Součin dlouhých čísel v Haskellu

-- Soucin dlouhych cisel
--   parametry: znamenko (1/-1), cifry [Int], pozice des. carky od 1. cifry
--   test -123.14*1.2:   multBig (-1,[1,2,3,1,4],3) (1,[1,2],1)
type BigNum = (Int, [Int], Int)
multBig :: BigNum -> BigNum -> BigNum
multBig (sign1,s1,d1) (sign2,s2,d2) = (sign1 * sign2, finalNum, finalD)
    where finalNum = reverse $ multB (reverse s1) (reverse s2) 0
          finalD = length finalNum - (length s1 - d1 + length s2 - d2)

-- Vynasobi dva otocene seznamy cisel
--   parametry: prvni seznam (odzadu), ukrajujici se druhy seznam (odzadu), posunuti od zacatku
multB :: [Int] -> [Int] -> Int -> [Int]
multB _ [] _ = []
multB s1 (n:ns) p = normNums [a+b | (a,b) <- zipPlus line otherLines] 0
    where line = [0|_<-[1..p]] ++ multByNum s1 n
          otherLines = multB s1 ns (p+1)

-- Vynasobi seznam cislem
multByNum :: [Int] -> Int -> [Int]
multByNum s n =
    let nums = [n*x | x <- s]
    in normNums nums 0

-- Posle dal desitkove prebytky (druhy parametr = preteceni)
normNums :: [Int] -> Int -> [Int]
normNums [] 0 = []
normNums [] n = [n]
normNums (x:xs) n = ((x+n) `mod` 10) : (normNums xs ((x+n) `div` 10))

-- zipPlus - jako zip (udela dvojice), plus doplni kratsi seznam nulami
zipPlus :: (Num a, Num b) => [a] -> [b] -> [(a,b)]
zipPlus [] [] = []
zipPlus [] (x:xs) = (0,x):(zipPlus [] xs)
zipPlus (x:xs) [] = (x,0):(zipPlus xs [])
zipPlus (x:xs) (y:ys) = (x,y):(zipPlus xs ys)