[Haskell-cafe] Explicit garbage collection
Edward Kmett
ekmett at gmail.com
Thu Jan 7 15:07:50 EST 2010
Thats a shame, I was rather fond of this monad. To get that last 'True',
you'll really have to rely on garbage collection to approximate reachability
for you, leaving the weak reference solution the best you can hope for.
-Edward Kmett
On Thu, Jan 7, 2010 at 12:39 PM, Miguel Mitrofanov <miguelimo38 at yandex.ru>wrote:
> Damn. Seems like I really need (True, False, True) as a result of "test".
>
>
> On 7 Jan 2010, at 08:52, Miguel Mitrofanov wrote:
>
> Seems very nice. Thanks.
>>
>> On 7 Jan 2010, at 08:01, Edward Kmett wrote:
>>
>> Here is a slightly nicer version using the Codensity monad of STM.
>>>
>>> Thanks go to Andrea Vezzosi for figuring out an annoying hanging bug I
>>> was having.
>>>
>>> -Edward Kmett
>>>
>>> {-# LANGUAGE Rank2Types, GeneralizedNewtypeDeriving, DeriveFunctor #-}
>>> module STMOracle
>>> ( Oracle, Ref
>>> , newRef, readRef, writeRef, modifyRef, needRef
>>> ) where
>>>
>>> import Control.Applicative
>>> import Control.Monad
>>> import Control.Concurrent.STM
>>>
>>> instance Applicative STM where
>>> pure = return
>>> (<*>) = ap
>>>
>>> newtype Ref s a = Ref (TVar (Maybe a))
>>> newtype Oracle s a = Oracle { unOracle :: forall r. (a -> STM r) -> STM r
>>> } deriving (Functor)
>>>
>>> instance Monad (Oracle s) where
>>> return x = Oracle (\k -> k x)
>>> Oracle m >>= f = Oracle (\k -> m (\a -> unOracle (f a) k))
>>>
>>> mkOracle m = Oracle (m >>=)
>>>
>>> runOracle :: (forall s. Oracle s a) -> IO a
>>> runOracle t = atomically (unOracle t return)
>>>
>>> newRef :: a -> Oracle s (Ref s a)
>>> newRef a = mkOracle $ Ref <$> newTVar (Just a)
>>>
>>> readRef :: Ref s a -> Oracle s a
>>> readRef (Ref r) = mkOracle $ do
>>> m <- readTVar r
>>> maybe retry return m
>>>
>>> writeRef :: a -> Ref s a -> Oracle s a
>>> writeRef a (Ref r) = mkOracle $ do
>>> writeTVar r (Just a)
>>> return a
>>>
>>> modifyRef :: (a -> a) -> Ref s a -> Oracle s a
>>> modifyRef f r = do
>>> a <- readRef r
>>> writeRef (f a) r
>>>
>>> needRef :: Ref s a -> Oracle s Bool
>>> needRef (Ref slot) = Oracle $ \k ->
>>> (writeTVar slot Nothing >> k False)
>>> `orElse` k True
>>>
>>> -- test case:
>>> refMaybe b
>>> dflt ref = if b then readRef ref else return dflt
>>> refIgnore ref = return "blablabla"
>>> refFst ref = fst `fmap` readRef ref
>>> test = do
>>> a <- newRef "x"
>>> b <- newRef 1
>>> c <- newRef ('z', Just 0)
>>> -- no performLocalGC required
>>> x <- needRef a
>>> y <- needRef b
>>> z <- needRef c
>>> u <- refMaybe y "t" a -- note that it wouldn't actually read "a",
>>> -- but it won't be known until runtime.
>>> w <- refIgnore b
>>> v <- refFst c
>>> return (x, y, z)
>>>
>>>
>>>
>>> On Wed, Jan 6, 2010 at 10:28 PM, Edward Kmett <ekmett at gmail.com> wrote:
>>> I don't believe you can get quite the semantics you want. However, you
>>> can get reasonably close, by building a manual store and backtracking.
>>>
>>> {-# LANGUAGE Rank2Types #-}
>>> -- lets define an Oracle that tracks whether or not you might need the
>>> reference, by backtracking.
>>> module Oracle
>>> ( Oracle, Ref
>>> , newRef, readRef, writeRef, modifyRef, needRef
>>> ) where
>>>
>>> import Control.Applicative
>>> import Control.Arrow (first)
>>> import Control.Monad
>>> import Data.IntMap (IntMap)
>>> import qualified Data.IntMap as M
>>> import Unsafe.Coerce (unsafeCoerce)
>>> import GHC.Prim (Any)
>>>
>>> -- we need to track our own worlds, otherwise we'd have to build over ST,
>>> change optimistically, and track how to backtrack the state of the Store.
>>> Much uglier.
>>> -- values are stored as 'Any's for safety, see GHC.Prim for a discussion
>>> on the hazards of risking the storage of function types using unsafeCoerce
>>> as anything else.
>>> data World s = World { store :: !(IntMap Any), hwm :: !Int }
>>>
>>> -- references into our store
>>> newtype Ref s a = Ref Int deriving (Eq)
>>>
>>> -- our monad that can 'see the future' ~ StateT (World s) []newtype
>>> Oracle s a = Oracle { unOracle :: World s -> [(a, World s)] }
>>>
>>> -- we rely on the fact that the list is always non-empty for any oracle
>>> you can run. we are only allowed to backtrack if we thought we wouldn't need
>>> the reference, and wound up needing it, so head will always succeed.
>>> runOracle :: (forall s. Oracle s a) -> a
>>> runOracle f = fst $ head $ unOracle f $ World M.empty 1
>>>
>>>
>>> instance Monad (Oracle s) where
>>> return a = Oracle $ \w -> [(a,w)]
>>> Oracle m >>= k = Oracle $ \s -> do
>>> (a,s') <- m s
>>> unOracle (k a) s'
>>>
>>> -- note: you cannot safely define fail here without risking a crash in
>>> runOracle
>>> -- Similarly, we're not a MonadPlus instance because we always want to
>>> succeed eventually.
>>>
>>> instance Functor (Oracle s) where
>>> fmap f (Oracle g) = Oracle $ \w -> first f <$> g w
>>>
>>> instance Applicative (Oracle s) where
>>> pure = return
>>> (<*>) = ap
>>>
>>> -- new ref allocates a fresh slot and inserts the value into the store.
>>> the type level brand 's' keeps us safe, and we don't export the Ref
>>> constructor.
>>> newRef :: a -> Oracle s (Ref s a)
>>> newRef a = Oracle $ \(World w t) ->
>>> [(Ref t, World (M.insert t (unsafeCoerce a) w) (t + 1))]
>>>
>>> -- readRef is the only thing that ever backtracks, if we try to read a
>>> reference we claimed we wouldn't need, then we backtrack to when we decided
>>> we didn't need the reference, and continue with its value.
>>> readRef :: Ref s a -> Oracle s a
>>> readRef (Ref slot) = Oracle $ \world ->
>>> maybe [] (\a -> [(unsafeCoerce a, world)]) $ M.lookup slot (store
>>> world)
>>>
>>> -- note, writeRef dfoesn't 'need' the ref's current value, so needRef
>>> will report False if you writeRef before you read it after this.
>>> writeRef :: a -> Ref s a -> Oracle s a
>>> writeRef a (Ref slot) = Oracle $ \world ->
>>> [(a, world { store = M.insert slot (unsafeCoerce a) $ store world
>>> })]
>>>
>>> {-
>>> -- alternate writeRef where writing 'needs' the ref.
>>> writeRef :: a -> Ref s a -> Oracle s a
>>> writeRef a (Ref slot) = Oracle $ \World store v -> do
>>> (Just _, store') <- return $ updateLookupWithKey replace slot store
>>> [(a, World store' v)]
>>> where
>>> replace _ _ = Just (unsafeCoerce a)
>>> -}
>>>
>>> -- modifying a reference of course needs its current value.
>>> modifyRef :: (a -> a) -> Ref s a -> Oracle s a
>>> modifyRef f r = do
>>> a <- readRef r
>>> writeRef (f a) r
>>>
>>> -- needRef tries to continue executing the world without the element in
>>> the store in question. if that fails, then we'll backtrack to here, and try
>>> again with the original world, and report that the element was in fact
>>> needed.
>>> needRef :: Ref s a -> Oracle s Bool
>>> needRef (Ref slot) = Oracle $ \world ->
>>> [ (False, world { store = M.delete slot $ store world })
>>> , (True, world)
>>> ]
>>>
>>> -- test case:
>>> refMaybe b dflt ref = if b then readRef ref else return dflt
>>> refIgnore ref = return "blablabla"
>>> refFst ref = fst <$> readRef ref
>>> test = do
>>> a <- newRef "x"
>>> b <- newRef 1
>>> c <- newRef ('z', Just 0)
>>> -- no performLocalGC required
>>> x <- needRef a
>>> y <- needRef b
>>> z <- needRef c
>>> u <- refMaybe y "t" a -- note that it wouldn't actually read "a",
>>> -- but it won't be known until runtime.
>>> w <- refIgnore b
>>> v <- refFst c
>>> return (x, y, z)
>>>
>>> -- This will disagree with your desired answer, returning:
>>>
>>> *Oracle> runOracle test
>>> Loading package syb ... linking ... done.
>>> Loading package array-0.2.0.0 ... linking ... done.
>>> Loading package containers-0.2.0.1 ... linking ... done.
>>> (False,False,True)
>>>
>>> rather than (True, False, True), because the oracle is able to see into
>>> the future (via backtracking) to see that refMaybe doesn't use the reference
>>> after all.
>>>
>>> This probably won't suit your needs, but it was a fun little exercise.
>>>
>>> -Edward Kmett
>>>
>>> On Wed, Jan 6, 2010 at 4:05 PM, Miguel Mitrofanov <miguelimo38 at yandex.ru>
>>> wrote:
>>>
>>> On 6 Jan 2010, at 23:21, Edward Kmett wrote:
>>>
>>> You probably just want to hold onto weak references for your
>>> 'isStillNeeded' checks.
>>>
>>> That's what I do now. But I want to minimize the network traffic, so I
>>> want referenced values to be garbage collected as soon as possible - and I
>>> couldn't find anything except System.Mem.performIO to do the job - which is
>>> a bit too global for me.
>>>
>>> Otherwise the isStillNeeded check itself will keep you from garbage
>>> collecting!
>>>
>>> Not necessary. What I'm imagining is that there is essentially only one
>>> way to access the value stored in the reference - with readRef. So, if there
>>> isn't any chance that readRef would be called, the value can be garbage
>>> collected; "isStillNeeded" function only needs the reference, not the value.
>>>
>>> Well, yeah, that's kinda like weak references.
>>>
>>>
>>> http://cvs.haskell.org/Hugs/pages/libraries/base/System-Mem-Weak.html
>>>
>>> -Edward Kmett
>>>
>>> On Wed, Jan 6, 2010 at 9:39 AM, Miguel Mitrofanov <miguelimo38 at yandex.ru>
>>> wrote:
>>> I'll take a look at them.
>>>
>>> I want something like this:
>>>
>>> refMaybe b dflt ref = if b then readRef ref else return dflt
>>> refIgnore ref = return "blablabla"
>>> refFst ref =
>>> do
>>> (v, w) <- readRef ref
>>> return v
>>> test =
>>> do
>>> a <- newRef "x"
>>> b <- newRef 1
>>> c <- newRef ('z', Just 0)
>>> performLocalGC -- if necessary
>>> x <- isStillNeeded a
>>> y <- isStillNeeded b
>>> z <- isStillNeeded c
>>> u <- refMaybe y "t" a -- note that it wouldn't actually read "a",
>>> -- but it won't be known until runtime.
>>> w <- refIgnore b
>>> v <- refFst c
>>> return (x, y, z)
>>>
>>> so that "run test" returns (True, False, True).
>>>
>>>
>>> Dan Doel wrote:
>>> On Wednesday 06 January 2010 8:52:10 am Miguel Mitrofanov wrote:
>>> Is there any kind of "ST" monad that allows to know if some STRef is no
>>> longer needed?
>>>
>>> The problem is, I want to send some data to an external storage over a
>>> network and get it back later, but I don't want to send unnecessary data.
>>>
>>> I've managed to do something like that with weak pointers,
>>> System.Mem.performGC and unsafePerformIO, but it seems to me that
>>> invoking
>>> GC every time is an overkill.
>>>
>>> Oh, and I'm ready to trade the purity of runST for that, if necessary.
>>>
>>> You may be able to use something like Oleg's Lightweight Monadic Regions
>>> to get this effect. I suppose it depends somewhat on what qualifies a
>>> reference as "no longer needed".
>>>
>>> http://www.cs.rutgers.edu/~ccshan/capability/region-io.pdf<http://www.cs.rutgers.edu/%7Eccshan/capability/region-io.pdf>
>>>
>>> I'm not aware of anything out-of-the-box that does what you want, though.
>>>
>>> -- Dan
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>>
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