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Clojure - idiomatic refactor from recursion

June 17, 2015

I’ve become addicted to solving programming challenges that you might find at hackerrank or 4clojure. I spend most of my day job in emberjs and as much as I find myself agreeing with reactjs's take on things, I simply don’t have the energy or the will to learn another javascript framework. I also think it is bad to learn another way to do the same thing in your spare time. You need to push your brain into interesting and new avenues.

I’m using clojure to solve these programs and it is quite nice to not have to worry about persisting data or learning the latest du jour framework.

I have found myself using the same pattern over and over again when iterating a sequence that might decrease due to some condition and accumulating a value while doing this. Below was my bog standard way of doing this:

(defn process [acc sticks]
  (let [smallest (apply min sticks)
        cuts (filter #(> % 0) (map #(- % smallest) sticks))]

    (if (empty? cuts)
      (process (conj acc (count cuts)) cuts))))

The above code reduces each item in the sequence by the smallest value and then filters out any values that are zero or less (lines 2 and 3). If there are any values left, the function recursively calls itself until the sequence is empty. On each call to the function, the count of remaining elements in the sequence is added to an accumulator argument acc.

I have been using this pattern for a while now but it did not seem to me to be idiomatic clojure. I did some research and the refactoring below is a huge improvement in terms of modularity, readibility and being more idiomatic:

(defn cuts [sticks]
  (let [smallest (apply min sticks)]
    (filter pos? (map #(- % smallest) sticks))))

(defn process [sticks]
  (->> (iterate cuts sticks)
       (map count)         ;; count each sticks
       (take-while pos?))) ;; accumulate while counts are positive

The first step was to isolate in a new function the logic that performs the filtering on the sequence. I have created a cuts function on line 1. This improves the modularity.

The process function on line 5 uses the thread last macro. Both the thread macro -> and the thread last macro ->> make the code easier to read by allowing you to use an imperative style rather than nesting the calls. The thread last macro inserts the first argument that you pass to it as the last argument to each of the forms. Below are some simpler examples of the thread last macro that should illustrate how the argument fills in the last argument for each of the forms:

(->> x
;; (f x)
(->> x
;; (g (f x))
(->> x
     (f y))
;; (f y x)

If we return to our example, we can use macroexpand to expand out the macro to its nested reality:

(macroexpand '(->>
              (iterate cuts [1 2 3 4 3 3 2 1])
              (map count)
              (take-while pos?)))

Below is the output of the macroexpand call:

(take-while pos? (map count (iterate cuts [1 2 3 4 3 3 2 1])))

The first argument in this case is supplied via the iterate function on line 6 that generates an infinite lazy sequence by taking two parameters, a function and a seed value. The seed value in this case is the vector [1 2 3 4 3 3 2 1] and this is initially passed to the cuts function. iterate will then infinitely call cuts passing in as an argument, the previous call to cuts.

map is used to apply a function to each item returned from the iterate form and we use count as the function to transform the result of each call to cuts. take-while is used to constrain the infinite nature of iterate and as soon as the pos? function returns false, the results are returned to the calling function. We use pos? to check whether there are are any elements that are not zero. take-while will also accumulate the count results and return them to the calling function instead of using an accumulator argument like I used in my original example.

I really like how this turned out and it is definitely more idiomatic clojure than my original version and I think this is a good barrier to get past in my journey into clojure.

Paul Cowan

Nomadic cattle rustler and inventor of the electric lasso.
Company Website
Follow me on twitter
Contact me for frontend answers.