How to deal with application state in Gtk2Hs

Question

Trying to learn to write applications with Gtk2Hs I\'m getting difficulties bridging the gap between the event driven Gtk2HS and the persistent state of my model. So to simp

Answer 1

There's basically two approaches:

1. Use a pointer of some kind. This is your IORef or MVar approach. You can hide this behind a MonadState-like interface if you like:

   newtype GtkT s m a = GtkT { unGtkT :: ReaderT (IORef s) m a } deriving (Functor, Applicative, Monad, MonadIO)
   runGtkT = runReaderT . unGtkT
   
   instance MonadIO m => MonadState s (GtkT s m) where
       get   = GtkT (ask >>= liftIO . readIORef)
       put s = GtkT (ask >>= liftIO . flip writeIORef s)
   

2. Pull an "inversion of control" style trick. Write a callback that prints a number, then replaces itself with a new callback that prints a higher number.

If you try to use State or StateT directly, you're gonna have a bad time.