import System.Environment
import System.IO
import Data.List (delete)
import qualified Data.ByteString as B
import qualified Data.ByteString.Char8 as W

add :: String -> String -> IO ()
add f t = appendFile f ("[ ] " ++ t ++ "\n")

remove :: String -> Int -> IO ()
remove f t = do list <- B.readFile f
                let tasks = W.lines list
                let newlist = W.unlines $ delete (tasks !! t) tasks
                B.writeFile f newlist

complete :: String -> Int -> IO ()
complete f t = do list <- B.readFile f
                  let tasks = W.lines list
                  let task = tasks !! t
                  let newlist = W.unlines $ (delete task tasks) ++ [(W.append (W.pack "[*]") (W.drop 3 task))]
                  B.writeFile f newlist

main = do args <- getArgs
          case args of
               [f, "add", task] -> add f task
               [f, "complete", task] -> complete f (read task :: Int)
               [f, "remove", task] -> remove f (read task :: Int)
               _ -> putStrLn "Invalid arguments."