John Meacham wrote:
Well, the actual problem I am trying to solve involves properly
reclaiming elements in a circularly linked list (next and prev pointers
are TVars). I have a linked list and I need to be able to remove values
from the list when all references to the node no longer exist, not
counting the linked list references themselves.
Using Weak pointers inside the list itself doesn't work, since if an
element is collected, you also lose the information needed to stitch up
the list.
Originally, I had a wacky plan involving weak pointers in the nodes
themselves pointing to sentinal nodes, when the sentinal was collected,
I then know I can delete the node. The idea was that I can lazily delete
entire chains of nodes rather than one by one. I gave up on that idea.
(deRefWeak not working in STM was sort of a show stopper, and it was
overly complicated)
So now I have a scheme whereby I attach a finalizer to a proxy thunk.
data TimeStamp = TimeStamp TS
data TS = TS {
tsWord :: TVar Word64,
tsPrev :: TVar TS,
tsNext :: TVar TS
}
so, the finalizer attached to 'TimeStamp' values simply deletes the
value it points to from the linked list.
What you want here is to attach the finalizer to one of the TVars, probably
tsWord. Attaching the finalizer to Timestamp is very risky: the compiler
is free to optimise the Timestamp wrapper away, regardless of how much you
hide in the module API. For example, consider an operation on Timestamp:
once the operation has looked inside the Timestamp wrapper, it no longer
holds a pointer to it, so the finalizer might run, even though the
operation is still working on the TS. A function that is strict in
Timestamp will have a worker that takes the unboxed TS, and might even
re-wrap it in a new Timestamp (with no finalizer, of course).
You can work around this using touch#, but that's a bit inelegant, and I'm
not certain it solves all the problems.
This is why we have mkWeakIORef and addMVarFinalizer - they attach
finalizers to the primitive objects inside the IORef/MVar respectively, so
you can be sure that compiler optimisations aren't going to affect the
finalization properties you want.
Adding finalizers to arbitrary objects was useful for the memo table
application we had in mind when weak pointers were introduced, but for all
the other applications I've come across since then, we really want to add
finalizers to objects whose lifetimes are under programmer control. Notice
how ForeignPtrs attach the finalizer carefully to the MutVar# inside the
ForeignPtr, not the ForeignPtr itself.
The module interface ensures
that only 'TimeStamp' values can escape and each has a finalizer
attached. the interface is quite simple:
newTimeStamp :: IO TimeStamp
insertAfter :: TimeStamp -> IO TimeStamp
now, the problem is that I want to export insertAfter in the 'STM'
monad, not the IO one. however, I am not sure how to pull it off. I
cannot ensure the finalizer is only attached once to the node, if I use
'unsafeIOToSTM' then STM retrying could end up created multiple finalized
nodes, possibly prematurely deleting the element from the linked list.
basically, what would be really nice is if there were something like
registerCommitIO :: IO () -> STM ()
Yes, we ought to have this. As others have pointed out, you can do this by
adding another monad on top of STM, but you can't really do registerRetry
that way.
where all IO actions registered with this function (within an atomically
block) are executed exactly once if and only if the atomically block
commits. The IO action is not run within the STM block, notably
atomically $ do foo; registerCommitIO bar; baz
is equivalent to
atomically (do foo; baz) >> bar
I found I needed the equivalent of 'touchForeignPtr' for arbitrary
objects (which I was able to do with the touch# primitive, but
touch :: a -> IO ()
touchSTM :: a -> STM ()
would both be at home in System.Mem.Weak.
with appropriate caveats, of course!
While I am wishing for things,
unsafePerformSTM :: STM a -> a
would be really handy too :)
The trouble with that is that it can lead to nested transactions, and the
RTS isn't set up to handle that. It's probably a fair bit of work.
> insertAfter :: TimeStamp -> IO TimeStamp
> insertAfter t@(TimeStamp ts) = do
> (tts,ts) <- atomically $ insertAfter' ts
> touchTS t
> addFinalizer ts (deleteTimeStamp tts)
> return ts
ah, I see you're adding the finalizer to the TS, not the Timestamp. Same
arguments apply, though.
> touchTS :: TimeStamp -> IO ()
> touchTS ts = touchForeignPtr (unsafeCoerce# ts)
*blink* that can't possibly work!
Cheers,
Simon
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