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

[Merijn Verstraaten]

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
>> 
>> 
>> 
>> _______________________________________________
>> Libraries mailing list
>> Libraries at 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 at gu.se
> http://www.cse.chalmers.se/~abela/

-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 842 bytes
Desc: Message signed with OpenPGP using GPGMail
URL: <http://mail.haskell.org/pipermail/libraries/attachments/20170324/7499c069/attachment.sig>