[Haskell-cafe] Explicit garbage collection

Is there any kind of "ST" monad that allows to know if some STRef is no longer needed? The problem is, I want to send some data to an external storage over a network and get it back later, but I don't want to send unnecessary data. I've managed to do something like that with weak pointers, System.Mem.performGC and unsafePerformIO, but it seems to me that invoking GC every time is an overkill. Oh, and I'm ready to trade the purity of runST for that, if necessary. Thanks.