Difference between blog post and Hs2048.

hs2048.hs

import Data.List (group)
import Data.Maybe (isJust)
import Data.Monoid ((<>))

empty :: Int -> [Maybe Int]
empty = flip replicate Nothing

shift :: [Maybe Int] -> [Maybe Int]
shift v = take n (v' <> empty n)
  where
    n = length v
    v' = group (filter isJust v) >>= go
    go (Just a : Just b : ts) = Just (a + b) : go ts
    go ts = ts

main :: IO ()
main = print $ shift $ replicate 4 $ Just 2
-- [Just 4,Just 4,Nothing,Nothing]

post.hs

import Data.Maybe (catMaybes)
import Data.List (group)

add :: [Int] -> [Int]
add (x : y : rest) = x + y : rest
add ts = ts

pad :: [a] -> a -> Int -> [a]
pad xs x n = take n (xs ++ repeat x)

shift :: [Maybe Int] -> [Maybe Int]
shift v = pad ts Nothing (length v)
  where
    ts = map Just (concatMap add (group (catMaybes v)))

main :: IO ()
main = print $ shift $ replicate 4 $ Just 2
-- [Just 4,Just 2,Just 2,Nothing]