Discussion on: Daily Challenge #287 - 16+18=214

import Numeric.Natural (Natural)

(^+^) :: Natural -> Natural -> Natural
(^+^) = reducer 1
  where
    reducer c 0 0 = 0
    reducer c a b =
      let sum = a `mod` 10 + b `mod` 10
          delta = if sum < 10 then 10 else 100
      in c * sum + reducer (c * delta) (shift a) (shift b)
    shift x = floor $ fromIntegral x / 10

import Numeric.Natural (Natural)
import Data.Char (digitToInt)

(^+^) :: Natural -> Natural -> Natural
(^+^) n1 n2 = read $ sumHead n1' n2' <> sumTail n1' n2'
  where
    sumHead x y = maybe "" reverse $ zipRemain x y
    sumTail x y = concat . reverse . fmap (show . add) $ zip x y
    add (x,y) = digitToInt x + digitToInt y
    n1' = reverse $ show n1
    n2' = reverse $ show n2

zipRemain :: [a] -> [a] -> Maybe [a]
zipRemain [] [] = Nothing
zipRemain xs [] = Just xs
zipRemain [] ys = Just ys
zipRemain (_:xs) (_:ys) = zipRemain xs ys

a = ["1", "2", "3", "4", "5"]
b = ["a", "b", "c"]
zipAB = zip a b -- [("1","a"), ("2","b"), ("3","c")] 
zipRemainAB = zipRemain a b -- ["4", "5"]