Validating credit card numbers in Haskell

This Haskell code can validate a credit card number:

	import Data.Char
	toDigits :: Integer -> [Integer]
	toDigits n = map (\ x -> toInteger (digitToInt x)) (show n)
	toDigitsRev :: Integer -> [Integer]
	toDigitsRev n = reverse (toDigits n)
	doubleSecond :: [Integer] -> [Integer]
	doubleSecond [] = []
	doubleSecond [a] = a : []
	doubleSecond (b : c : xs) = b : 2 * c : doubleSecond xs
	sumDigits :: [Integer] -> Integer
	sumDigits [] = 0
	sumDigits (x : xs) = sum (toDigits x) + sumDigits xs
	isValid :: Integer -> Bool
	isValid n = mod (sumDigits (doubleSecond (toDigitsRev n))) 10 == 0

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