My attempt not only compiled, but seems to run OK. (Really, a just gave it a couple of trivial tests, and said good enough. I didn't really mess with corner cases like empty lists. It was late, and I didn't want to tempt fate.) THUS, I am mostly looking for style feedback, although if there are any obvious logic errors, I'd be "happy" to learn about those too. {-- 7. Modify the binary string transmitter example to detect simple transmission errors using the concept of parity bits. That is, each eight-bit binary number produced during encoding is extended with a parity bit, set to one if the number contains an odd number of ones, and to zero otherwise. In turn, each resulting nine-bit binary number consumed during decoding is checked to ensure that its parity bit is correct, with the parity bit being discarded if this is the case, and a parity error being reported otherwise. Hint: the library function error :: String -> a displays the given string as an error message and terminates the program; the polymorphic result type ensures that error can be used in any context. 8. Test your new string transmitter program from the previous exercise using a faulty communication channel that forgets the first bit, which can be modelled using the tail function on lists of bits. Hutton, Graham. Programming in Haskell (Kindle Locations 2842-2851). Cambridge University Press. Kindle Edition. --} import Data.Char type Bit = Int byte :: Int byte = 8 parityByte :: Int parityByte = 9 bin2int :: [Bit] -> Int bin2int = foldr (\x y -> x + 2 * y) 0 -- Hutton, Graham. Programming in Haskell (Kindle Locations 2647-2649). Cambridge University Press. Kindle Edition. int2bin :: Int -> [Bit] int2bin 0 = [] int2bin n = n `mod` 2 : int2bin (n `div` 2) -- Hutton, Graham. Programming in Haskell (Kindle Locations 2654-2656). Cambridge University Press. Kindle Edition. make8 :: [Bit] -> [Bit] make8 bits = addParity (take byte (bits ++ repeat 0)) -- Parity functions addParity :: [Bit] -> [Bit] addParity xs = if even (sum xs) then xs ++ [0] else xs ++ [1] checkParity :: [Bit] -> Bool checkParity xs = (((even . sum) (take ((length xs) - 1) xs)) == ((even . last) xs)) || (((odd . sum) (take ((length xs) - 1) xs)) == ((odd . last) xs)) errorParity :: [Bit] -> ([Bit], Bool) errorParity xs = if checkParity xs then (xs, checkParity xs) else error "Parity error" dropParity :: [Bit] -> [Bit] dropParity xs = take ((length xs) - 1) xs -- Hutton, Graham. Programming in Haskell (Kindle Locations 2662-2663). Cambridge University Press. Kindle Edition. -- TRANSMISSION encode :: String -> [Bit] encode = concat . map (make8 . int2bin . ord) -- Hutton, Graham. Programming in Haskell (Kindle Locations 2673-2675). Cambridge University Press. Kindle Edition. chop8 :: [Bit] -> [[Bit]] chop8 [] = [] chop8 bits = (dropParity . fst . errorParity) (take parityByte bits) : chop8 (drop parityByte bits) -- Hutton, Graham. Programming in Haskell (Kindle Locations 2681-2683). Cambridge University Press. Kindle Edition. decode :: [Bit] -> String decode = map (chr . bin2int) . chop8 -- Hutton, Graham. Programming in Haskell (Kindle Locations 2686-2688). Cambridge University Press. Kindle Edition. -- channel :: [Bit] -> [Bit] -- channel = id channel :: [Bit] -> [Bit] channel = tail -- Hutton, Graham. Programming in Haskell (Kindle Locations 2696-2697). Cambridge University Press. Kindle Edition. transmit :: String -> String transmit = decode . channel . encode -- Hutton, Graham. Programming in Haskell (Kindle Locations 2694-2695). Cambridge University Press. Kindle Edition.