an answer of https://github.com/haskell-jp/makeMistakesToLearnHaskell/blob/master/assets/2.5.md

2-5.hs

import Control.Monad (forM_)
import Data.List.Split (splitOn)
import qualified Data.Map as M
import Data.Maybe (maybe)
import System.Environment (getArgs)
import System.IO (readFile)

parseLine :: String -> (String, Int)
parseLine str = let (key:count:_) = splitOn "\t" str
                in (key, read count)

main :: IO ()
main = do
  (path:_) <- getArgs
  ls <- lines <$> readFile path
  let ps = map parseLine ls
      summary = foldr collector M.empty ps
  forM_ (M.toList summary) $ \(k, c) -> do
    putStrLn $ k ++ "\t" ++ show c
  where
    collector (k, c) = M.alter (Just . maybe c (c +)) k