Paul Johnson wrote:
> class (Eq a) => AppEq f a where
> instance (Applicative f, Eq a) => AppEq f a where
> instance (Ord a) => AppEq Interval a where
In Haskell, instances are selected based solely on the types in the
head. Constraints like `Applicative f' are not consulted when the
instance is being selected. Rather, constraints are checked after the
instance has been selected. The above two instances are overlapping
since Interval is a particular case of `f'.
That said, it is possible to select an instance based on
constraints. The approach is described at
http://haskell.org/haskellwiki/GHC/AdvancedOverlap
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