I'm guessing it's not safe to use "newIORef" as the function argument to withStorableArray, but in a simple case [1], it seems to work okay. I suppose the danger is if the RTS decides to move the array. Does this happen IRL? - Hal [1]: the example program is something like: module Main where import Data.Array.Storable import Foreign import Data.IORef mkArray :: Int -> IO (StorableArray Int Int) mkArray sz = newArray_ (1,sz) >>= \arr -> mapM_ (\i -> writeArray arr i i) [1..sz] >> return arr foo :: Ptr Int -> IO Int foo ptr = peek ptr >>= \v -> poke ptr (v+1) >> return v bar = do arr <- mkArray 10 ptr <- withStorableArray arr newIORef >>= readIORef foo ptr >> foo ptr >> foo ptr withStorableArray arr foo main = bar >>= print >> bar >>= print >> bar >>= print -- Hal Daume III "Computer science is no more about computers | hdaume@isi.edu than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume