Re: PROPOSAL: Add Map/IntMap newtypes for different Monoid to containers

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 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 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

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

-- 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/

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries