Meta-point: backward compatibility

[Ross Paterson]

On Thu, Apr 24, 2008 at 08:18:10PM +0000, Aaron Denney wrote:
> And there is a lot that clearly isn't battle tested in a reasonable new
> form, though the current practice is widely agreed upon to be broken.
> Examples include all monads having fail, rather than only those in a
> subclass, monad not being a subclass of functor, and the whole numeric
> hierarchy issue (which I don't think can be properly designed unless we
> know whether it's going to be FDs or ATs, though, of course, designing
> it for either would provide valuable experience for the limitations
> of both),

I don't think any of these need involve multi-parameter type classes
(we don't need vector spaces in the Prelude), but they're often presented
as use cases for things like class aliases.  Even then, the numeric
hierarchy is probably easier to fix that the others: one can define a
finer-grain hierarchy with the existing classes as a facade, and leave
clients untouched.  The change would be felt by the minority defining
instances of numeric classes, but they are exactly the people who find
the present hierarchy inadequate.