As I was preparing for my presentation on Clojure data structures I flashed back to a recent programming task I had to overcome. I don’t remember exactly what it was but it involved grouping data by particular keys and having an array of values for each key. When such a task rears its head I always have to play around in the REPL (err, the browser console) to see the exact working of each function I am about to rely on.

Which one changes the array I call it on? Which one returns the new value and leaves the original intact? What about taking two arrays to make a third one? Javascript being slightly OOish, may I possibly use the + operator to join them together?

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x = [1,2]; y = x.push(3);      // x = [1,2,3]; y = 3;
x = [1,2]; y = x.concat([3]);  // x = [1,2]; y = [1,2,3]
x = [1,2]; y = x.concat(3);    // x = [1,2]; y = [1,2,3]
x = [1,2]; y = [3]; z = x + y; // z = "1,23" WAT?

I spend a considerable amount of time trying these out and since I discover no overarching design behind how they operate I come away none-the-wiser and so repeat the same process on a slighly different problem some time later. (slice vs. splice, anyone?)

Before I bash on javascript some more, I find it important to state that it’s an enormous feat to design and implement a language in ten days, hat tip to Brendan Eich.

The point I want to get across is that not having a solid design principle behind a language is actually a hindrance. It slows you down by having to try the pieces (functions, in this case) individually unless you have memorized them.

On the other hand, when you work with a language that has a design philosophy you gain the benefit of faster development. In Clojure, in the case of collection types, that principle is immutability. You don’t have to read the docs or fiddle around in the REPL. The original never changes, you get the “modified” version in the returned value:

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(def x [1 2]) (def y (conj x 3)) ; x = [1,2]; y=[1,2,3]
(def x [1 2]) (def y (concat x [3]); x [1,2]; y=[1,2,3]

Let’s look at another language, Ruby.

Called on an array, push, concat, shift, pop and unshift all modify their caller. Push, concat and unshift return the modified array, while shift and pop return the removed element. So the rule seems to be that methods that expand the array return the new array, methods that take away an element return that element. So far so good.

What about take and drop? They both take away a number of elements, from the beginning or from the end, and the number of elements being passed in as a parameter. I’d totally expect these methods to change the original in place. After all, few names sound more destructive than drop. We’re in for a surprise. Both of them leave the original intact and return the taken (or dropped elements). In fact, you can pass in a number to both shift and pop which makes them identical to take and drop with the important distinction of whether they mutate or not the array they are called on.

To add to the confusion, there is delete which deletes all occurrences of the element that is given as the parameter and there is compact which does exactly the same for a special case, nil. (So compact takes no parameter). However, it turns out that delete is mutating, but compact is not, preferring to return the array with no nils. compact! comes to the rescue which does mutation and so x.compact! is the same x.delete(nil).

To give an analogy from natural languages, learning operations on collections in Ruby and Javascript is like learning the article for each noun in German (der, die or das). There are rules but they are hard to remember in the first place and are often violated.

Learning the same thing in Clojure is like learning articles in English: there is one true way to do it.