Re: [Haskell-cafe] Custome monoid mappend on HashMap and HashSet values

Hi, John. You can always use newtype wrapper if you need to overload existing method behavior: newtype MyHashMap a b = MyHashMap { unMy :: HashMap a b} instance Monoid (MyHasMap a b) where mempty = MyHasMap mempty mappend a b = your_overloaded_function Then just wrap and unwrap your data to do a custom mappend, also you can write a wrapper function, in case if you'll restrict types then it may work only for the types you need: (<~>) :: HashMap Int (HashMap Int Int) -> HashMap Int (HashMap Int Int) -> HashMap Int (HashMap Int Int) a <~> b = unMy $ (<>) `on` MyHashMap a b -- Alexander