Let's TDD clojure.core/reduce

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Summary: clojure.core/reduce is a powerful function, yet the code for it is so simple. It's four lines! We TDD our own implementation.

A couple of weeks ago I introduced a TDD-in-the-browser system and applied it to build map. reduce is another cornerstone of functional programming. I explained how reduce works recently, so that should help understand it from the outside.

But how does it work from the inside? Let's write a version of reduce.


Define a function my-reduce. It should take three arguments. The first argument, f, is a function. The second argument is the initial value. And the third argument is a collection of values. f is a function of two arguments. my-reduce should take one value from the collection and apply f to the initial argument and the first value. The return value of f will become the new initial value, and then


(and js/window
(js-delete js/window.cljs.user "my_reduce"))
;; This is a live editor. Make changes to turn the tests above green.
(defn my-reduce [f init coll])

empty list returns init

(my-reduce + 0 [])

0 + 1

(my-reduce + 0 [1])

0 + 1 + 2

(my-reduce + 0 [1 2]){.test-expression} 3{.expected}

0 + 1 + 2 + 3

(my-reduce + 0 [1 2 3]){.test-expression} 6{.expected}

(sum (range 100))

(my-reduce + 0 (range 100)){.test-expression} 4950{.expected}

1 * 1

(my-reduce * 1 [1]){.test-expression} 1{.expected}

1 * 1 * 2

(my-reduce * 1 [1 2]){.test-expression} 2{.expected}

1 * 1 * 2 * 3

(my-reduce * 1 [1 2 3]){.test-expression} 6{.expected}


clojure.core/reduce is a very useful function. It's one of those functions I wish I had started using earlier. And building a version of reduce yourself can help you understand how the recursion works.

I hope you like it. If you liked this, you may like LispCast Introduction to Clojure. It's a video course about Clojure, starting from the ground up. It has exercises, animations, visuals, and code. Try it!