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

[Nickolay Kudasov]

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 at 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 at 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 at gu.se
> > http://www.cse.chalmers.se/~abela/
>
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