Hi - I've got the beginnings of an API for a GUI system as follows: data DWindow a data DEdit a type Window = DWindow type Edit a = DWindow (DEdit a) foreign import ccall duma_init :: IO () foreign import ccall duma_run :: IO () foreign import ccall duma_release :: FunPtr (Ptr (Window a) -> IO ()) foreign import ccall duma_createEdit :: IO (Ptr (Edit a)) foreign import ccall duma_addTop :: Ptr (Window a) -> IO () createEdit :: IO (ForeignPtr (Edit a)) createEdit = do edit <- duma_createEdit newForeignPtr duma_release edit addTop :: ForeignPtr (Window a) -> IO () addTop w = withForeignPtr w duma_addTop This works, but it seems a bit of a pain to have to manually convert between ForeignPtr's and Ptr's all the time. In particular, for the definition of addTop, I tried: foreign import ccall "duma_addTop" addTop :: ForeignPtr (Window a) -> IO () but got an error because ForeignPtr's are not allowed as part of the type of a foreign function. Since the definition of ForeignPtr is just void *, I wonder why this restriction exists - ie is a ForeignPtr not just the same address as the corresponding Ptr? My other question is what happens if I want to have a function that takes more than one ForeignPtr as argument ie foreign import ccall duma_test :: Ptr (Window a) -> Ptr (Window a) -> IO () test :: ForeignPtr (Window a) -> ForeignPtr (Window a) -> IO () test p q = withForeignPtr p (\p' -> withForeignPtr q $ duma_test p') Is this the only way to achieve this? It seems a bit long-winded and possibly a bit inefficient... One other question: if I use forkIO within Haskell, am I right in thinking that the lightweight concurrent threads are safe to use with my single threaded C code ie that there is no danger of a thread being preemptively halted while it is inside a foreign function? Thanks, Brian.