todolist.hs

import System.Environment
import System.Directory
import System.IO
import Data.List
import Control.Exception

dispatch :: String -> [String] -> IO ()
dispatch "add" = add
dispatch "view" = view
dispatch "remove" = remove
dispatch command = doesntExist command

doesntExist :: String -> [String] -> IO ()
doesntExist command _ =
  putStrLn $ "The " ++ command ++ " command doesn't exist"

main = do
  (command: argList) <- getArgs
  dispatch command argList

add :: [String] -> IO ()
add [fileName, todoItem] = appendFile fileName (todoItem ++ "\n")
add _ = putStrLn "The add command takes exactly two arguments."

view :: [String] -> IO ()
view [fileName] = do
  contens <- readFile fileName
  let todoTasks = lines contens
      numberedTasks = zipWith (\ n line -> show n ++ " - " ++ line)
                      [0..] todoTasks
  putStrLn $ unlines numberedTasks


remove :: [String] -> IO ()
remove [fileName, numberString] = do
  contens <- readFile fileName
  let todoTasks = lines contens
      numberedTasks = zipWith (\ n line -> show n ++ " - " ++ line)
                      [0..] todoTasks
  putStrLn "These are your TO-DO items: "
  mapM_ putStrLn numberedTasks
  let number = read numberString
      newTodoItems = unlines $ delete (todoTasks !! number) todoTasks
  bracketOnError (openTempFile "." "temp")
    (\ (tempName, tempHandle) ->
      hClose tempHandle
      removeFile tempName)

    (\ (tempName, tempHandle) ->
      hPutStr tempHandle newTodoItems
      hClose tempHandle
      removeFile "todo.txt"
      renameFile tempName "todo.txt")