G'day all. On Tue, Aug 26, 2003 at 02:33:28PM +1000, Thomas L. Bevan wrote:
I'd like some help building an IO StateTransformer which can be escaped midway through without losing the state accummulated up to that point.
A simple way to do this is to use a ReaderT monad stacked on top of IO, where the reader state is an IORef to whatever your "real" state is, then write wrapper functions to access the IORef like a state monad. For example, if you had: newtype PersistentStateT s m a = PersistentStateT (ReaderT (IORef s) m a) then you could write: -- WARNING: Unchecked code follows! instance (MonadIO m) => MonadState s (PersistentState s m) where get = PersistentStateT $ do r <- ask liftIO (readIORef r) set s = PersistentStateT $ do r <- ask liftIO (writeIORef r s) The drawback is that the state is no longer backtrackable, say, if you use a nondeterminism monad transformer stacked over this. Cheers, Andrew Bromage