b

red.hs:13:23:
    Couldn't match expected type `IO a0'
                with actual type `Data.Map.Map k0 a1'
    In the return type of a call of `Data.Map.insert'
    In the expression: Data.Map.insert file (processFile file) acc
    In the first argument of `foldr', namely
      `(\ file acc -> Data.Map.insert file (processFile file) acc)'

red.hs

import System.Environment
import Data.List.Split
import qualified Data.Map

main = do
  (action:args) <- getArgs
  case action of
    "merge" -> merge args
    otherwise -> putStrLn "unkown command"

merge :: [String] -> IO ()
merge files = do
  foldr (\file acc -> Data.Map.insert file (processFile file) acc) Data.Map.empty (init files)
  return ()

processFile :: String -> IO [(String, Float)]
processFile path = do
  text <- readFile path
  return (foldr (\line acc -> (processLine line) : acc) [] (lines text))

processLine :: String -> (String, Float)
processLine line =
  let [date, price] = splitOn "," line
  in (date, (read price :: Float))