dispatch.hs

    import System.Environment   
    import System.Directory  
    import System.IO  
    import Data.List  
      
    dispatch :: [(String, [String] -> IO ())]  
    dispatch =  [ ("add", add)  
                , ("view", view)  
                , ("remove", remove)  
                ]  
       
    main = do  
        (command:args) <- getArgs  
        let (Just action) = lookup command dispatch  
        action args  
      
    add :: [String] -> IO ()  
    add [fileName, todoItem] = appendFile fileName (todoItem ++ "\n")  
      
    view :: [String] -> IO ()  
    view [fileName] = do  
        contents <- readFile fileName  
        let todoTasks = lines contents  
            numberedTasks = zipWith (\n line -> show n ++ " - " ++ line) [0..] todoTasks  
        putStr $ unlines numberedTasks  
      
    remove :: [String] -> IO ()  
    remove [fileName, numberString] = do  
        handle <- openFile fileName ReadMode  
        (tempName, tempHandle) <- openTempFile "." "temp"  
        contents <- hGetContents handle  
        let number = read numberString  
            todoTasks = lines contents  
            newTodoItems = delete (todoTasks !! number) todoTasks  
        hPutStr tempHandle $ unlines newTodoItems  
        hClose handle  
        hClose tempHandle  
        removeFile fileName  
        renameFile tempName fileName