todo.hs

import System.Environment
import System.Directory ( removeFile, renameFile )
import System.IO
    ( hClose,
      openFile,
      hGetContents,
      hPutStr,
      openTempFile,
      IOMode(ReadMode) )
import Data.List ( delete )
import Control.Exception ( bracketOnError )

main = do
  (cmd: args) <- getArgs 
  dispatch cmd args

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

add :: [String] -> IO()
add [fileName, todoItem] = appendFile fileName todoItem

view :: [String] -> IO()
view [fileName]= do
  handle <- openFile fileName ReadMode 
  contents <- hGetContents handle
  let todoTasks = lines contents
      nTodoTasks = zipWith (\n line -> show n ++ line ++ "\n") [0..] todoTasks
  putStr $ unlines nTodoTasks

remove :: [String] -> IO()
remove [fileName, numberString] = do
  handle <- openFile fileName ReadMode 
  contents <- hGetContents handle
  let todoTasks = lines contents
      number = read numberString
      newTodoTasks = unlines $ delete (todoTasks !! number) todoTasks
  bracketOnError (openTempFile "." "temp")
      (\ (tempName, tempHandle) -> do
        hClose tempHandle
        removeFile tempName)
      (\ (tempName, tempHandle) -> do
        hPutStr tempHandle newTodoTasks
        hClose tempHandle
        removeFile fileName
        renameFile tempName fileName)

doesNotExist :: String -> [String] -> IO()
doesNotExist command _ = do
  putStrLn $ command ++ " does not exist"