| Suppose I want to extend this datatype to the one including | possibility of neither Left or Right (i. e. None). Currently I have to | use Maybe Either, so my options are: | | Just (Left a) | Just (Right b) | Nothing | | If I could extend the Either datatype I might have (syntax may be | different, this is just an example) | | data NEither a b = <Either a b> | None As Ben says, this is altogether a bigger deal. You can imagine two main approaches 1. NEither simply "macro-includes" the constructors of Either in NEither, but they are different constructors. So Either.Left and NEither.Left would be different, and incompatible constructors. This is simple, but clunky; for example, now you'd need to use a qualified name to disambiguate which you meant, whenever you used Left. 2. Use sub-typing, so that a value (Left x) is *both* in type Either and in type NEither. This opens a very large and complicated design space, as Ben mentioned. Simon