Luhn Checksummed Numbers

gistfile1.hs

import Data.Char

validateCC :: [Char] -> Bool
validateCC ccString = 
  let sum = luhnSum $ ccDigits ccString
  in 0 == mod sum 10

ccDigits digits = reverse $ map digitToInt digits

luhnSum digits = 
  let 
    toSumTuple = digitsToSum digits
    toSum = fst toSumTuple ++ snd toSumTuple
  in sum $ concatMap toDigits toSum
    
digitsToSum digits = 
  let
    split = splitDigits digits
    products = map (* 2) $ snd split
  in
   ( (fst split) , products )
   
toDigits x | x < 10 = [x]
           | x >= 10  = [ 1 , ( x - 10 ) ]          
    
splitDigits digits = (splitDigits' ([],[]) True digits)
  where                     
    splitDigits' acc _ [] = acc
    splitDigits' (a,b) True (x:xs) = splitDigits' ( x : a , b ) False xs 
    splitDigits' (a,b) False (x:xs) = splitDigits' ( a , x:b ) True xs