@hackage monoidal-containers0.6.4.0

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)