Hello first a big thanks your for all your comment :) I end-up with this solution data Diffractometer = Diffractometer { difEngineList :: (ForeignPtr HklEngineList) , difGeometry :: (ForeignPtr HklGeometry) , difDetector :: (ForeignPtr HklDetector) , difSample :: (ForeignPtr HklSample) } deriving (Show) newDiffractometer :: Factory -> Geometry -> Detector -> Sample -> IO Diffractometer newDiffractometer f g d s = do f_engines <- newEngineList f f_geometry <- newGeometry f g f_detector <- newDetector d f_sample <- newSample s withForeignPtr f_sample $ \sample -> withForeignPtr f_detector $ \detector -> withForeignPtr f_geometry $ \geometry -> withForeignPtr f_engines $ \engines -> do c_hkl_engine_list_init engines geometry detector sample return $ Diffractometer f_engines f_geometry f_detector f_sample solve' :: Ptr HklEngine -> CSize -> Engine -> IO (ForeignPtr HklGeometryList) solve' engine n (Engine _ ps _) = do let positions = [v | (Parameter _ v _) <- ps] withArray positions $ \values -> c_hkl_engine_pseudo_axis_values_set engine values n unit nullPtr >>= newForeignPtr c_hkl_geometry_list_free solveTrajPipe' :: Diffractometer -> Pipe Engine Geometry IO () solveTrajPipe' dif = forever $ do -- Inside here we are using `StateT Int (Consumer a IO) r` e <- await let name = engineName e solutions <- lift $ withForeignPtr (difEngineList dif) $ \engines -> withCString name $ \cname -> do engine <- c_hkl_engine_list_engine_get_by_name engines cname nullPtr n <- c_hkl_engine_pseudo_axis_names_get engine >>= darrayStringLen solutions <- solve' engine n e >>= getSolution0 return solutions yield solutions solveTrajPipe :: Factory -> Geometry -> Detector -> Sample -> Pipe Engine Geometry IO () solveTrajPipe f g d s = do dif <- lift $ newDiffractometer f g d s solveTrajPipe' dif so, I created a data type with contain all my foreignPtr. instanciate them at the begining. this newDiffractometer function also initialise the C library objects c_hkl_engine_list_init engines geometry detector sample then I just need to do a forever loop and use this type data to keep the C internal state. what's worring me is that I have a sort of internal state but this is not expressed anywhere in the type system... Cheers Frederic