[Haskell-beginners] which typefor this FFI call

[PICCA Frederic-Emmanuel]

Hello, I end up with this and it works :) thanks

type H5Iterate a = HId_t -> CString -> In H5L_info_t -> InOut a -> IO HErr_t

foreign import ccall "wrapper" mkOp :: H5Iterate a -> IO (FunPtr (H5Iterate a))

nxEntries ∷ FilePath → IO [String]
nxEntries f = withH5File f $ \h → do
  state <- newIORef []
  statePtr <- newStablePtr state
  let opData = InOut $ castStablePtrToPtr statePtr
  let startIndex = Nothing
  let indexType = ByName
  let order = Native
  iop <- mkOp callback
  _ <- withInOut_ (maybe 0 hSize startIndex) $ \ioStartIndex ->
      h5l_iterate (hid h) (indexTypeCode indexType) (iterOrderCode order) ioStartIndex iop opData
              
  freeHaskellFunPtr iop
  freeStablePtr statePtr

  -- retrieve the final state
  readIORef state
    where
      callback ∷ H5Iterate a
      callback _g n _i (InOut dataptr) =
          do
            let opData = castWrappedPtr dataptr
            -- get the state
            stRef <- deRefStablePtr (castPtrToStablePtr opData)
            st <- readIORef stRef

            -- compute the new state
            name <- peekCString n
            let newSt = st ++ [name]
            print st
            print name
            print newSt
                                                            
            -- store the new state
            writeIORef stRef newSt
            return $ HErr_t 0


BUT I lose the type checking at the callback interface.
the ffi call of the h5l_iterate method is

#ccall H5Literate, <hid_t> -> <H5_index_t> -> <H5_iter_order_t> -> InOut <hsize_t> -> H5L_iterate_t a -> InOut a -> IO <herr_t>


where H5L_iterate_t a = H5Iterate

So the call back should keep the information of the type via a


my question is, when I write this

  let opData = InOut $ castStablePtrToPtr statePtr

I have InOut (Ptr ()) is it possible to have somthing more like InOut (Ptr a) instead ?

Thanks

Fred