I have a query which is asked out of interest's sake... I'm essentially looking for an affirmation of what I think I already understand (or some info if I'm deluded ;)). To put this in context... I have some C code... typedef int func(void *); void from_maybe_int(void *val, func *my_func) { int i; i = my_func(val); printf("value within C: %d\n", i); } And some Haskell code... type FromMaybeInt = StablePtr (Maybe Int) -> IO CInt foreign import ccall "wrapper" wrapFromMaybeInt :: FromMaybeInt -> IO (FunPtr FromMaybeInt) foreign import ccall "from_maybe_int" cFromMaybeInt :: StablePtr (Maybe Int) -> FunPtr GetJust -> IO () functionToPass :: FromMaybeInt functionToPass sPtr = do m <- deRefStablePtr sPtr case m of Nothing -> return (-1) Just i -> return (fromIntegral i) main :: IO () main = do sPtr <- newStablePtr (Just 3) -- for example funPtr <- wrapFromMaybeInt functionToPass cFromMaybeInt sPtr funPtr freeStablePtr sPtr The compiled program works fine. However I wanted to check this was correct usage. As in, is perfectly fine to pass a value of type StablePtr a into C? Am I correct in thinking that StablePtr is defined as a void pointer in C? From my very limited understanding of C, it is also the case that you can implicitly cast a void pointer to any other pointer type and vice-versa. Some appear to deem it bad practice if you explictly give the cast. So therefore I am somewhat hazy on the use for castStablePtrToPtr. I found the ghc docs to be quite cryptic for this function. Google dug up a few examples of it's use in PUGS... which has lead me to think that the function is purely for type 'convience' in Haskell. Is this the case or am I missing a use case here? Disclaimer: My knowledge and experience of C is somewhat limited. Ironically I have started playing with C by way of Haskell. (Sick and twisted I know). Thanks, Daniel James P.S. I must say, I really like the Haskell FFI. Very clean and enjoyable to work with.