) -> do
        r <- generateResetPeriods cpds rds
        o <- generateOtherPeriods cpds ods
        return (r ++ o)
      (None, None) -> error "expecting calculation period"  

  -- Define payment calculation period
  let adj = optional noAdj (\x -> get "paymentDatesAdjustments" x.paymentDatesAdjustments) pds
  let payRelativeTo = optional PayRelativeToEnumCalculationPeriodEndDate (\x -> get "payRelativeTo" x.payRelativeTo) pds
  let offset = (\x -> x.paymentDaysOffset) =<< pds
  mapA (generateSinglePeriod payRelativeTo offset adj . replicate 1) cps

  where
    noAdj = BusinessDayAdjustments
              { id = None
              , businessCenters = None
              , businessDayConvention = BusinessDayConventionEnumNONE
              }

    generateSinglePeriod :
      (Fetch f)
      => PayRelativeToEnum
      -> Optional Offset
      -> BusinessDayAdjustments
      -> [CalculationPeriod]
      -> f PaymentCalculationPeriod
    generateSinglePeriod _ _ _ [] = error "expecting at least one calculation period"
    generateSinglePeriod payRelativeTo offset adj cps = do
      let baseDate = case payRelativeTo of
                      PayRelativeToEnumCalculationPeriodStartDate ->
                        (get "adjustedStartDate" . (\cp -> cp.adjustedStartDate) . head) cps
                      PayRelativeToEnumCalculationPeriodEndDate ->
                        (get "adjustedEndDate" . (\cp -> cp.adjustedEndDate) . last) $ cps
                      otherwise -> throwNotSupportedError (show payRelativeTo)

      uPayDate <- optional (pure baseDate) (\o -> applyOffset o adj.businessCenters baseDate) offset
      aPayDate <- adjustDate adj uPayDate
      return PaymentCalculationPeriod
                { id = None
                , adjustedPaymentDate = Some aPayDate
                , calculationPeriod = cps
                , discountFactor = None
                , fixedPaymentAmount = None
                , forecastPaymentAmount = None
                , presentValueAmount = None
                , unadjustedPaymentDate = Some uPayDate
                }

The PaymentCalculationPeriod details for the floatingRateDefinition:

populatePCP : [(Optional Text, ResetPrimitive)] -> [(Optional Text, EventPrimitive)] -> InterestRatePayout -> PaymentCalculationPeriod -> (PaymentCalculationPeriod, Lineage)
populatePCP existingResets existingEvents irp pcp =
  let (resetReferences, eventReferences, cpsFull) = unzip3 $ map (setInterestRate . setDayCountFraction . setQuantity) pcp.calculationPeriod
      lineage = emptyLineage
                  { interestRatePayoutReference = [referenceWithEmptyMeta irp.primeKey]
                  , eventReference = map referenceWithEmptyMeta $ concat resetReferences
                  , eventReference = map referenceWithEmptyMeta $ concat eventReferences
                  }
      pcpFull = pcp { calculationPeriod = cpsFull }
  in (pcpFull, lineage)
  where
    ...

    setInterestRate : CalculationPeriod -> ([Text], [Text], CalculationPeriod)
    setInterestRate cp
      ...    

      | Some floatingRate <- irp.rateSpecification.floatingRate
      , Some otherValue <- irp.rateSpecification.otherValue
      = let frDef = get "floatingRateDefinition" cp.floatingRateDefinition
            frdNew = FR.calcFloatingRate existingResets floatingRate frDef
            ovDef = get "otherValueDefinition" cp.otherValueDefinition
            ovdNew = OV.calcOtherValue existingEvents otherValue ovDef
        in (fst frdNew, fst ovdNew, cp { floatingRateDefinition = Some (snd frdNew)
                                       ; otherValueDefinition = Some (snd ovdNew) })

      | otherwise = error "expecting exactly two 'rateSpecification'"