RULES pragma with class constraint

[John Meacham]

Is it possible to create a RULES that fires only if a type has a given
class constraint? something like:

> snub :: Ord a => [a] -> [a]
> snub xs = f Set.empty xs where
>     f _ [] = []
>     f (x:xs) set
>         | x `Set.member` set = f xs
>         | otherwise = x:f xs (Set.insert x set)
>
>
> {-# RULES "nub/snub" Ord a => forall a xs .  nub (xs::[a]) = snub xs #-}

Such a rule would probably only be able to fire on monotypes with known
instances since otherwise the dictionary won't necessarily be available.
but it seems like it might be a useful feature even with that
restriction.

        John

--
John Meacham - ⑆repetae.net⑆john⑈