问题
I've solved the prev question partially.
Right now I'm able to register GObject subtype via bindings-gobject (see hpase)
I can implement SourceCompletionProvider using c'g_type_add_interface_static function (but didn't tried yet).
The only issue is to convert Ptr (), returned by c'g_object_newv, to gtk2hs data type SourceCompletionProvider. How can I do it? Any hints?
SourceCompletionProvider is defined like:
newtype SourceCompletionProvider = SourceCompletionProvider (ForeignPtr (SourceCompletionProvider))
What does this definition means? Why it is recursive? Provider is a ForeignPtr to provider -- looks strange for me.
Thanks.
Solution:
makeNewGObject mkGObject $ castPtr <$> c'g_object_newv myObType 0 nullPtr
回答1:
The outer SourceCompletionProvider is required since this is a newtype, and the inner SourceCompletionProvider is just a marker to distinguish this foreign pointer from pointers to other types. If you look at the definition of Ptr a, it's data Ptr a = Ptr Addr# - a is a phantom type that doesn't appear on the right-hand side, so the definition is not actually recursive.
You can convert a Ptr () to ForeignPtr () using newForeignPtr_ and then cast it to ForeignPtr SourceCompletionProvider with castForeignPtr.
edit: After looking at this a bit more, I think that to make this work you'll need to first convert your Ptr to GObject with the method outlined above and then use unsafeCastGObject. Not tested, though.
来源：https://stackoverflow.com/questions/12509431/convert-ptr-to-sourcecompletionprovider