Thanks Alexander and Tran,

So I went through the whole process of defining newtype, but it was quite a long process.  My code below.

Surely it would make more sense if the HashMap monoid were defined in terms of the monoid of its value type?

In that case you could choose the monoid for the value to take the left value, which would be the equivalent of the current behaviour.

Cheers,

-John

import qualified  Control.Applicative as A

import            Data.Hashable

import qualified  Data.HashMap.Lazy as M

import            Data.Monoid

newtype HashMap k v = HashMap (M.HashMap k v)

instance (Eq k, Hashable k, Monoid v) => Monoid (HashMap k v) where

  mempty = empty

  mappend (HashMap a) (HashMap b) = HashMap (M.unionWith mappend a b)

empty :: HashMap k v

empty = HashMap M.empty

singleton :: Hashable k => k -> v -> HashMap k v

singleton k v = HashMap (M.singleton k v)

null :: HashMap k v -> Bool

null (HashMap m) = M.null m

size :: HashMap k v -> Int

size (HashMap m) = M.size $ m

member :: (Eq k, Hashable k) => k -> HashMap k a -> Bool

member k (HashMap m) = M.member k m

lookup :: (Eq k, Hashable k) => k -> HashMap k v -> Maybe v

lookup k (HashMap m) = M.lookup k m

lookupDefault :: (Eq k, Hashable k) => v -> k -> HashMap k v -> v

lookupDefault v k (HashMap m) = M.lookupDefault v k m

(!) :: (Eq k, Hashable k) => HashMap k v -> k -> v

(!) (HashMap m) k = (M.!) m k

insert :: (Eq k, Hashable k) => k -> v -> HashMap k v -> HashMap k v

insert k v (HashMap m) = HashMap (M.insert k v m)

insertWith :: (Eq k, Hashable k) => (v -> v -> v) -> k -> v -> HashMap k v -> HashMap k v

insertWith f k v (HashMap m) = HashMap (M.insertWith f k v m)

delete :: (Eq k, Hashable k) => k -> HashMap k v -> HashMap k v

delete k (HashMap m) = HashMap $ M.delete k m

adjust :: (Eq k, Hashable k) => (v -> v) -> k -> HashMap k v -> HashMap k v

adjust f k (HashMap m) = HashMap $ M.adjust f k m

union :: (Eq k, Hashable k) => HashMap k v -> HashMap k v -> HashMap k v

union (HashMap a) (HashMap b) = HashMap (M.union a b)

unionWith :: (Eq k, Hashable k) => (v -> v -> v) -> HashMap k v -> HashMap k v -> HashMap k v

unionWith f (HashMap a) (HashMap b) = HashMap (M.unionWith f a b)

unions :: (Eq k, Hashable k) => [HashMap k v] -> HashMap k v

unions ms = HashMap (M.unions [un m | m <- ms])

  where un (HashMap m) = m

map  :: (v1 -> v2) -> HashMap k v1 -> HashMap k v2

map f (HashMap m) = HashMap (M.map f m)

--mapWithKey :: (k -> v1 -> v2) -> HashMap k v1 -> HashMap k v2

--mapWithKey f (HashMap m) = HashMap (M.mapWithKey f m)

traverseWithKey :: A.Applicative f => (k -> v1 -> f v2) -> HashMap k v1 -> f (HashMap k v2)

traverseWithKey f (HashMap m) = HashMap `fmap` (M.traverseWithKey f m)

difference :: (Eq k, Hashable k) => HashMap k v -> HashMap k w -> HashMap k v

difference (HashMap a) (HashMap b) = HashMap (M.difference a b)

intersection :: (Eq k, Hashable k) => HashMap k v -> HashMap k w -> HashMap k v

intersection (HashMap a) (HashMap b) = HashMap (M.intersection a b)

intersectionWith :: (Eq k, Hashable k) => (v1 -> v2 -> v3) -> HashMap k v1 -> HashMap k v2 -> HashMap k v3

intersectionWith f (HashMap a) (HashMap b) = HashMap (M.intersectionWith f a b)

foldl' :: (a -> v -> a) -> a -> HashMap k v -> a

foldl' f v (HashMap m) = M.foldl' f v m

foldlWithKey' :: (a -> k -> v -> a) -> a -> HashMap k v -> a

foldlWithKey' f v (HashMap m) = M.foldlWithKey' f v m

foldr :: (v -> a -> a) -> a -> HashMap k v -> a

foldr f v (HashMap m) = M.foldr f v m

foldrWithKey :: (k -> v -> a -> a) -> a -> HashMap k v -> a

foldrWithKey f v (HashMap m) = M.foldrWithKey f v m

filter :: (v -> Bool) -> HashMap k v -> HashMap k v

filter f (HashMap m) = HashMap (M.filter f m)

filterWithKey :: (k -> v -> Bool) -> HashMap k v -> HashMap k v

filterWithKey f (HashMap m) = HashMap (M.filterWithKey f m)

keys :: HashMap k v -> [k]

keys (HashMap m) = M.keys m

elems :: HashMap k v -> [v]

elems (HashMap m) = M.elems m

toList :: HashMap k v -> [(k, v)]

toList (HashMap m) = M.toList m

fromList :: (Eq k, Hashable k) => [(k, v)] -> HashMap k v

fromList kvs = HashMap (M.fromList kvs)

fromListWith :: (Eq k, Hashable k) => (v -> v -> v) -> [(k, v)] -> HashMap k v

fromListWith f kvs = HashMap (M.fromListWith f kvs)

On 10 May 2014 16:07, Alexander V Vershilov <alexander.vershilov@gmail.com> wrote:

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