validate :: Integer -> Bool
validate n
  | n > 0 = mod (sumDigits . doubleEveryOther . toDigits $ n) 10 == 0
  | otherwise = False
validate :: Integer -> Bool
validate n
  | n > 0 = mod (sumDigits . doubleEveryOther . toDigits $ n) 10 == 0
  | otherwise = False
validate n
  | n > 0 = mod (sumDigits . doubleEveryOther . toDigits $ n) 10 == 0
  | otherwise = False