A Data.Map wrapper with a Semigroup and Monoid instances that delegate to the individual keys.

import qualified Data.Map as Map
import Data.Map.Append
> one = AppendMap $ Map.fromList [(1, "hello"), (2, "goodbye")]
> two = AppendMap $ Map.fromList [(1, "world"), (3, "again")]
> unAppendMap (one <> two)
fromList [(1, "helloworld"), (2, "goodbye"), (3, "again")]

Motivation

Data.Map has a Semigroup instance that keeps the values from the first argument in case of a key conflict:

import qualified Data.Map as Map
import Data.Map.Append
> Map.fromList [(1, "hello")] <> Map.fromList [(1, "goodbye")]
fromList [(1, "hello")]

A different instance has been suggested for a long time (1, 2), but this is a breaking change and hasn't happened yet (as of 2018-08).

Development

Source code is formatted with hindent then stylish-haskell.

Releasing

• Run ./bumpversion patch|minor|major. This will create a new version and push it to the repository to be released.