Aggregation Using merge-with
Clojure provides the merge-with function as a way of conjoining a series
of maps. You must provide merge-with a function that it can use to merge
two values for matching keys. For instance, imagine you have a bunch of maps
that contain counts for entities identified by keywords. You can consolidate
the sum of all the counts into a single map using the merge-with function
combined with the + function.
> (merge-with + {:a 1 :b 3} {:b 2 :c 3} {:c 1 :d 4})
{:a 1, :b 5, :c 4, :d 4}
For different kinds of data, a different function argument may be more
appropriate. For instance, aggregating lists instead of integers calls for
the concat function:
> (merge-with concat {:a '(1 2) :b '(3 4)} {:c '(3 4) :a '(5 4 1)})
{:a (1 2 5 4 1), :b (3 4), :c (3 4)}