Challenge 197 easy

ch197e.hs

validISBN x y
  | x > 9 = validISBN (reduce (fromIntegral x)) (extra (fromIntegral x) y)
  | otherwise = elevens (fromIntegral x) y

reduce x = round(x) `mod` (tens x)

lvl x = floor $ logBase 10 x

extra x y = y + ((round(x) `div` (tens x)) * ((lvl x) + 1))

tens x = round(10 ^^ (lvl x))

elevens x y = ((round(x) + y) `mod` 11) == 0