[Haskell-cafe] Instances that shouldn't overlap

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