IO StateTransformer with an escape clause
Andrew J Bromage
Wed, 27 Aug 2003 18:23:19 +1000
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.