Containers with merging via monoidal accumulation. The Monoid instances
provided by the "containers" and "unordered-containers" packages merge
structures in a left-biased manner instead of using the underlying monoidal
structure of the value.

This package wraps the types provided by these packages, but provides
Monoid instances implemented in terms of the value type's mappend. For
instance, the Monoid Map instance looks like,

  instance (Ord k, Semigroup a) => Monoid (MonoidalMap k a)

Homepage:
https://hackage.haskell.org/package/monoidal-containers
