SumTwo.hs

sumTwo :: [Int] -> [Int] -> [Int]
sumTwo x y = doit x y [] 0
    where
        doit [] [] sum 0                    = reverse sum
        doit [] [] sum carry                = doit [] [] (carry : sum) 0
        doit (x : xs) (y : ys) sum carry    =
            let tot = x + y + carry
                (n, nc) =
                    if tot > 9
                        then (tot - 10, 1)
                        else (tot, 0)
                in doit xs ys (n : sum) nc