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Re: [Haskell-cafe] Explicit garbage collection

Subject: Re: [Haskell-cafe] Explicit garbage collection
From: Miguel Mitrofanov
Date: Thu, 7 Jan 2010 20:39:10 +0300
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 <[email protected]> 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 <[email protected] > 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 <[email protected] > 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

I'm not aware of anything out-of-the-box that does what you want, though.
-- Dan
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