(2017) Haskell, Task 3. Supermarket Billing. This assignment was taken from the book, "Haskell: The Craft of Functional Programming Second Edition" by Simon Thompson. This is my poor attempt at functional programming. At least it worked out well albeit a rather ugly coding.

Haskell-Supermarket-Billing.hs

{-
    Supermarket Billing.

    Use produceBill to output the complete receipt.
    eg. produceBill [1234, 4719, 3814, 1112, 1113, 1234, 0000, 666]
-}

-- Hide lookup from Prelude so we can use our own.
import Prelude hiding (lookup)

-- Types
type Name     = String
type Price    = Int
type Barcode  = Int

type Database = [ (Barcode, Name, Price) ]
type TillType = [ Barcode ]
type BillType = [ (Name, Price) ]

-- Initialise database
codeIndex :: Database
codeIndex = [ (4719, "Fish Fingers", 121),
              (5643, "Nappies", 1010),
              (3814, "Orange Jelly", 56),
              (1111, "Hula Hoops", 21),
              (1112, "Hula Hoops (Giants)", 133),
              (1234, "Dry Sherry, 1lt", 540) ]

-- Update database
addItem :: Database -> (Barcode, Name, Price) -> Database
addItem db (barcode, name, price) = [(barcode, name, price)] ++ db

removeItem :: Database -> (Barcode, Name, Price) -> Database
removeItem db (barcode, name, price) = [ item | item <- db, item /= (barcode, name, price) ]

updateDb :: Database -> (Barcode, Name, Price) -> Database
updateDb db (barcode, name, price) = (removeItem db (barcode, name, price)) >> (addItem db (barcode, name, price))

-- Maximum length of a string
lineLength :: Int
lineLength = 30

-- String formatting methods
formatPence :: Price -> String
formatPence price = (show (price `div` 100)) ++ "." ++ (show (price `mod` 100))

formatLine :: (Name, Price) -> String
formatLine (name, price) = name ++ (replicate (lineLength - length name - length (formatPence price)) '.') ++ (formatPence price) ++ "\n"

formatLines :: [(Name, Price)] -> String
formatLines xs = concat [formatLine(name, price) | (name, price) <- xs]

makeDiscount :: BillType -> Int
makeDiscount bill = count `div` 2 * 100
    where count = (length [ (name) | (name, price) <- bill, name == "Dry Sherry, 1lt"])

formatDiscount :: Int -> String
formatDiscount n = "Discount" ++ (replicate (lineLength - 8 - (length (formatPence n))) '.') ++ (formatPence n)

makeTotal :: BillType -> Price
makeTotal bill = (sum [ price | (name, price) <- bill ]) - (makeDiscount bill)

formatTotal :: Price -> String
formatTotal total = "Total" ++ (replicate(lineLength - 5 - length (formatPence total)) '.') ++ (formatPence total)

formatBill :: BillType -> String
formatBill bill = (replicate 8 ' ') ++ "My Store" ++ (replicate 8 ' ') ++ "\n\n" ++ formatLines bill ++ "\n" ++ (formatDiscount (makeDiscount bill)) ++ "\n\n" ++ (formatTotal (makeTotal bill))

-- Item lookup methods
look :: Database -> Barcode -> (Name, Price)
look db bc
    | len >= 1 = head item
    | otherwise = ("Unknown Item", 0)
    where
      item = [ (name, price) | (barcode, name, price) <- db, bc == barcode ]
      len = length item

lookup :: Barcode -> (Name, Price)
lookup bc = look codeIndex bc

-- List producing methods
makeBill :: TillType -> BillType
makeBill barcodes = [ lookup barcode | barcode <- barcodes, (lookup barcode) /= ("Unknown Item", 0) ]

-- Output final bill.
produceBill :: TillType -> IO ()
produceBill barcodes = putStrLn(formatBill (makeBill barcodes))