So why not use one? 

 

Simon


If I do

    instance (Data p) => Data (Experiment p)

then GHC does not know that the type instances for


     type instance XEOverLit (GhcPass 'Parsed     ) = PlaceHolder
     type instance XEOverLit (GhcPass 'Renamed    ) = PlaceHolder
     type instance XEOverLit (GhcPass 'Typechecked) = PlaceHolder

apply.

Or do I misunderstand your advice?

Alan