Hi! -- Background: -- I have some foreign objects that are mapped to ForeignPtr's, say newtype Obj = O (ForeignPtr Obj) and a foreign "collection" where those objects are added. The adding is something on the lines of addObj, removeObj :: Collection -> Obj -> IO () id est, the C code of the collection maintains a reference to all objects added. That means I have to keep all the ForeignPtr's added alive and that I must not prevent garbage collection of those removed. Of course the 'Collection' isn't just a simple collection, otherwise I would be using normal Haskell counterparts. -- Question: -- What is the best way to keep the added objects alive? Using a list will keep the addition cheap and a low memory overhead, while a Data.Set will make removeObj work in sublinear time (on the Haskell side). Are there any other options? I'm currently heading towards data Collection = C !(ForeignPtr Collection) !(IORef [Obj]) like ForeignPtr itself (although in the case of ForeignPtr's there is no way to remove finalizers). Thanks! -- Felipe.