+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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