Hello Li-yao thanks a lot for you explanations, it helps me a lot. I end up with this error (beware the long error message...) src/Hkl/Binoculars/Projections/Config/Sample.hs:68:14: error: [GHC-39999] • Could not deduce ‘Hkl.DataSource.GDataSourceAcq (GHC.Generics.C1 (GHC.Generics.MetaCons "DataSourceT'Sample" GHC.Generics.PrefixI False) (((GHC.Generics.S1 (GHC.Generics.MetaSel Nothing GHC.Generics.NoSourceUnpackedness GHC.Generics.NoSourceStrictness GHC.Generics.DecidedLazy) (GHC.Generics.Rec0 (DataSourceT DSAcq Double)) GHC.Generics.:*: GHC.Generics.S1 (GHC.Generics.MetaSel Nothing GHC.Generics.NoSourceUnpackedness GHC.Generics.NoSourceStrictness GHC.Generics.DecidedLazy) (GHC.Generics.Rec0 (DataSourceT DSAcq Double))) GHC.Generics.:*: (GHC.Generics.S1 (GHC.Generics.MetaSel Nothing GHC.Generics.NoSourceUnpackedness GHC.Generics.NoSourceStrictness GHC.Generics.DecidedLazy) (GHC.Generics.Rec0 (DataSourceT DSAcq Double)) GHC.Generics.:*: GHC.Generics.S1 (GHC.Generics.MetaSel Nothing GHC.Generics.NoSourceUnpackedness GHC.Generics.NoSourceStrictness GHC.Generics.DecidedLazy) (GHC.Generics.Rec0 (DataSourceT DSAcq Degree)))) GHC.Generics.:*: ((GHC.Generics.S1 (GHC.Generics.MetaSel Nothing GHC.Generics.NoSourceUnpackedness GHC.Generics.NoSourceStrictness GHC.Generics.DecidedLazy) (GHC.Generics.Rec0 (DataSourceT DSAcq Degree)) GHC.Generics.:*: GHC.Generics.S1 (GHC.Generics.MetaSel Nothing GHC.Generics.NoSourceUnpackedness GHC.Generics.NoSourceStrictness GHC.Generics.DecidedLazy) (GHC.Generics.Rec0 (DataSourceT DSAcq Degree))) GHC.Generics.:*: (GHC.Generics.S1 (GHC.Generics.MetaSel Nothing GHC.Generics.NoSourceUnpackedness GHC.Generics.NoSourceStrictness GHC.Generics.DecidedLazy) (GHC.Generics.Rec0 (DataSourceT DSAcq Degree)) GHC.Generics.:*: (GHC.Generics.S1 (GHC.Generics.MetaSel GHC.Generics.NoSourceUnpackedness GHC.Generics.NoSourceStrictness GHC.Generics.DecidedLazy) (GHC.Generics.Rec0 (DataSourceT DSAcq Degree)) GHC.Generics.:*: GHC.Generics.S1 (GHC.Generics.MetaSel Nothing GHC.Generics.NoSourceUnpackedness GHC.Generics.NoSourceStrictness GHC.Generics.DecidedLazy) (GHC.Generics.Rec0 (DataSourceT DSAcq Degree)))))) GHC.Generics.:+: GHC.Generics.C1 (GHC.Generics.MetaCons "DataSourceT'Sample'Or" GHC.Generics.PrefixI False) (GHC.Generics.S1 (GHC.Generics.MetaSel Nothing GHC.Generics.NoSourceUnpackedness GHC.Generics.NoSourceStrictness GHC.Generics.DecidedLazy) (GHC.Generics.Rec0 (DataSourceT DSAcq Sample)) GHC.Generics.:*: GHC.Generics.S1 (GHC.Generics.MetaSel Nothing GHC.Generics.NoSourceUnpackedness GHC.Generics.NoSourceStrictness GHC.Generics.DecidedLazy) (GHC.Generics.Rec0 (DataSourceT DSAcq Sample))))’ Nothing arising from a use of ‘generic'ds'Shape’ from the context: Pipes.Safe.MonadSafe m bound by the type signature for: ds'Shape :: forall (m :: * -> *). Pipes.Safe.MonadSafe m => DataSourceT DSAcq Sample -> m DataSourceShape at src/Hkl/Binoculars/Projections/Config/Sample.hs:68:3-10 • In the expression: generic'ds'Shape In an equation for ‘ds'Shape’: ds'Shape = generic'ds'Shape In the instance declaration for ‘DataSource Sample’ | 68 | ds'Shape = generic'ds'Shape | ^^^^^^^^^^^^^^^^ It works great if I have this type. instance DataSource Sample where data DataSourceT k Sample = DataSourceT'Sample (DataSourceT k Double) -- a (DataSourceT k Double) -- b (DataSourceT k Double) -- c (DataSourceT k Degree) -- alpha (DataSourceT k Degree) -- beta (DataSourceT k Degree) -- gamma (DataSourceT k Degree) -- ux (DataSourceT k Degree) -- uy (DataSourceT k Degree) -- uz deriving (Generic) ds'Shape = generic'ds'Shape but not If I need to add this other constructor | DataSourceT'Sample'Or (DataSourceT k Sample) (DataSourceT k Sample) is it connected to this ? GHC.Generics.:+: GHC.Generics.C1 There is no instance for :+: in your proposition. These `Or` Constructor are usefull with the default withDatasourcePOr of the cladd DataSource , it is a sort of fallback. try the first and if something goes wrong try the second one. withDataSourcePOr ∷ (Location l, MonadSafe m) ⇒ ScanFile l → DataSourceT DSPath a → DataSourceT DSPath a → (DataSourceT DSAcq a → m r) → m r withDataSourcePOr f l r g = withDataSourceP f l g `catch` λexl → withDataSourceP f r g `catch` λexr → throwM $ CanNotOpenDataSource'Or exl exr This is how I define the fallback when declaring the instance from this Or constructor withDataSourceP f (DataSourcePath'Sample'Or l r) g = withDataSourcePOr f l r g I don ot know if this is the right design... thanks Fred