Hello, I have this data DataSourceShape = DataSourceShape ([HSize], [Maybe HSize]) | DataSourceShape'Range !DIM1 !DIM1 a function which combine 2 DataSourceShape and produce an new one (monoid operation ?) combine'Shape ∷ DataSourceShape → DataSourceShape → DataSourceShape I use this in this class with familly types. class DataSource a where data DataSourcePath a ∷ Type data DataSourceAcq a ∷ Type ds'Shape ∷ MonadSafe m ⇒ DataSourceAcq a → m DataSourceShape withDataSourceP ∷ (Location l, MonadSafe m) ⇒ ScanFile l → DataSourcePath a → (DataSourceAcq a → m r) → m r and here an instance for one of my type DataFrameQCustom (I have plenty of them). they are all constructed the same way. data DataSourceAcq DataFrameQCustom = DataSourceAcq'DataFrameQCustom (DataSourceAcq Attenuation) (DataSourceAcq Geometry) (DataSourceAcq Image) (DataSourceAcq Mask) (DataSourceAcq Timestamp) (DataSourceAcq Timescan0) (DataSourceAcq Scannumber) ds'Shape(DataSourceAcq'DataFrameQCustom a g i m idx t0 s) = do sa ← ds'Shape a sg ← ds'Shape g si ← ds'Shape i sm ← ds'Shape m sidx ← ds'Shape idx st0 ← ds'Shape t0 ss ← ds'Shape s pure $ foldl1 combine'Shape [sa, sg, si, sm, sidx, st0, ss] withDataSourceP f (DataSourcePath'DataFrameQCustom a g i m idx t0 s) gg = withDataSourceP f a $ λa' → withDataSourceP f g $ λg' → withDataSourceP f i $ λi' → withDataSourceP f m $ λm' → withDataSourceP f idx $ λidx' → withDataSourceP f s $ λs' → withDataSourceP f t0 $ λt0' → gg (DataSourceAcq'DataFrameQCustom a' g' i' m' idx' t0' s') My question is, how should avoid writting by hand all these ds'Shape / withDatasourceP implementations, which seems quite mechanical. thanks for your help Frederic