ST monad and monad tranformers

[Josef Svenningsson]

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 <twhitehead at gmail.com>:
> 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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