gistfile1.hs

lastDigit :: Integer -> Integer
lastDigit n = if n >= 10 then n `mod` 10
              else n;

dropLastDigit :: Integer -> Integer
dropLastDigit n = n `div` 10

toDigits :: Integer -> [Integer]
toDigits 0 = []
toDigits n = if n < 0 then []
             else if n < 10 then [n]
                  else toDigits(n `div` 10)++[n `mod` 10]

ccreverse :: [Integer] -> [Integer] -> [Integer]
ccreverse [] ys = ys
ccreverse (x:xs) ys  = ccreverse xs (x:ys)

doubleEveryOther :: [Integer] -> [Integer]
doubleEveryOther [] = []
doubleEveryOther (x:[]) = [x]
doubleEveryOther (x:y:zs) = [x]++[y*2]++doubleEveryOther(zs)

sumDigits :: [Integer] -> Integer
sumDigits [] = 0
sumDigits (x:xs) = sum (toDigits x) + sumDigits xs

validate :: Integer -> Bool
validate n = (sumDigits (doubleEveryOther (toDigits n))) `mod` 10 == 0