「Elem b ~ a」は「Elem bとaは同じ型である」って意味ですね
ただまあ832の例は関数従属と多引数型クラスでも複雑にはならない

{-# LANGUAGE FlexibleInstances, MultiParamTypeClasses, FunctionalDependencies #-}
module Mult where
import Prelude hiding (filter)
import qualified Prelude as P
import qualified Data.List as L

class FilterContext a b | b -> a where
　　filter :: (a -> Bool) -> [a] -> b

instance FilterContext a [a] where
　　filter = P.filter

instance FilterContext a ([a], [a]) where
　　filter = L.partition

*> filter (> 5) [1..10] :: [Int]
[6,7,8,9,10]
*> filter (> 5) [1..10] :: ([Int], [Int])
([6,7,8,9,10],[1,2,3,4,5])