Re: [Haskell-cafe] ANNOUNCE: pqueue-mtl, stateful-mtl

I just posted stateful-mtl and pqueue-mtl 1.0.2, making use of the new approach to single-threaded ST wrapping. I discovered while making the modifications to both packages that the MonadSTTrans type class was unnecessary, enabling a cleaner integration with mtl proper. I'm pretty confident that this approach is airtight, but let me know if you encounter contradictions or problems. Louis Wasserman wasserman.louis@gmail.com On Mon, Feb 16, 2009 at 10:21 AM, Sittampalam, Ganesh < ganesh.sittampalam@credit-suisse.com> wrote:

Oh, I see, every derived monad has to have an 's' in its type somewhere.

------------------------------ *From:* Louis Wasserman [mailto:wasserman.louis@gmail.com] *Sent:* 16 February 2009 16:17

*To:* Sittampalam, Ganesh *Cc:* Dan Doel; Henning Thielemann; haskell-cafe@haskell.org *Subject:* Re: [Haskell-cafe] ANNOUNCE: pqueue-mtl, stateful-mtl

But the m -> s dependency will have been removed by the time runST gets a hold of it! It works, I just tested it.

*Control.Monad.Array.ArrayM> :t runST (runArrayT 5 Nothing getContents) runST (runArrayT 5 Nothing getContents) :: [Maybe a] *Control.Monad.Array.ArrayM> runST (runArrayT 5 Nothing getContents) [Nothing,Nothing,Nothing,Nothing,Nothing]

There is, unfortunately, one last key point needed in this approach: the transformer cannot implement MonadTrans, which requires that it work for all monads. The hack I added is

class MonadSTTrans s t where stLift :: MonadST s m => m a -> t m a

instance MonadTrans t => MonadSTTrans s t where stLift = lift

which, as a side effect, makes explicit the distinction between normal monad transformers and ST-wrapped monad transformers.

Louis Wasserman wasserman.louis@gmail.com

On Mon, Feb 16, 2009 at 10:04 AM, Sittampalam, Ganesh < ganesh.sittampalam@credit-suisse.com> wrote:

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I don't think this can be right, because the m -> s dependency will contradict the universal quantification of s required by runST. In other words, unwrapping the transformers will leave you with an ST computation for a specific s, which runST will reject.

------------------------------ *From:* Louis Wasserman [mailto:wasserman.louis@gmail.com] *Sent:* 16 February 2009 16:01 *To:* Sittampalam, Ganesh *Cc:* Dan Doel; Henning Thielemann; haskell-cafe@haskell.org

*Subject:* Re: [Haskell-cafe] ANNOUNCE: pqueue-mtl, stateful-mtl

Overnight I had the following thought, which I think could work rather well. The most basic implementation of the idea is as follows:

class MonadST s m | m -> s where liftST :: ST s a -> m a

instance MonadST s (ST s) where ... instance MonadST s m => MonadST ...

newtype FooT m e = FooT (StateT Foo m e)

instance (Monad m, MonadST s m) => Monad (FooT m) where ...

instance (Monad m, MonadST s m) => MonadBar (FooT m) where <operations using an ST state>

instance (Monad m, MonadST s m) => MonadST s (FooT m) where ...

The point here is that a MonadST instance guarantees that the bottom monad is an ST -- and therefore single-threaded of necessity -- and grants any ST-based monad transformers on top of it access to its single state thread.

The more fully general approach to guaranteeing an underlying monad is single-threaded would be to create a dummy state parameter version of each single-threaded monad -- State, Writer, and Reader -- and add a typeclass called MonadThreaded or something.

The real question with this approach would be how to go about unwrapping ST-based monad transformers in this fashion: I'm thinking that you would essentially perform unwrapping of the outer monad using an ST computation which gets lifted to the next-higher monad. So, say, for example:

newtype MonadST s m => ArrayT e m a = ArrayT {execArrayT :: StateT (STArray s Int e) m a}

runArrayT :: (Monad m, MonadST s m) => Int -> ArrayT e m a -> m a runArrayT n m = liftST (newArray_ (0, n-1)) >>= evalStateT (execArrayT m)

Key points: - A MonadST s m instance should *always* imply that the bottom-level monad is of type ST s, preferably a bottom level provided when defining a monad by stacking transformers. The fact that the bottom monad is in ST should guarantee single-threaded, referentially transparent behavior. - A non-transformer implementation of an ST-bound monad transformer would simply involve setting the bottom monad to ST, rather than Identity as for most monad transformers. - Unwrapping an ST-bound monad transformer involves no universal quantification on the state type. After all transformers have been unwrapped, it should be possible to invoke runST on the final ST s a. - Both normal transformers and ST-bound transformers should propagate MonadST.

I'm going to go try implementing this idea in stateful-mtl now...

Louis Wasserman wasserman.louis@gmail.com

On Mon, Feb 16, 2009 at 3:07 AM, Sittampalam, Ganesh < ganesh.sittampalam@credit-suisse.com> wrote:

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Well, I think a type system like Clean's that had linear/uniqueness types could "fix" the issue by actually checking that the state is single-threaded (and thus stop you from applying it to a "forking" monad). But there's a fundamental operational problem that ST makes destructive updates, so to support it as a monad transformer in general you'd need a type system that actually introduced fork operations (which "linear implicit parameters" used to do in GHC , but they were removed because they were quite complicated semantically and noone really used them).

------------------------------ *From:* haskell-cafe-bounces@haskell.org [mailto: haskell-cafe-bounces@haskell.org] *On Behalf Of *Louis Wasserman *Sent:* 16 February 2009 03:31 *To:* Dan Doel *Cc:* Henning Thielemann; haskell-cafe@haskell.org *Subject:* Re: [Haskell-cafe] ANNOUNCE: pqueue-mtl, stateful-mtl

Okay, I tested it out and the arrow transformer has the same problem. I realized this after I sent the last message -- the point is that at any particular point, intuitively there should be exactly one copy of a State# s for each state thread, and it should never get duplicated; allowing other monads or arrows to hold a State# s in any form allows them to hold more than one, violating that goal.

I'm not entirely convinced yet that there *isn't* some really gorgeous type system magic to fix this issue, like the type-system magic that motivates the type of runST in the first place, but that's not an argument that such magic exists...it's certainly an interesting topic to mull.

Louis Wasserman wasserman.louis@gmail.com

On Sun, Feb 15, 2009 at 9:20 PM, Dan Doel wrote:

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Hello all,

I just uploaded stateful-mtl and pqueue-mtl 1.0.1. The ST monad transformer and array transformer have been removed -- I've convinced myself that a heap transformer backed by an ST array cannot be referentially transparent -- and the heap monad is now available only as a basic monad and not a transformer, though it still provides priority queue functionality to any of the mtl wrappers around it. stateful-mtl retains a MonadST typeclass which is implemented by ST and monad transformers around it, allowing computations in the the ST-bound heap monad to perform ST operations in its thread.

Since this discussion had largely led to the conclusion that ST can only be used as a bottom-level monad, it would be pretty uncool if ST computations couldn't be performed in a monad using ST internally because the ST

On Sunday 15 February 2009 9:44:42 pm Louis Wasserman wrote: thread

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was hidden and there was no way to place ST computations 'under' the outer monad. Anyway, it's essentially just like the MonadIO typeclass, except with a functional dependency on the state type.

There was a question I asked that never got answered, and I'm still curious: would an ST *arrow* transformer be valid? Arrows impose sequencing on their operations that monads don't... I'm going to test out some ideas, I think.

Your proposed type:

State (Kleisli []) x y = (s, x) -> [(s, y)]

is (roughly) isomorphic to:

x -> StateT s [] y = x -> s -> [(s, y)]

The problem with an ST transformer is that the state parameter needs to be used linearly, because that's the only condition under which the optimization of mutable update is safe. ST ensures this by construction, as opposed to other languages (Clean) that have type systems that can express this kind of constraint directly. However, with STT, whether the state parameter is used linearly is a function of the wrapped monad. You'd have to give a more fleshed out version of your proposed state arrow transformer, but off the top of my head, I'm not sure it'd be any better.

-- Dan

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