Simon Peyton-Jones wrote:

I was about to suggest almost exactly the same. In particular, John's proposal could be decomposed into three parts:

1. Allow instance declarations to define methods of superclasses. These are simply converted into the appropriate instance declarations for the superclasses.

2. Allow class declarations to give defaults for methods in superclasses. Together with (1) they are used in the obvious way.

3. Allow empty instance declarations to be implicitly generated.

As a nice side effect, (1) and (2) together would allow us to cleanly get rid of the fmap/liftM annoyance:

*> class Functor f where { fmap :: ... } *> class Functor m => Monad m where { fmap = liftM }

If the instance for every type were allowed, Foo and Bar would be indistinguishable from true synonyms. Further, if classes with no methods have no use currently, this "universal instance" could be compiler generated whenever a class without methods is declared. Or the empty class may be treated as a synonym, if that's simpler. Does this make any sense?