I wrote 103 lines of imperative code in Haskell :P

Main.hs

module Main where

{-
I wrote this to get a bettern understanding of Haskell way of doing
side effects, which turns out, is not much fun :)
-}

import Todo

import System.Environment
import System.FilePath
import System.IO
import Data.List
import Data.Time

{-
Beware! Most complex syntax since C++ ahead !
-}

main = do
    handle <- openFile "todos.txt" ReadMode
    todos <- readTodos handle []
    hClose handle
    app todos

menu = [ ("Add Todo", addTodo)
       , ("Remove Todo", removeTodo)
       , ("Save", writeTodos)
       ]

app :: [Todo] -> IO ()
app todos = do
    showTodos todos
    showMenu
    input <- getLine
    let selection = read input :: Int
    newTodos <- snd (menu !! selection) todos
    app newTodos

showTodos todos = do
    let todoStrs = map show todos
    mapM_ (\todo -> putStr $ todo ++
                concat (take 50 (repeat "-")) ++ "\n") todoStrs

today :: IO Day
today = do 
    t <- getZonedTime
    return (localDay $ zonedTimeToLocalTime t)

hGetNonEmptyLine :: Handle -> IO String
hGetNonEmptyLine handle = do
    line <- hGetLine handle
    if line == ""
        then hGetNonEmptyLine handle
        else return (line)

readTodo :: Handle -> IO Todo
readTodo handle = do
    desc <- hGetNonEmptyLine handle
    priorityS <- hGetNonEmptyLine handle
    deadlineS <- hGetNonEmptyLine handle
    let priority = read priorityS :: Int
    let deadline = read deadlineS :: Day
    return (Todo desc priority deadline)

readTodos :: Handle -> [Todo] -> IO [Todo]
readTodos handle todos = do
    eofp <- hIsEOF handle
    if eofp
        then return todos
        else do nextTodo <- readTodo handle
                readTodos handle (insert nextTodo todos)

writeTodos :: [Todo] -> IO [Todo]
writeTodos todos = do
    handle <- openFile "todos.txt" WriteMode
    mapM_ (\todo -> let str = desc todo ++ "\n" ++ show (priority todo)
                              ++ "\n" ++ show (deadline todo)
                    in hPutStrLn handle str) todos
    hClose handle
    return todos

removeTodo :: [Todo] -> IO [Todo]
removeTodo todos = do
    idx <- readId
    return $ removeIdx idx todos

addTodo :: [Todo] -> IO [Todo]
addTodo todos = do
    putStr "Todo description: "
    descS <- getLine
    putStr "Todo priority: "
    priS <- getLine
    let priority = read priS :: Int
    putStr "Todo deadline(YYYY-MM-DD): "
    dayS <- getLine
    let day = read dayS :: Day
    return $ insert (Todo descS priority day) todos

removeIdx n lst = (take n lst) ++ (drop (n+1) lst)
                        
readId :: IO Int
readId = do
    putStr "Enter index of Todo item: "
    input <- getLine
    let id = read input :: Int
    return id

showMenu = do
    mapM_ (\menuItem -> do putStrLn $ "[" ++ (show $ fst menuItem) ++ "] "
                            ++ (fst $ snd menuItem)) (zip [0..] menu)
    putStr "> "

Todo.hs

module Todo where

import Data.Time
import Data.List

data Todo = Todo { desc :: String
                 , priority :: Int
                 , deadline :: Day }

instance Show Todo where
    show (Todo desc pri dl) = "Description: " ++ desc ++ "\nPriority: " ++ show pri
        ++ "\nDeadline: " ++ show dl ++ "\n"

instance Eq Todo where
    Todo _ _ p1 == Todo _ _ p2 = p1 == p2
    
instance Ord Todo where
    compare (Todo _ _ p1) (Todo _ _ p2) = compare p2 p1