todo.hs

import Control.Monad.Trans.State
import Data.List (intercalate)
import Control.Monad.IO.Class (liftIO)

type TodoState a = StateT [Todo] IO a
type Category = String

data Todo = Todo { text :: String
                 , categories :: [Category]
                 } deriving (Show)

addItem :: String -> [Category] -> TodoState ()
addItem txt cs = modify (Todo txt cs:)

updateItem :: String -> String -> TodoState ()
updateItem old new = modify (map (\x -> if text x == old then x {text = new} else x))

viewList :: [Category] -> TodoState ()
viewList cs = do
    list <- get
    liftIO $ do
        putStrLn $ "# " ++ intercalate " & " cs
        mapM_ (putStrLn . ("- " ++) . text) $ filterCategory cs list
        putStr "\n"

filterCategory :: [Category] -> [Todo] -> [Todo]
filterCategory cs = filter (any (`elem` cs) . categories)

runTodo :: TodoState () -> IO ()
runTodo = flip evalStateT []

main :: IO ()
main = runTodo $ do
    addItem "Go to work" ["Programming"]
    addItem "Create Sine Waves in C" ["Music", "Programming"]
    addItem "Play my synth" ["Music"]
    viewList ["Music"]
    updateItem "Create Sine Waves in C" "Create Sine Waves in Python"
    viewList ["Programming"]
    viewList ["Programming", "Music"]