As the result of a conversation on haskell-gui, I have tried to implement the disallocation of resources when a stream is garbage collected. To explain myself: I have a function f :: IO [a] which returns a lazy stream after allocating some resource to feed it (say installing a callback). I wish that the resource could be disallocated when it's no longer used. I did the obvious implementation with Weak.addFinalizer; results are encouraging but not completely satisfying; the scheme I used is: f = do allocateResource l <- makeTheStream addFinalizer l (disallocateResource) return l The problem is that if no memory is allocated, no garbage collection happens; of course finalization is not guaranteed, as the manual states. Another alternative is to make f return an esplicit "close stream" action: f :: IO ([a],IO ()) Is anyone willing to explain me other alternatives if there are, or to tell me that there aren't? Thanks for attention Vincenzo