IO StateTransformer with an escape clause
Hi, I'd like some help building an IO StateTransformer which can be escaped midway through without losing the state accummulated up to that point. I tried constructing a StateT s MaybeIO a monad but the state is lost when the rest of the monad collapses. Tom
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
On Tue, 26 Aug 2003 14:33:28 +1000, "Thomas L. Bevan"
Hi,
I'd like some help building an IO StateTransformer which can be escaped midway through without losing the state accummulated up to that point. I tried constructing a StateT s MaybeIO a monad but the state is lost when the rest of the monad collapses.
How is your MaybeIO type constructed? If with a monad transformer, then you could consider putting the MaybeT transformer outside the StateT transformer (I think that should work). Ganesh
participants (3)
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Andrew J Bromage -
Ganesh Sittampalam -
Thomas L. Bevan