[Haskell] using the ffi with callbacks

[Anatoly Zaretsky]

On 7/20/06, Evan Martin <martine at danga.com> wrote:
> To elaborate, the code setting this up looks something like this:
>   callback_fcn <- ... -- get a FunPtr using "wrapper" from the ffi
>   free_fcn <- ... -- as above
>   -- the callback data is just stuff that needs freeing
>   callback_data <- newStablePtr (callback_fcn, free_fcn)
>   register_callback callback_fcn callback_data free_fcn
> And my plan was: within the function free_fcn wraps, free
> callback_fcn, free the StablePtr, and then finally free free_fcn
> itself.

As Taral mentioned there's no need in creating different free_fcn's
for each new callback.
The following may be considered as a linker hack but I see no reason
why it could not work:

> freeCallback :: StablePtr (FunPtr a) -> IO ()
> freeCallback sPtr = do
>   fPtr <- deRefStablePtr sPtr
>   freeStablePtr sPtr
>   freeHaskellFunPtr fPtr
>
> foreign export ccall "_hs_some_really_private_symbol"
>   freeCallback :: StablePtr (FunPtr a) -> IO ()
> foreign import ccall "&_hs_some_really_private_symbol"
>   free_fcn :: FunPtr (StablePtr (FunPtr a) -> IO ())

And then you can use it like this:
  ...
  callback_data <- newStablePtr callback_fcn
  register_callback callback_fcn callback_data free_fcn

--
Tolik