https://soupi.github.io/rfc/writing_simple_haskell/ - typed as I read the tutorial.

todo.hs

module Main where

data Command
  = QuitApp
  | DisplayItems
  | Help
  | AddItem String
  | Complete Int

type Item = String
type Items = [Item]

addItem :: Item -> Items -> Items
addItem item items = item : items

removeItem :: Int -> Items -> Either String Items
removeItem reverseIndex allItems =
    impl (length allItems - reverseIndex) allItems
  where
    impl index items =
      case (index, items) of
        (0, item : rest) ->
          Right rest
        (n, []) ->
          Left "Index out of bounds"
        (n, item : rest) ->
          case impl (n - 1) rest of
            Right newItems ->
              Right (item : newItems)
            Left errMsg ->
              Left errMsg

displayItems :: Items -> String
displayItems items =
  let
    displayItem index item = show index ++ ": " ++ item
    reversedList = reverse items
    displayedItemsList = zipWith displayItem [1..] reversedList
  in
    unlines displayedItemsList


parseCommand :: String -> Either String Command
parseCommand line = case words line of
  ["quit"]             -> Right QuitApp
  ["items"]            -> Right DisplayItems
  ["help"]             -> Right Help
  ["complete", idxStr] ->
    if all (\c -> elem c "0123456789") idxStr
      then Right (Complete (read idxStr))
      else Left "Invalid index"
  "add" : "-" : item   -> Right (AddItem (unwords item))
  _                    -> Left "Unknown command, try 'help'"

interactWithUser :: Items -> IO ()
interactWithUser items = do
  line <- getLine
  case parseCommand line of
    Right DisplayItems -> do
      putStrLn "Current Items:"
      putStrLn (displayItems items)
      interactWithUser items

    Right (AddItem item) -> do
      let newItems = addItem item items
      putStrLn "Item added.\n"
      interactWithUser newItems

    Right QuitApp -> do
      pure ()

    Right Help -> do
      putStrLn "Commands: items, help, quit, add - <item to add>"
      interactWithUser items

    Right (Complete index) -> do
      let result = removeItem index items
      case result of
        Left errMsg -> do
          putStrLn ("Error: " ++ errMsg)
          interactWithUser items
        Right newItems -> do
          putStrLn "Item complete."
          interactWithUser newItems
         
    Left errorMessage -> do
      interactWithUser items
      
main :: IO ()
main = do
  putStrLn " --| TODO |-- "
  putStrLn "Commands: items, help, quit, add - <item to add>"
  let initialList = []
  interactWithUser initialList
  putStrLn "Thanks, see you soon!"