Hi Merijn,

I have seen unionWith (<>) used many times and it is indeed a useful operation.
I also do not recall ever relying on the Monoid instance that's defined currently.
However, I never needed a newtype wrapper.
Moreover, introducing a newtype just for the sake of the right Monoid instance does not seem to be worth it.

For one I'd want to have all the other useful Map/Set functions (e.g. lookup) defined on the newtype to not have to deal with wrapping/unwrapping newtype. So for the newtype to really be useful, I guess you would have to copy all the functions. I'm guessing this will introduce some maintenance burden.

Besides you can't really add one newtype for all strict/lazy Map/IntMap. Since `unionWith` is not a typeclass member, you'd have to resort to 4 (or more) newtypes, one for each Map type. And you'd have to copy over all the functions for each newtype...

I'd be really happy to see default Monoid instances changed, but I too think that's not going to happen (at least anytime soon).
In the meantime unionWith (<>) seems reasonable enough.

Can you elaborate on how you benefited from a newtype?
Why unionWith (<>) was not enough?

Kind regards,
Nick

On Fri, 24 Mar 2017 at 14:39 Merijn Verstraaten <merijn@inconsistent.nl> wrote:
Specifically, it's to add (let's use Foo as non-bikeshed name):

newtype Foo k v = Foo (Map k v) deriving (...)

instance (Ord k, Semigroup v) => Semigroup (Foo k v) where
    Foo m1 <> Foo m2 = Foo (Map.unionWith (<>) m1 m2)

and corresponding Monoid (which would have 'mempty = Foo Map.empty'), deriving all other instances from Map. And the same for IntMap. I don't particularly care whether the Monoid instance should have a Semigroup or Monoid constraint on 'v'.

I've personally created this newtype a bunch of times (as have others in #haskell, apparently), but I've never needed/wanted the current Monoid instance. Now I realise that changing the Monoid instance is not happening, so instead I would simply like to add a newtype that has the (to me) more useful Monoid instance.

Cheers,
Merijn

> On 24 Mar 2017, at 11:59, Andreas Abel <abela@chalmers.se> wrote:
>
> Please make your proposal more specific.
> (So that we can tear it apart, haha!)
>
> Seriously, I think it is to vague for starting a discussion.
>
> Best,
> Andreas
>
> On 24.03.2017 11:36, Merijn Verstraaten wrote:
>> I would like to propose adding a newtype wrapper (with all relevant instances) for Map and IntMap (if I missed any other applicable type in containers, let me know).
>>
>> This newtype should differ in Monoid/Semigroup instance from Map/IntMap by switching:
>>
>> Ord k => Semigroup (Map k v)
>> Ord k => Monoid (Map k v)
>>
>> to:
>>
>> (Ord k, Semigroup v) => Semigroup (Map k v)
>> (Ord k, Monoid v) => Monoid (Map k v) or (Ord k, Semigroup v) => Monoid (Map k v)
>>
>> Any opinions on the overall idea? Opinions on which Monoid instance? Bikeshed for the newtype names?
>>
>> Cheers,
>> Merijn
>>
>>
>>
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>>
>
>
> --
> Andreas Abel  <><      Du bist der geliebte Mensch.
>
> Department of Computer Science and Engineering
> Chalmers and Gothenburg University, Sweden
>
> andreas.abel@gu.se
> http://www.cse.chalmers.se/~abela/

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