Suppose I have a C function like this: void register_callback( void (*callback_fcn)(void *data), void *callback_data, void (*free_fcn)(void *data)); I think this is pretty common in C libraries. The idea is that you can register a callback along with a pointer to some data to pass to it, and when the C code is done with the callback+data it'll call your free_fcn, again passing the same data.
I'd like to wrap this in Haskell, allowing me to pass in a Haskell function as a callback. The tricky part is that to pass in Haskell functions, I need to use the FFI "wrapper" import, which means I need to later free them. But the only place I can free them is within the "free" callback, and I've just discovered this isn't allowed! 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. However, I discovered this thread, which indicates that having a function free its FunPtr is not allowed: http://www.haskell.org//pipermail/glasgow-haskell-users/2006-March/009907.html I've poked around a bit online, and so far, the only solutions I've seen are 1) http://darcs.haskell.org/packages/GLUT/Graphics/UI/GLUT/Callbacks/Registration.hs , which seems to involve three extra IORefs and unsafePerformIOs as well as an external library function for registering the scavenger. 2) http://darcs.haskell.org/gtk2hs/glib/System/Glib/ , which seems to either use a bunch of Haskell-specific C code I don't quite follow or use the self-delete pattern I described above (see mkFunPtrDestroyNotify in http://darcs.haskell.org/gtk2hs/glib/System/Glib/GObject.chs.pp). So my question is: is there really no simple way to support this pattern (which I would broadly refer to as "callbacks in general"), or am I missing something? _______________________________________________ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
