On Tue, 2006-07-25 at 22:16 -0400, DeeJay-G615 wrote:

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); }

For greater clarity you might like to use HsStablePtr in place of void*.

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?

Yes, that's exactly the point of a StablePtr, to be able to pass it to C and then get it back and get a Haskell value out of it. Your usage of it looks fine to me.

Am I correct in thinking that StablePtr is defined as a void pointer in C?

Right. The FFI spec defines it in HsFFI.h as: typedef void * HsStablePtr;

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.

True. In this case it hardly matters because you are not allowed to directly dereference the pointer in C anyway as it's not guaranteed to point to any valid location. Indeed in GHC's implementation it doesn't. It's just a token which you can pass back to Haskell land.

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?

So when you export a StablePtr to C using an FFI declaration like your one: foreign import ccall "from_maybe_int" cFromMaybeInt :: StablePtr (Maybe Int) -> FunPtr GetJust -> IO () The StablePtr is automatically converted to the C void *. The castStablePtrToPtr function just allows you to do that same conversion but inside Haskell. This might be useful if the StablePtr has to be put into or removed from a C structure.

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).

I find C & Haskell a good combination for practical stuff. Their strengths and weaknesses complement each other quite well. Duncan