Hi Tyson,
I also needed something like this a while ago so I knocked up a really
simple module and put it on hackage:
http://hackage.haskell.org/cgi-bin/hackage-scripts/package/STMonadTrans
If you have any suggestions for improvement they are most welcome.
Patches even more so.
Josef
2009/2/2 Tyson Whitehead
I have a situation in which I believe I need a parameterizable version of the strict ST monad. My computation type is "StateT s' (STT s (ErrorT e m)) a" (i.e., fails or succeeds and has an internal state involving a state thread).
The STT type above is a version of ST like the ReaderT, StateT, etc. types.
newtype STT s m a = STT ( State# s -> m (STTBox s a) ) data STTBox s a = STTBox {-#UNPACK#-} !(State# s) {-#UNPACK#-} !a
(I'm guessing on the UNPACK paragmas here) with
runSTT :: (Monad m) => (forall s. STT s m a) -> m a runSTT m = case m of STT m' -> do STTBox _ x <- m' realWorld# return x
(writing this as "runSTT (STT m') = ..." doesn't typecheck with ghc 6.8.2)
instance Monad m => Monad (STT s m) where return x = STT $ \s -> return $ STTBox s x (STT m) >>= k = STT $ \s -> do STTBox s' x <- m s case k x of STT k' -> k' s'
plus all the assorted instances for Functor, MonadPlus, MonadFix, MonadTrans, MonadReader, MonadState, etc. For example,
instance MonadWriter w m => MonadWriter w (STT s m) where tell = lift . tell listen (STT m) = STT $ \s -> do (STTBox s' x,w) <- listen $ m s return $ STTBox s' (x,w) pass (STT m) = STT $ \s -> pass $ do STTBox s' (x,f) <- m s return (STTBox s' x,f)
I was looking for any comments, wondering if there is a reason for this not existing in the library already, and what I should do in terms of paragmas and such for speed? I see the GHC-ST file has a mix of INLINE and NOINLINE.
http://www.haskell.org/ghc/dist/current/docs/libraries/base/src/GHC-ST.html
In particular, return, >>=, >>, and runST are marked INLINE, but there is a "regrettably delicate" comment that goes with the runST method. Also, what about the Functor, MonadPlus, MonadFix, MonadTrans, MonadReader, etc. methods?
Thanks! -Tyson
PS: I would be happy to provide the whole works to be added to the library if it is something that should be there.
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