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