deletetodo.hs

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

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