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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)}