Hello, I wrote this code data DataFrameH5 a = DataFrameH5 (Nxs a) -- Nexus file (DataSource H5) -- gamma (DataSource H5) -- delta (DataSource H5) -- wavelength PoniGenerator -- ponie generator class Frame t a where len :: t -> IO (Maybe Int) row :: t -> Int -> MaybeT IO (DifTomoFrame a DIM1) instance Frame (DataFrameH5 DataFrameH5Path) DataFrameH5Path where len (DataFrameH5 _ _ (DataSourceH5 _ d) _ _) = lenH5Dataspace d row d@(DataFrameH5 nxs' g d' w ponigen) idx = do n <- lift $ len d let eof = fromJust n - 1 == idx let mu = 0.0 let komega = 0.0 let kappa = 0.0 let kphi = 0.0 gamma <- g `atIndex'` (ix1 0) delta <- d' `atIndex'` (ix1 idx) wavelength <- w `atIndex'` (ix1 0) let source = Source (head wavelength *~ nano meter) let positions = concat [mu, komega, kappa, kphi, gamma, delta] -- print positions let geometry = Geometry K6c source positions Nothing let detector = ZeroD m <- lift $ geometryDetectorRotationGet geometry detector poniext <- lift $ ponigen (MyMatrix HklB m) idx return $ DifTomoFrame { difTomoFrameNxs = nxs' , difTomoFrameIdx = idx , difTomoFrameEOF = eof , difTomoFrameGeometry = geometry , difTomoFramePoniExt = poniext } has you can see my t type contains also the a reference to the a one So when I create the instance, I need to write two times the DataFrameH5Path I would like to know how to write the same class with only class Frame t where len :: t -> IO (Maybe Int) row :: t -> Int -> MaybeT IO (DifTomoFrame <extract type a from type t> DIM1) thanks for your help Frederic