[Haskell-cafe] Combining ST with STM

[Jonas Scholl]

On 02/09/2016 01:46 PM, Thomas Koster wrote:
> On 9 February 2016 at 14:43, Thomas Koster <tkoster at gmail.com> wrote:
>> I have an STM transaction that needs some private, temporary state.
>> The most obvious way is to simply pass pure state as arguments, but
>> for efficiency, I would like this state to be some kind of mutable
>> array, like STArray.
>>
>> The private state is, by definition, not shared, so
>> including it in the STM log and commit process is, as far as I can
>> tell, pointless.
>>
>> ST and STArray still appear to be the most appropriate tools for the
>> private state, because STRefs and STArrays really, really are private.
>>
>> So this basically means I want to interleave ST and STM in a "safe"
>> way. That is, if the STM transaction retries, I want the ST state to
>> be vaporised as well.
>>
>> Ideally, I would love to be able to say something like this:
>>
>> -- | Copy the value from the shared TVar into the private STRef.
>> load :: TVar a -> STRef a -> STSTM s ()
>> load shared private = do
>>   value <- liftSTM (readTVar shared)
>>   liftST (writeSTRef private value)
>>
>> Naturally, that STRef must originate from a call to newSTRef earlier
>> in the same transaction and is private to it, just like the real ST
>> monad. As far as I can tell, I am not trying to weaken either ST or
>> STM in any way here.
> 
> Please forgive the typo in the type signature of "load", which should
> have been:
> 
> load :: TVar a -> STRef s a -> STSTM s ()
> 
> Let me elaborate on STSTM, a monad I made up for this example that
> combines the characteristics of ST and STM in the way that I want.
> If my requirements were unclear from my prose, perhaps the code below
> will illuminate them better.
> 
> An STSTM transaction is intended to be an STM transaction imbued with a
> state token that encapsulates additional, transaction-local state in the
> spirit of ST.
> 
> It is not intended to secretly perform IO inside STM, a la
> GHC.Conc.unsafeIOToSTM.
> 
> It is not intended to facilitate the leaking of state into or out of an
> STM transaction through STRefs, nor to communicate state between
> successive retries of an STM transaction.

I understand that, you just said, you wanted to sprinkle some runST
calls with unsafeThawArray and unsafeFreezeArray into your STM code. So
I assumed you wanted to share an (ST)Array between these STM actions.

> 
> Thanks to hints from Ryan and Jonas, I made an attempt at implementing
> it myself.
> 
> Below is my implementation of STSTM and associated operations. You will
> need to link with the "primitive" and "stm" packages. I used versions
> 0.6 and 2.4.4, resp., and GHC 7.10.2.
> 
> 
> {-# LANGUAGE GeneralizedNewtypeDeriving, Rank2Types #-}
> 
> module Control.Monad.STSTM
>   (
>     STSTM,
>     liftST,
>     liftSTM,
>     atomicallyRunST,
>     module Control.Monad.STM
>   )
> where
> 
> import Control.Monad.Primitive
> import Control.Monad.ST
> import Control.Monad.STM
> 
> -- | A computation of type @STSTM s a@ is an 'STM' computation that
> -- also transforms a transaction-local internal state indexed by @s@, as
> -- in the 'ST' monad, and returns a value of type @a at .
> newtype STSTM s a = STSTM { unSTSTM :: STM a }
>   deriving (Functor, Applicative, Monad)
> 
> -- | Lift an 'ST' computation into the 'STSTM' transaction.
> liftST :: ST s a -> STSTM s a
> {-# INLINE liftST #-}
> liftST x = STSTM $
>   let y = unsafeInlineST x
>   in  y `seq` return y

This is highly unsafe and will not do what you think it does!
unsafeInlineST provides an ST action with a realWorld# token out of thin
air and thus can float outside liftST, especially because you inline it.
This produces exactly the bug I reported against STMonadTrans.

A safe version could take the state token from the STM action, pass it
into the ST action and carry on with the returned state token (look at
GHC.Conc.Sync). Or convert the ST action to IO and then just run the IO
action in STM. This should be fine if you do not use unsafeThaw - any
garbage written to some STRef/STArray will be thrown away after the
runtime sees the STM action will fail and restarts it.

> 
> -- | Lift an 'STM' computation into the 'STSTM' transaction.
> liftSTM :: STM a -> STSTM s a
> {-# INLINE liftSTM #-}
> liftSTM = STSTM
> 
> -- | Perform a series of 'STSTM' actions atomically.
> --
> -- The 'ST' state is discarded when the 'STM' transaction commits or
> -- retries.
> atomicallyRunST :: (forall s. STSTM s a) -> IO a
> {-# INLINE atomicallyRunST #-}
> atomicallyRunST x = atomically (unSTSTM x)
> 
> 
> Some commentary follows:
> 
> Some initial sanity testing with the GHC threaded runtime shows that it
> does what I want, but I am not familiar enough with Core or the RTS to
> predict whether or not it will launch nuclear missiles at the next
> transit of Venus. I would be grateful for any feedback.
> 
> The use of rank-2 polymorphism in the type of atomicallyRunST is
> intended to encapsulate the ST state exactly like it does for runST,
> and that the ST state cannot leak into or out of the transaction.

What you still can not use is unsafeThaw. Consider this:

foo :: Array Int Val -> TVar Int -> IO someResult
foo arr var = atomicallyRunST $ do
    marr <- liftST $ unsafeThaw arr
    val <- liftSTM $ readTVar var
    liftST $ writeArray marr val someOtherVal
    ... do something more...

What happens if the transaction is restarted after the write? You've
written into arr (unsafeThaw did not copy it), but have no log to revert
the write. Now you see a different immutable array. This is bad.

So you can not use unsafeThaw. Even if only one transaction gets a hold
on this array and it would be safe to use unsafeThaw with plain ST (as
this can not retry), because the transaction has to depend on other
TVars etc, otherwise there would be no need for STM.

And now I am wondering what happens if a thread evaluates something like
runST ... unsafeThawArray ... unsafeFreezeArray ... and is hit by an
asynchronous exception... The computation is restated the next time the
thunk is demanded, but this could have already changed the array, right?
So can runST ... unsafeThawArray ... be used in a safe way or is this
combination inherently broken?

Anyway, I think the following holds true:
 - using STRefs: These must have been created in the transaction, so it
works.
 - using STArrays: unsafeThawing an incoming Array will break
referential transparency sooner or later. Thawing (and thus copying) the
incoming array or creating a fresh one should work.
 - using TArrays: You can return these from the STM action and start
another one later with them without breaking referential transparency as
always. If you have to modify incoming arguments, even if only one STM
action has a reference to them at a time, these can be faster as you do
not have to copy everything - instead they will have a log of the
writes, so you would have to benchmark copying against transaction logs.

> 
> STSTM is not a monad transformer (visibly or internally). I hope that
> any potential problems that might afflict the STMonadTrans package are
> irrelevant here.

You won't have problems with lists as underlying monad, yes.

> 
> I use seq in liftST to force the unsafe inline ST computation to occur
> before bind proceeds to the next computation. Without seq, ST
> computations returning () (or anything else that is not evaluated)
> appear to stay as thunks and never transform any state. I suspect this
> may cause problems with bottoms, but I am not sure if that is any
> different from real ST/runST.

Keep in mind that a `seq` b does not guarantee that a is evaluated
before b. I think this is not a problem here, as there are more severe
problems anyway (see above), but this is generally good to have in mind
when writing such code.

Jonas

> 
> --
> Thomas Koster
> _______________________________________________
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> Haskell-Cafe at haskell.org
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> 


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