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

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

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