Re: Adding a finalizer to a TVar

22 Dec 2013


      +1 on adding to stm, I need this too.

Cheers,
Merijn

On Dec 22, 2013, at 21:10 , John Lato wrote:
...
On Dec 22, 2013 4:56 AM, "Sebastiaan Visser"  wrote:
...
Hi,
Both MVar and IORef have specialized functions to attach a finalizer to there inner identifiable (stable) reference cell. I happen to need something similar for TVars, but that one doesn't seem to exist.
The documentation of System.Mem.Weak tells me:
"Finalizers can be used reliably for types that are created explicitly and have identity, such as IORef and MVar. However, to place a finalizer on one of these types, you should use the specific operation provided for that type, e.g. mkWeakIORef and addMVarFinalizer respectively (the non-uniformity is accidental). These operations attach the finalizer to the primitive object inside the box (e.g. MutVar# in the case of IORef), because attaching the finalizer to the box itself fails when the outer box is optimised away by the compiler."
Does it make sense to add a similar function to the STM package?
What I do now is hack around it by adding an IORef next to my TVar which I attach the finalizer to. Because my outer datatype is opaque I expect both variables to go out of scope and get garbage collected simultaneously. As you might expect, this isn't a very satisfiable solution.
Thanks,
Sebastiaan
I needed something similar a while ago, and wrote this (I think it's correct, haven't had any problems at  any rate):
...
{-# LANGUAGE MagicHash #-}
{-# LANGUAGE UnboxedTuples #-}
{-# OPTIONS_GHC -Wall #-}
module Weak.TVar (
  mkWeakTVar
, mkWeakTVarKey
) where
import GHC.Conc.Sync (TVar (..))
import GHC.Weak
import GHC.Base
mkWeakTVar :: TVar a -> Maybe (IO ()) -> IO (Weak (TVar a))
mkWeakTVar t f = mkWeakTVarKey t t f
-- | Create a Weak reference keyed off a TVar.
mkWeakTVarKey :: TVar b -> a -> Maybe (IO ()) -> IO (Weak a)
mkWeakTVarKey (TVar r#) v (Just f) = IO $ \s ->
      case mkWeak# r# v f s of (# s1, w #) -> (# s1, Weak w #)
mkWeakTVarKey (TVar r#) v Nothing = IO $ \s ->
      case mkWeakNoFinalizer# r# v s of (# s1, w #) -> (# s1, Weak w #)
Perhaps something similar would work for you until such a function is added?
(FWIW I support adding this function to the stm package)
John L.

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Re: Adding a finalizer to a TVar

Merijn Verstraaten