b

λ ghc red.hs 
[1 of 1] Compiling Main             ( red.hs, red.o )

red.hs:8:5:
    Couldn't match expected type `Char' with actual type `[Char]'
    In the pattern: "merge"
    In a case alternative: "merge" -> merge args putStrLn "done"
    In the expression:
      case action of {
        "merge" -> merge args putStrLn "done"
        otherwise -> putStrLn "unkown command" }

red.hs:15:23:
    Couldn't match expected type `IO ()'
                with actual type `Data.Map.Map k0 a0'
    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:19:11:
    Couldn't match expected type `[t0]' with actual type `IO String'
    In the return type of a call of `readFile'
    In a stmt of a 'do' expression: text <- readFile path
    In the expression:
      do { text <- readFile path;
           foldr (\ line acc -> (processLine line) : acc) [] (lines text) }

red.hs:25:13:
    Couldn't match expected type `Float' with actual type `[Char]'
    In the expression: price
    In the expression: (date, price)
    In the expression:
      let [date, price] = splitOn "," line in (date, price)
λ 

gistfile1.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 =
  foldr (\file acc -> Data.Map.insert file (processFile file) acc) Data.Map.empty (init files)

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

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