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  1. import Data.Function ((&))
  2. import Text.Printf
  3.  
  4. data SaleSum = SaleSum {
  5.   totalCost     :: Double,
  6.   totalQuantity :: Int
  7. }
  8.  
  9. class SaleSummary s where
  10.   summary :: s -> SaleSum
  11.  
  12. instance Semigroup SaleSum where
  13.   (SaleSum c1 q1) <> (SaleSum c2 q2) = SaleSum (c1 + c2) (q1 + q2)
  14.  
  15. instance Monoid SaleSum where
  16.   mempty = SaleSum 0.0 0
  17.  
  18. instance Show SaleSum where
  19.   show (SaleSum cost quantity) =
  20.     "Total cost = " ++ printf "%.2f" cost ++ ", quantity = " ++ show quantity
  21.  
  22. totalSummary :: SaleSummary s => [s] -> SaleSum
  23. totalSummary xs = xs & map summary & mconcat
  24.  
  25. ----------------------------------------------------------------
  26.  
  27. data Sale = Sale {
  28.   itemID   :: String,
  29.   price    :: Double,
  30.   quantity :: Int
  31.   -- maybe some other fields
  32. }
  33.  
  34. instance SaleSummary Sale where
  35.   summary s = SaleSum (price s * fromIntegral (quantity s)) (quantity s)
  36.  
  37. testSales = [
  38.   Sale "Foo" 0.20 4,
  39.   Sale "Bar" 1.10 3
  40.   ]
  41.  
  42. main = print $ totalSummary testSales
Success #stdin #stdout 0s 4360KB
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Standard input is empty
stdout
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Total cost = 4.10, quantity = 7
https://ideone.com/xcshAS
language:
Haskell (ghc 8.4.4)
created:
3 years ago
visibility:
public

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