Game Development with Clojure/ClojureScript

This weekend it’s Ludum Dare again, the world’s longest running game jam. The idea is that, alone or with a team, you build a game in a weekend based on a certain theme.

We got a little team together here in Berlin, and so I’ve been reviewing what options there are for someone wanting to build a game in Clojure or Clojurescript.

The good news is there are plenty of options, as you’ll see from the list below. You can do desktop games, browser based games with canvas or webgl, and you can even create Unity 3D games, all from your comfortable Clojure parentheses.

That’s more or less where the good news ends. Most of these frameworks and libraries are still alpha-level software, many have a “use at your own risk, you’re on your own” kind of warning on the project page. And the documentation… well… let’s just say that you’re likely better off studying cracked turtle bones to figure out how these libraries work.

Disclaimer: most of these I haven’t actually tried, certainly not in anger, so much of what follows is from what I could gather looking at the project page and perusing some code samples and (if present) docs. Feel free to “well actually” me on Twitter if you feel I’m misrepresenting things.

Desktop/mobile offerings


Platforms: desktop, web
Based on: Processing / Processing.js
Maturity: 👴
Documentation: 😊

Quil is Clojure’s version of the graphics programming toolkit Processing. It’s beginner friendly, and really lends itself to exploration, which is also why ClojureBridge chose it for the official curriculum.

It’s been around for a while and “just works”. The project site has decent API docs, there’s a wiki with useful info, and you can find plenty of examples floating around, like the ones I created last year for The Land of Quil, or the ones that are being worked on as part of the ClojureBridge Berlin Study Group Curriculum.

It’s also the only offering on this list that’s actually cross platform. You do have to keep a few small things in mind, but it’s very doable to have a single code base work for both desktop and in the browser, which is pretty cool.

The only downside is that for game development Quil is pretty limited. Its sweet spot lies more in generative art and interactive graphics. You get a bunch of drawing primitives, and some functions for mouse and keyboard input, but beyond that you’re on your own. No sprites, physics, collision detection, scenes, layers, sound, tweening, or event system. It’s still a lot of fun to play around with, but if you only have a weekend then maybe tone down your ambitions.


Platforms: desktop, Android
Based on: libGDX
Maturity: 👩
Documentation: 😊

Play-clj is Zach Oakes game framework for Clojure. There’s a big warning on the README that he’s focusing his efforts now on Play-cljs, which I’ll cover further down. Zach is the author of Nightcode, a beginner friendly editor/IDE for Clojure. He’s also created a Nightcode spin-off called Nightmod, specifically for game development using Play-clj. It calls itself “a tool for making live-moddable games”. The idea is clear: bring REPL-like interactive development to game dev.

Despite the warning Play-clj seems pretty solid. It’s the only offering in this list that explicitly mentions mobile as a target. Play-clj games use a Entity-Component System which is pretty standard in game development nowadays. It’s based on libGDX which is a feature rich game dev framework, and as such provides many features you’d expect, like sprites or physics simulation.

There’s also comprehensive documentation, not just API docs but also (bless the author) and actual step-by-step tutorial. It doesn’t seem like much to ask but it’s a rare offering in the documentation wasteland you’ll encounter further down.

Oh and Zach (I can say Zach, right?) spoke about Play-clj at Clojure/conj! Check out the talk: Making Games at Runtime With Clojure.


Platforms: desktop
Based on: Unity
Maturity: 👦
Documentation: 🤔

Arcadia is definitely the odd in one out in this list, but it’s such a cool and promising project that it wouldn’t stand not to mention it. Most readers of this blog will know that there is a JVM and a Javascript based version of Clojure, but there’s a lesser known implementation called ClojureCLR, targeting Microsoft’s Common Language Runtime, the virtual machine that powers .NET.

This is what enabled Arcadia to integrate Clojure with Unity, a bona fide A-level professional game development environment. This also means the workflow is a bit other than usual. Unity comes with its own coding environment, and to get started with Arcadia you clone the Arcadia source into your Unity project, although this might change as the project matures.

Unity is a commercial offering, it is not free software/open source, but there’s a free plan for hobby use.

Documentation is still limited, but there are a handful of wiki pages that do seem to contain all you need to get rolling.

Arcadia is development by Tim Gardner and Ramsey Nasser from Kitchen Table Coders, the same group that David Nolen is part of.

Browser based

Now we come to the web part, games that are playable directly in your browser. Note that we already mentioned Quil at the top. Since it’s cross platform it really belongs in both categories.

To evaluate these libraries it’s good to first have a look at what powers them under the hood. Whatever superpowers they have will be inherited directly from whatever javascript library is driving them.

Some of these only provide a thin wrapper for Clojurescript, meaning they don’t bring anything new to the table, but merely provide a more idiomatic syntax, avoiding lots of interop calls. Others try to add value of their own.


Platforms: browser
Based on: p5.js Maturity: 👶
Documentation: 🤔

P5.js is a reimagination of Processing.js, the library Quil is based on. It wants to “make coding accessible for artists, designers, educators, and beginners”. That’s a great goal, but just like with Quil this means that handy features specifically for game development are a bit thin on the ground.

Play-cljs seems to mostly be a thin wrapper around p5.js, although it’s possible the author plans to build more gaming features in. Note that despite the one letter difference, play-clj and play-cljs are completely different animals, with completely different feature sets and APIs.

Zach tends to do a pretty good job at documenting things, although there’s no step-by-step tutorial in this case. You do get a bunch of example games, and generated API docs, which for such a minimal library really might be enough.


Platforms: browser
Based on: Pixi.js
Maturity: 👶
Documentation: 😠

Chocolatier seems pretty impressive, it’s based on Pixi.js which prizes itself at being “the fastest, most flexible 2D WebGL renderer”. It has few hundred Github stars, and it seems a lot of thought has gone into its design. The author even did a conj talk about Functional Game Design, and the README talks a lot about its “modified entity component system”, but I’m really missing a gentler introduction on how to actually use the thing.

There’s a big example in the README that’s more or less provided without comment. There’s a documentation folder, with a single file, containing a single line: “TODO: write great documentation”.

It’s really unfortunate, I feel like if I had a day or two to sink my teeth into this, preferrably well rested and with few distractions, I could get to a point where I could have a lot of fun with it. Sigh… oh well, I guess as a creator of accessible technical content it’s good to know there’s still plenty of work to be done.


Platforms: browser
Based on: Phaser
Maturity: 👶
Documentation: 🤔

Phzr is just a thin wrapper around Phaser, one of the more popular Javascript game engines. Documentation is limited, and instead you’re simply referred to the Phaser docs. This of course is the benefit of thin wrappers, as long as the underlying library is well documented you’re all golden. So I think this actually would make a pretty good choice, although the question is if you really need a wrapper then.

re-frame + SVG

Platforms: browser
Based on: Reagent/React
Maturity: 👦
Documentation: 🤔

Not a game library as such, but still something I would consider a valid option. Browser support for SVG has improved rapidly in recent years, and React hasn’t missed the boat either. The result is that you get drawing primitives right in your DOM, which can then be styled and animated, and which generate DOM events that you can respond to.

In that sense it’s no less basic than Quil or play-cljs. You could use and React wrapper like Reagent, Om, Quiescent, or Rum, but my favorite by far is re-frame, and I believe its event system is uniquely suited for use with games.

And re-frame has some pretty decent docs. It’s a bit of a weird style, and the README is a bit of work to get through, but it does explain well how it works, and there are wiki pages explaining all the core concepts. Some even say the re-frame docs are like the hidden documentation for Reagent, the library re-frame is built on. Some Reagent concepts are only properly explained here.

You might have to be a bit more performance-conscious than with other options, but make good use of React/Reagent’s optimizations to avoid unnecessary rendering, and keep in mind that some things are cheaper to animate than others.

Finally if I would go this route I would combine it with the excellent 2D/3D geometry toolkit for Clojure/Clojurescript. The same goes for Quil or Play-cljs.

Javascript engines

Platforms: browser
Maturity: 👵
Documentation: 😃

Finally a very compelling option is to simply forget about Clojurescript specific libraries, and instead pick any of the popular Javascript game engines. There are literally dozens of high quality libraries out there. Forget about being idiomatic, just interop the shit out of it!

The benefits are obvious: you get a battle tested foundation backed by a large community, and actual documentation. Just use it as is, and wrap as you go where it makes sense.

More blog posts

Fork This Conference

Last weekend Heart of Clojure took place in Leuven, Belgium. As one of the core organizers it was extremely gratifying to see this event come to life. We started with a vision of a particular type of event we wanted to create, and I feel like we delivered on all fronts.

For an impression of what it was like you can check out Malwine’s comic summary, or Manuel’s blog post.

It seems people had a good time, and a lot of people are already asking about the next edition. However we don’t intend to make this a yearly recurring conference. We might be back in two years, maybe with another Heart of Clojure, maybe with something else. We need to think about that.

Advice to My Younger Self

When I was 16 I was visited by a man who said he had come from the future. He had traveled twenty years back to 1999 to sit down with me and have a chat.

We talked for an hour or so, and in the end he gave me a few pieces of advice. I have lived by these and they have served me well, and now I dispense this advice to you.

Become allergic to The Churn

ClojureScript logging with goog.log

This post explores goog.log, and builds an idiomatic ClojureScript wrapper, with support for cljs-devtools, cross-platform logging (by being API-compatible with Pedestal Log), and logging in production.

This deep dive into GCL’s logging functionality was inspired by work done with Nextjournal, whose support greatly helped in putting this library together.

Clojure’s standard library isn’t as “batteries included” as, say, Python. This is because Clojure and ClojureScript are hosted languages. They rely on a host platform to provide the lower level runtime functionality, which also allows them to tap into the host language’s standard library and ecosystem. That’s your batteries right there.

The Art of Tree Shaping with Clojure Zippers

This is a talk I did for the “Den of Clojure” meetup in Denver, Colorado. Enjoy!

Captions (subtitles) are available, and you can find the transcript below, as well as slides over here.

For comments and discussion please refer to this post on r/Clojure.

Test Wars: A New Hope

Yesterday was the first day for me on a new job, thanks to Clojurists Together I will be able to dedicate the coming three months to improving Kaocha, a next generation test runner for Clojure.

A number of projects applied for grants this quarter, some much more established than Kaocha. Clojurists Together has been asking people through their surveys if it would be cool to also fund “speculative” projects, and it seems people agreed.

I am extremely grateful for this opportunity. I hope to demonstrate in the coming months that Kaocha holds a lot of potential, and to deliver some of that potential in the form of a tool people love to use.

Two Years of Lambda Island, A Healthy Pace and Things to Come

It’s been just over two years since Lambda Island first launched, and just like last year I’d like to give you all an update about what’s been happening, where we are, and where things are going.

To recap: the first year was rough. I’d been self-employed for nearly a decade, but I’d always done stable contracting work, which provided a steady stream of income, and made it easy for me to unplug at the end of the day.

Lambda Island was, as the Dutch expression goes, “a different pair of sleeves”. I really underestimated what switching to a one-man product business in a niche market would mean, and within months I was struggling with symptoms of burnout, so most of year one was characterised by trying to keep things going and stay afloat financially, while looking after myself and trying to get back to a good place, physically and mentally.

D3 and ClojureScript

This is a guest post by Joanne Cheng (twitter), a freelance software engineer and visualization consultant based in Denver, Colorado. She has taught workshops and spoken at conferences about visualizing data with D3. Turns out ClojureScript and D3 are a great fit, in this post she’ll show you how to create your own visualization using the power of D3 and the elegance of ClojureScript.

I use D3.js for drawing custom data visualizations. I love using the library, but I wanted to try one of the several compile to JavaScript options, and I decided to look into ClojureScript. It ended up working out well for me, so I’m going to show you how I created a D3.js visualization using ClojureScript!

What we’re visualizing

Reloading Woes

Setting the Stage

When doing client work I put a lot of emphasis on tooling and workflow. By coaching people on their workflow, and by making sure the tooling is there to support it, a team can become many times more effective and productive.

An important part of that is having a good story for code reloading. Real world projects tend to have many dependencies and a large amount of code, making them slow to boot up, so we want to avoid having to restart the process.

The Bare Minimum, or Making Mayonnaise with Clojure

Making Mayonnaise

Imagine you have a grandfather who’s great at making mayonnaise. He’s been making mayonnaise since before the war, and the result is truly excellent. What’s more, he does this with a good old fashioned whisk. He’s kept his right arm in shape throughout decades just by beating those eggs and oil and vinegar.

Now he’s bought himself a handheld electric mixer after hearing his friend rave about hers, but after a few tries he gives up and goes back to his whisk. He says he just can’t get the same result. This seems slightly odd, so the next time you go over you ask him to show you how he uses the mixer.

Clojure Gotchas: "contains?" and Associative Collections

When learning a programming language we rarely read the reference documentation front to back. Instead someone might follow some tutorials, and look at sample code, until they’re confident enough to start a little project for practice.

From that point on the learning process is largely “just in time”, looking up exactly the things you need to perform the task at hand. As this goes on you might start to recognize some patterns, some internal logic that allows you to “intuit” how one part of the language works, based on experience with another part.

Developing this “intuition” — understanding this internal logic — is key to using a language effectively, but occasionally our intuition will be off. Some things are simply not obvious, unless someone has explained them to us. In this post I will look at something that frequently trips people up, and attempt to explain the underlying reasoning.

Dates in Clojure: Making Sense of the Mess

Update 2018-11-27: while most of this article is still relevant, I no longer recommend using JodaTime as the main date/time representation for new projects. Even existing projects that aren’t too invested in JodaTime/clj-time should consider migrating to java.time and across the board.

Update 2 2019-05-29: Also check out the talk Cross Platform DateTime Awesomeness by Henry Widd, given at Clojure/north 2019

You can always count on human culture to make programming messy. To find out if a person is a programmer just have them say “encodings” or “timezones” and watch their face.

Clojure Gotchas: Surrogate Pairs

tl;dr: both Java and JavaScript have trouble dealing with unicode characters from Supplementary Planes, like emoji 😱💣.

Today I started working on the next feature of lambdaisland/uri, URI normalization. I worked test-first, you’ll get to see how that went in the next Lambda Island episode.

One of the design goals for this library is to have 100% parity between Clojure and ClojureScript. Learn once, use anywhere. The code is all written in .cljc files, so it can be treated as either Clojure or ClojureScript. Only where necessary am I using a small amount of reader conditionals.

Simple and Happy; is Clojure dying, and what has Ruby got to do with it?

The past week or so a lot of discussion and introspection has been happening in the Clojure community. Eric Normand responded to my one year Lambda Island post with some reflections on the size and growth of the community.

And then Zack Maril lamented on Twitter: “I’m calling it, clojure’s dying more than it is growing”. This sparked a mega-thread, which was still raging four days later. A parallel discussion thread formed on Reddit. Someone asked if their were any Clojure failure stories. They were pointed at this talk from RubyConf 2016, which seemed to hit a lot of people right in the feels, and sparked a subthread with a life of its own.

Meanwhile Ray, one of the hosts of the defn podcast reacted to the original tweet: “I’m calling it: Clojure is alive and well with excellent defaults for productive and sustainable software development.” This sparked another big thread.

Loading Clojure Libraries Directly From Github

Did you ever fix a bug in an open source library, and then had to wait until the maintainer released an updated version?

It’s happened to me many times, the latest one being Toucan. I had run into a limitation, and found out that there was already an open ticket. It wasn’t a big change so I decided to dive in and address it. Just a little yak shave so I could get on with my life.

Now this pull request needs to be reviewed, and merged, and eventually be released to Clojars, but ain’t nobody got time for that stuff. No sir-ee.

Lambda Island Turns One, The Story of a Rocky Ride

One year ago to date I launched Lambda Island, a service that offers high quality video tutorials on web development with Clojure and ClojureScript. It’s been quite a ride. In this post I want to look back at the past year, provide some insight into how this experience has been for me, and give you a glimpse of what the future has in store.

This story really starts in December 2015. After three years of doing contract work for Ticketsolve I decided it was time for a change. I have been self-employed for many years, but I knew that sooner or later I wanted to try my hand at selling a product, rather than selling my time.

In January and February I took some time for soul-searching, and recharging. I went to speak at RubyConf Australia, and got to hang out with some old friends around Australia and New Zealand. Once back in Berlin I got busy creating Lambda Island.

Writing Node.js scripts with ClojureScript

In the two most recent  Lambda Island episodes I covered in-depth how to create command line utilities based on Lumo, how to combine them with third party libraries, and how to deploy them to

However there’s a different way to create tools with ClojureScript and distribute them through NPM, without relying on Lumo. In this blog post I want to quickly demostrate how to do just that.

To recap, Lumo is a ClojureScript environment based on Node.js, using bootstrapped (self-hosted) ClojureScript. This means the ClojureScript compiler, which is written in Clojure and runs on the JVM, is used to compile itself to JavaScript. This way the JVM is no longer needed, all you need is a JavaScript runtime to compile and run ClojureScript code, which in this case is provided by Node.js. On top of that Lumo uses nexe, so Lumo can be distributed as a single compact and fast executable binary.

Announcing lambdaisland/uri 1.0.0

I just released lambdaisland/uri, a pure Clojure/ClojureScript URI library. It is available on Github and Clojars.

This is a small piece of the code base that powers It’s inspired by Ruby’s Addressable::URI, the most solid URI implementation I’ve seen to date, although it only offers a small part of the functionality that library offers.

It’s written in pure Clojure/ClojureScript, with only minimal use of .cljc reader conditionals to smooth over differences in regular expression syntax, and differences in core protocols. It does not rely on any URI functionality offered by the host, such as, so it’s usable across all current and future Clojure implementations (Clojure, ClojureScript, ClojureCLR).

re-frame Subscriptions Got Even Better

Up until recently, to use re-frame subscriptions in Reagent views, you had to use a form-2 component.

A form-2 component is a function that returns another function, which does the actual rendering of the component to hiccup. In contrast, a form-1 component renders the hiccup directly.

;; form-1
(defn todo-item [todo]
   [todo-checkbox (:id todo) (:completed todo)]
   [:label {:unselectable "on"} title]
   [:button.destroy {:on-click #(dispatch [:todos/remove (:id todo)])}]])

;; form-2
(defn todo-item [todo]
  (fn [todo]
     [todo-checkbox (:id todo) (:completed todo)]
     [:label {:unselectable "on"} title]
     [:button.destroy {:on-click #(dispatch [:todos/remove (:id todo)])}]]))

Union Types with Clojure.Spec

Elm and other statically typed languages have a great feature called Union Types (also called Sum Types or Algebraic Data Types).

Here’s an example taken from Elm. Suppose your system used to represent users as integers, maybe just an auto-incrementing primary key, but then switched to UUIDs represented as strings.

To correctly model this situation, you need a way to create a type that can be either an integer or a string, that’s what union types give you.