A Little Clojure Trick to Brighten Your Day

På norsk på Kodemaker-bloggen.

The most prominent and central ideas in Clojure might be pure functions and immutable data, but today I write about a little thing – just a neat little feature – that puts a smile on my face.

Here is a small code snippet:

(defn add-new-zombie [zombies]
  (conj zombies (create-zombie)))

(update game :zombies add-new-zombie)

The update function takes a map game, a key :zombies in that map, and a function add-new-zombie that is used to update the value behind that key.

(conj adds a value to a list)

This add-new-zombie function doesn’t seem to carry its own weight. We prefer small building blocks over small functions.

Let’s bring the code in via an anonymous function instead:

(update game :zombies #(conj % (create-zombie)))

This might look strange if you’re not familiar with Clojure. There’s all sorts of syntax, what with hashes and percent signs.

We can forgo the shorthand and write a proper function literal instead:

(update game :zombies (fn [zombies] (conj zombies (create-zombie))))

Comparing the two, you might see that % acts as a sort of anaphoric parameter – special syntax for winning code golf tournaments. While sometimes handy, that is not the thing that brings me joy when I code.

Rather, it’s this

The usual form of update looks like this:

(update map key function)

The joyous part is update’s other form. It takes varargs and looks like this:

(update map key function args...)

These extra arguments are passed to the function. And the trick is: the value to be updated is passed in first, followed by the rest of the arguments.

So we can change this:

(update game :zombies (fn [zombies] (conj zombies (create-zombie))))

to this:

(update game :zombies conj (create-zombie))

Read: “Update the game’s zombies by adding a new zombie.”


But that’s not all

This trick also works with update-in and swap!. The latter updates Clojure’s atoms, a sort of mutable state containment facility.

We don’t need to write:

(swap! game-atom (fn [game] (update game :zombies (fn [zombies] (conj zombies (create-zombie))))))

That’s a mouthful. So many parentheses, right? We know that we can already simplify it to:

(swap! game-atom (fn [game] (update game :zombies conj (create-zombie))))

Or, we can simplify it to:

(swap! game-atom update :zombies (fn [zombies] (conj zombies (create-zombie))))

But here is the most beautiful part. Since these compose absolutely wonderfully, it all boils down to:

(swap! game-atom update :zombies conj (create-zombie))

It almost brings a tear to my eye.