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

[Merijn Verstraaten]

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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