Overlaps

module OverlapsFoo where

import Data.Set (Set)
import qualified Data.Set as S

newtype Foo a = Foo a
    deriving (Eq, Ord, Show)

mkFooSet :: Ord a => [a] -> Set (Foo a)
mkFooSet = S.fromList . fmap Foo

--

{-# LANGUAGE FlexibleInstances #-}

module OverlapsBar where

import Data.Monoid
import Data.Set (Set)
import qualified Data.Set as S

import OverlapsFoo

data Bar = Bar1 | Bar2 | Bar3
    deriving (Eq, Ord, Show)

instance {-# Overlaps #-} Ord (Foo Bar) where
    compare (Foo x) (Foo y) = compare y x

bar :: Set (Foo Bar)
bar = S.fromList [Foo Bar1, Foo Bar2, Foo Bar3] <> mkFooSet [Bar1, Bar2, Bar3]
-- fromList [Foo Bar3,Foo Bar1,Foo Bar2,Foo Bar3,Foo Bar1]