A basic TODO list application written in Haskell.

Todo.hs

import Data.Maybe
import System.IO

type Todos = [Todo]

type Todo = String

addTodo :: Todos -> Todo -> Todos
addTodo todos todo = todos ++ [todo]

removeTodoN :: Todos -> Int -> Todos
-- NOTE: remember to account for empty patterns
removeTodoN [] i = []
removeTodoN list@(t : ts) i
  | i < 1 = list
  | i == 1 = ts
  | otherwise = t : removeTodoN ts (i - 1)

removeTodoMatching :: Todos -> Todo -> Todos
removeTodoMatching [] s = []
removeTodoMatching (t : ts) s
  | s == t = ts
  | otherwise = t : removeTodoMatching ts s

readTodos :: FilePath -> IO Todos
readTodos path = do
  contents <- readFile path
  return $ lines contents

displayTodos :: Todos -> IO ()
displayTodos todos =
  mapM_ putStrLn $
    -- NOTE: use zipWith rather than map . zip
    zipWith
      (\i t -> show i ++ ". " ++ t)
      [1 ..]
      todos

data Action = Remove Int | Add Todo | Exit

parseUserAction' :: String -> Maybe Action
parseUserAction' input
  | null inputWords = Just Exit
  | action == "remove" =
    if numWords > 1
      then Just $ Remove $ read $ inputWords !! 1
      else Nothing
  | action == "add" = Just $ Add $ unwords $ tail inputWords
  | otherwise = Nothing
  where
    inputWords = words input
    numWords = length inputWords
    action = head inputWords

parseUserAction :: String -> IO Action
-- if `parseUseraction' <input>` yields Nothing, yields the result of
-- `getUserAction`; else, uses `return` to map the `Just a` (Action) value to
-- an `IO Action`
parseUserAction = maybe getUserAction return . parseUserAction'

getUserAction :: IO Action
getUserAction = do
  putStrLn "\nOptions: remove `i`, add `str` or empty to exit"
  -- source: https://stackoverflow.com/questions/55091150/change-exit-value-on-getline-exception-in-haskell
  --
  -- getLine will panic if it sees an EOF, so we need to check if there's an EOF beforehand and handle it accordingly
  isClosed <- isEOF
  if isClosed
    then return Exit
    else do
      input <- getLine
      parseUserAction input

run :: Todos -> IO Todos
run todos = do
  putStrLn "These are your existing TO-DO items:"
  -- display the todos
  displayTodos todos
  -- ask user what they want to do
  action <- getUserAction
  case action of
    Remove i -> run $ removeTodoN todos i
    Add s -> run $ addTodo todos s
    Exit -> return todos

main = do
  todos <- readTodos "./todo.txt"
  newTodos <- run todos
  writeFile "./todo.txt" $ unlines newTodos