Add Aggregation Using merge-with as a clojure til.

README.md

@@ -28,6 +28,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 
 ### clojure
 
+- [Aggregation Using merge-with](clojure/aggregation-using-merge-with.md)
 - [Evaluate One Liners With lein-exec](clojure/evaluate-one-liners-with-lein-exec.md)
 - [Expanding Macros](clojure/expanding-macros.md)
 - [Open JavaDocs](clojure/open-javadocs.md)

clojure/aggregation-using-merge-with.md

@@ -0,0 +1,22 @@
+# 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.
+
+```clojure
+> (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:
+
+```clojure
+> (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)}
+```