Hello, people, This is about possible extension in treating of overlapping instances. Here is a simple example. ------------------------------------------ import qualified Data.Set as Set (empty, member, insert) import List (nub) class Nub a where myNub :: a -> a instance Eq a => Nub [a] where myNub = nub instance Ord a => Nub [a] where myNub xs = nub' Set.empty xs where nub' _ [] = [] nub' set (x: xs) = if Set.member x set then nub' set xs else x: (nub' (Set.insert x set) xs) ------------------------------------------ The idea is that it is good to replace nub of the Haskell-98 library with myNub. Because: for Eq a, myNub costs O(n^2), for Ord a, myNub costs O(n*(log n)), and it is good to denote this operation with the same name. Compiling this by GHC under -fglasgow-exts -fallow-overlapping-instances -fallow-undecidable-instances yields the error report: Duplicate instance declarations: instance [overlap ok] (Eq a) => Nub [a] -- Defined at IOverlap2.hs:8:0-43 instance [overlap ok] (Ord a) => Nub [a] -- Defined at IOverlap2.hs:(9,0)-(15,71) Probably, as the context of Ord a is stronger than Eq a, then on the instance overlap, the compiler could choose the instance with the stronger context. Can there be a reasonably implemented extension of this kind? Thank you in advance for your comments, ----------------- Serge Mechveliani mechvel@botik.ru