There are some situations where we may want to use GND to derive some class methods when it's not applicable to others. For example, some people would very much like to add a join method to Monad, but doing so would prevent GND from working for Monad. Similarly, the distribute method of Data.Distributive.Distributive is incompatible with GND. In each of these cases, a perfectly valid, and efficient, way to derive the class would be to use the GND mechanism to derive the methods it works with and the default method definitions for the rest. To cover these cases, I think it would be nice to allow a method declaration to give a default definition intended to be used for GND. This may or may not be the same as the usual default. Some classes can support GND for some methods, but using defaults for the others would give poor implementations. To cover this case, I think it would be nice to add per-method GND-deriving syntax. This could look something like instance C T where deriving f g = ....