Add Reductions as a clojure til.

README.md

@@ -13,6 +13,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 - [Expanding Macros](clojure/expanding-macros.md)
 - [Open JavaDocs](clojure/open-javadocs.md)
 - [Quick Clojure Docs](clojure/quick-clojure-docs.md)
+- [Reductions](clojure/reductions.md)
 - [Specify the Directory of a Shell Command](clojure/specify-the-directory-of-a-shell-command.md)
 - [Splitting On Whitespace](clojure/splitting-on-whitespace.md)
 - [Swap Two Items in a Vector](clojure/swap-two-items-in-a-vector.md)

clojure/reductions.md

@@ -0,0 +1,25 @@
+# Reductions
+
+One of the core functions in Clojure is
+[`reduce`](https://clojuredocs.org/clojure.core/reduce). It allows you to
+build up some result based on applying a function to each value in a
+collection. Clojure provides a similar function that builds up a (lazy)
+sequence of intermediate values as it is performing a reduce.
+This function is
+[`reductions`](https://clojuredocs.org/clojure.core/reductions).
+
+Using `reduce` to sum a collection of integers looks like this
+
+```clojure
+> (reduce + 0 [1 2 3 4 5])
+=> 15
+```
+
+whereas `reductions` performing the same task will look like this
+
+```clojure
+> (reductions + 0 [1 2 3 4 5])
+=> (0 1 3 6 10 15)
+```
+
+h/t Josh Davey