Vaguely terrible Haskell

todo.hs

-- from http://www.haskellforall.com/2015/10/basic-haskell-examples.html
import System.Directory
import System.Environment 
import System.IO

putTodo :: (Int, String) -> IO ()
putTodo (n, todo) = putStrLn (show n ++ ": " ++ todo)

prompt :: [String] -> IO ()
prompt todos = do
    putStrLn ""
    putStrLn "Current TODO List:"
    mapM_ putTodo (zip [0..] todos)
    command <- getLine
    interpret command todos

saveToFile :: String -> [String] -> IO ()
saveToFile fileName todos = do
    writeFile fileName (unlines todos)
    return ()
    -- prompt todos

interpret :: String -> [String] -> IO ()
interpret ('+':' ':todo) todos = prompt (todo:todos)
interpret ('-':' ':num ) todos =
    case delete (read num) todos of
        Nothing -> do
            putStrLn "No TODO entry matches the given number"
            prompt todos
        Just todos' -> prompt todos'
interpret  "w"         todos = do
    saveToFile "todos.txt" todos
    prompt todos
interpret  "wq"         todos = saveToFile "todos.txt" todos
interpret  "q"          todos = return ()
interpret command       todos = do
    putStrLn ("Invalid command: `" ++ command ++ "`")
    prompt todos

delete :: Int -> [a] -> Maybe [a]
delete 0 (_:as) = Just as
delete n (a:as) = do
    let n' = n - 1
    as' <- n' `seq` delete n' as
    Just (a:as')
delete _ []     = Nothing

main = do
    putStrLn "Commands:"
    putStrLn "+ <String> - Adda TODO entry"
    putStrLn "- <Int>    - Delete the numbered entry"
    putStrLn "wq         - Save & Quit"
    putStrLn "q          - Quit"
    [userArgs] <- getArgs
    case userArgs of
      -- this is *trying* to handle missing filename arg...
        [] -> do
            prompt []
        fileName -> do
            fileExists <- doesFileExist fileName
            if fileExists
                then do
                    todos <- readFile fileName
                    prompt (lines todos)
                else
                    prompt []