Hi. As with a previous post, I think I have found a possible memory problem with the uvector package. I have this data structure (for, again, my Netflix Prize project): IntMap (UArr (Word16 :*: Word8)) I was adding elements to the map using something like: v = map singletonU (a :*: b) insertWith appendU k v m However doing this eats a *lot* of memory. Today I have rewritten my program to use `alter` and `snocU`: append Nothing = Just $ singletonU (a :*: b) append (Just u) = Just $ snocU u (a :*: b) alter append k m This, finally, works. Unfortunately I'm still not able to load the entire Netflix Prize training data set, grouping ratings by customers, because my PC has only 2 GB of RAM. The required memory is about 2 GB, but when the system start to swap, I have to kill the program. So the question is: why appending an array of only one element to an existing array causes memory problems? This should be pratically the same as adding an element. Thanks Manlio