[Haskell-cafe] Backward compatibility

[Guy]

David Thomas wrote:
> I'd also like to see these two.  It occurs to me there may be language tweak that could reduce breakage and add some
> convenience in both cases.  It would not surprise me at all if this has been thought of, examined, and discarded, but I
> don't have time to dig so I'll just lay it out quickly, and if it has been discussed before someone will probably know
> where.
>
> The idea is to allow definitions of superclass operations in subclass instances.  If there are such definitions, it's
> treated as if there were instances defined for each class (potentially a source translation, even).  I think default
> definitions of superclass operations in the subclass would be used last (that is, only for the automatic instance of the
> superclass, and only if the superclass didn't have a default for that).
>
> This should allow existing instances of Num to just work (and I think Monad too, with some care).  It would also mean if
> you're making something that's a Monad or a Num you could just lay out the one instance in all one place, reducing a bit
> of boilerplate.  At the same time, you'd have the flexibility to just use the superclasses where you just want pieces.

http://hackage.haskell.org/trac/ghc/wiki/DefaultSuperclassInstances

I'm surprised that the various superclass proposals haven't got more attention, seeing as it would allow for this kind 
of class hierarchy clean-up without breaking lots of code.