Add two big integers

BigInt.hs

add [] [] c           = show c
add ls [] c           = add ls (replicate (length ls) '0') c
add [] ls c           = add (replicate (length ls) '0') ls c
add (x:xs) (y:ys) c   = (add xs ys carry) ++ (show result)
  where
    addInt x y z = (read x :: Int) + (read y :: Int) + z
    addition     = addInt [x] [y] c
    carry        = addition `quot` 10
    result       = addition `mod` 10


addBigInts x y = add (reverse x) (reverse y) 0

addBigInts "999999999999999999" "99999999999999999"