A meditation on reduction

One of the people you should follow if you’re learning Clojure, and enjoy prose with a twist, is Daniel Higginbotham. He’s recently published Clojure for the Brave and True (aka “The Crazed Dwarf Riding a Warpig book”) and has a home at Brave Clojure.

In his post Do Things: a Clojure Crash Course, Daniel takes us on a journey through syntax, data structures, functions and more. In the section entitled Shorter Symmetrizer with Reduce, he offers a version of the reduce function written in Clojure.

Screen Shot 2015-10-19 at 17.49.19For those of you who are unfamiliar with what reduce is and represents, here’s a quick precis. Along with map and filter, which are often implemented together in many languages (even imperative ones), and discussed frequently together, reduce is a higher order function that operates on immutable sequences to produce a new value, which can be scalar or a more complex structure such as a map or a list.

You can play about with map, filter and reduce right now to learn more, as they’re natively implemented in JavaScript. One option would be to open up a Chrome Developer Tools console, and follow this guide: Programming in a more functional style in JavaScript – Tech Workshop Notes.

So, back to Daniel's implementation of reduce in Clojure. This is it, in its entirety:
(defn my-reduce
  ([f initial coll]
   (loop [result initial
          remaining coll]
     (if (empty? remaining)
       (recur (f result (first remaining)) (rest remaining)))))
  ([f [head & tail]]
   (my-reduce f head tail)))

Perhaps not empirically, but to me, it is a thing of beauty. Beyond the calm structure, there are so many nuggets to consume, enjoy and learn from, that if I covered them all in this post, it would be far too long to read in a coffee break (which is roughly what I have in mind when writing posts).

So I’ll leave you, dear reader, to meditate on this set of forms, and in the next post I’ll point out the nuggets that are reflecting the most golden light (to me): The use of the first-and-rest pattern, multi-arity function definition (and the typical pattern of calling itself with default arguments set) plus variadic function use, and the use of loop/recur. In an earlier version of this implementation, there was the use of destructuring too, which in itself was interesting, although it was a little overkill.