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