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

Hi Ben, I personally already have an implementation of this I keep reusing, but then I was demonstrating a use of Monoids in #haskell-beginners and found myself again needing this, Cale also told me they have a similar implementation of Monoid for map. So I'm now aware of at least 6 implementations of exactly this instance. To me this means the current Monoid is a rather unfortunate choice (but I'm preemptively conceding the fight to fix that one). But I feel that is a stronger argument to move such a newtype into containers. Cheers, Merijn

On 24 Mar 2017, at 18:34, Ben Gamari wrote:

Merijn Verstraaten writes:

...

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?

For what it's worth, I provide precisely this in my monoidal-containers package [1]. Also, Edward Kmett provides a similar idea in his reducers package [2].

Cheers,

- Ben

[1] https://hackage.haskell.org/package/monoidal-containers [2] https://hackage.haskell.org/package/reducers-3.12.1/docs/Data-Semigroup-Unio...