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.