import Data.Function ((&))
import Text.Printf

data SaleSum = SaleSum {
  totalCost     :: Double,
  totalQuantity :: Int
}

class SaleSummary s where
  summary :: s -> SaleSum

instance Semigroup SaleSum where
  (SaleSum c1 q1) <> (SaleSum c2 q2) = SaleSum (c1 + c2) (q1 + q2)

instance Monoid SaleSum where
  mempty = SaleSum 0.0 0

instance Show SaleSum where
  show (SaleSum cost quantity) =
    "Total cost = " ++ printf "%.2f" cost ++ ", quantity = " ++ show quantity

totalSummary :: SaleSummary s => [s] -> SaleSum
totalSummary xs = xs & map summary & mconcat

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data Sale = Sale {
  itemID   :: String,
  price    :: Double,
  quantity :: Int
  -- maybe some other fields
}

instance SaleSummary Sale where
  summary s = SaleSum (price s * fromIntegral (quantity s)) (quantity s)

testSales = [
  Sale "Foo" 0.20 4,
  Sale "Bar" 1.10 3
  ]

main = print $ totalSummary testSales