A simple haskell module to calculate utilties cost.

utilitiesCost.hs

module UtilitiesCost
( electricityCost
, gasCost
, waterCost
, utilitiesCost
) where

import Debug.Trace

data Reading = Reading Double Double deriving (Eq, Show, Read)
data ReadingRange = ReadingRange (Double, Double) deriving (Eq, Show, Read)
data WaterExtraCost = WaterExtraCost Double Double Double deriving (Eq, Show, Read)

electricityCost :: Reading -> Double -> Double -> Double -> Double
electricityCost (Reading initialReading finalReading) rate days daysPerMonth =
  let usage = finalReading - initialReading
      usagePerDay = usage / days
  in  usagePerDay * daysPerMonth * rate

gasCost :: Reading -> Double -> Double -> Double -> Double
gasCost (Reading initialReading finalReading) rate days daysPerMonth =
  let usage = finalReading - initialReading
      usagePerDay = usage / days
  in  usagePerDay * daysPerMonth * rate

waterCost :: Reading -> Double -> WaterExtraCost -> Double -> Double -> Double
waterCost (Reading initialReading finalReading) rate (WaterExtraCost waterborneRate taxRate sanitaryFee) days daysPerMonth =
  let usage = finalReading - initialReading
      usagePerDay = usage / days
      waterFee = usagePerDay * daysPerMonth * rate
      waterborneFee = usagePerDay * daysPerMonth * waterborneRate
      tax = waterFee * taxRate
  in waterFee + waterborneFee + sanitaryFee + tax

utilitiesCost :: ReadingRange -> ReadingRange -> ReadingRange -> Double -> Double -> Double
utilitiesCost (ReadingRange electricityReading) (ReadingRange gasReading) (ReadingRange waterReading) days daysPerMonth =
    traceShow (electricityFee, gasFee, waterFee, refuseFee, gst) $ totalFee + gst
  where
    totalFee = electricityFee + gasFee + waterFee + refuseFee

    refuseFee = 29
    gstRate = 0.07
    gst = gstRate * totalFee

    electricityFee = electricityCost (Reading initialReading finalReading) rate days daysPerMonth
      where initialReading = fst electricityReading
            finalReading = snd electricityReading
            rate = 0.2608

    gasFee = gasCost (Reading initialReading finalReading) rate days daysPerMonth
      where initialReading = fst gasReading
            finalReading = snd gasReading
            rate = 0.2118

    waterFee = waterCost (Reading initialReading finalReading) rate (WaterExtraCost waterborneRate taxRate sanitaryFee) days daysPerMonth
      where initialReading = fst waterReading
            finalReading = snd waterReading
            (rate, waterborneRate, taxRate) = (1.17, 0.5607, 0.3)
            sanitaryFee =
              let months = ceiling (daysPerMonth / 31.0)
              in 2.28037 * fromIntegral months

Yes, @sovanlandy2 we need to maximize our time to avoid complexity. So little time, so much to do.