riddle.hs

module Main where

import Data.Monoid(Sum(..), getSum)
import Data.Char(digitToInt)
import System.Environment(getArgs)

main :: IO ()
main = do
  (arg:_) <- getArgs
  let list = (Sum . digitToInt) <$> arg
  print $ (getSum . mysum) list

mysum :: (Eq a, Monoid a) => [a] -> a
mysum l@(first:_) =
  let
    mysum' (x:y:rest') = (if x == y then x else mempty) `mappend` mysum' (y:rest')
    mysum' [x] = if first == x then first else mempty
    mysum' [] = mempty
  in
    mysum' l
mysum [] = mempty

Now just define (:^+^:) = mappend and o = mempty and it almost looks like proper addition again! :D