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.
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