Why aren't classes like "Num" levity polymorphic?

[Clinton Mead]

Hi All

It seems to me to be a free win just to replace:

`class Num a where`

with

`class Num (a :: (r :: RuntimeRep)) where`

And then one could define `Num` instances for unlifted types.

This would make it possible to avoid using the ugly `+#` etc syntax for
operations on unlifted types.

`Int#` and `Word#` could have `Num` instances defined just as `Int` and
`Word` already have.

I presume there's a reason why this hasn't been done, but I was wondering
why?

Thanks,
Clinton
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