Programming

Clojurescript at London FunctionalJS

At the January’s London FunctionalJS meetup the technology under discussion and use was Clojurescript. There was an introduction to the language basics from Thomas Kristensen of Forward, which was really much more about the basics of the syntax. We then went into the dojo exercises: the choices were implementing the Todo list SPA (the Javascript world’s Pet Store), using Clojurescript with an existing Javascript framework people were already used to working with or doing some 4Clojure.

Everyone ended up doing the Todo list which is interesting in its own way. Clearly the SPA is seen as the benchmark for evaluating these kinds of technologies.

Most of the teams were able to get the basic Todo functionality done in terms of adding and removing things from the list and re-rendering it. Most teams seemed to abstract the rendering but most put the list management into the callback for the event.

Again I was interested to see that most people grasped the idea of an atom and were able to manipulate its value. Because that kind of stuff is second-nature to me now I was wondering if it would cause issue in terms of creating a modifying function rather than directly manipulating the value. The example in the setup functions of the dojo code using conj seemed to be straight-forward enough for everyone.

Identifying and deleting items seemed more problematic. Some people wanted to do it by index but for the most part matching the text of the todo-item seemed to be popular. Probably the sensible way to actually manage the items is to uuid the items to allow their underlying state to change away from the identity.

Laziness definitely caught people out, including myself! I’ve moaned about the fact that using map purely for side-effects in fact results in the form not executing. Despite this I fell into the trap again, however fortunately having encountered it before I could reverse into a quick doall.

Other teams imaginatively re-implemented doall using loop. Which I guess is testament to how easy it is to do things in a LISP.

One thing that was hellish in our team’s code and which I think cropped up in the other teams as well was the amount of set! we were applying to build up very low-level DOM calls. Right at the end I remembered that Google Closure was available to abstract some of that work away. However it still means that your knowledge of Clojure needs to be heavily supplemented by low-level DOM APIs as well as what is available in the Google Closure library (which is not the best known of libraries).

I was also wondering whether doto might not have cleaned up our code a lot. It’s an issue that a lot of Javascript mutable state is not easy to wrangle with things like threading macros that normally ease the pain. I’ve seen this in the WebGL dojos as well.

The final ugly issue of the evening was the project template that managed to both run on my machine and not run on my machine. The template was more complex that the standard SPA template as it used Compojure and Clojurescript (presumably using the former to serve static assets on localhost). Leiningen skeleton projects have to work and be reliable, otherwise potential adopters just get frustrated and quit.

The reactions were interesting, a guy on our team at the end asked why he would want to use Clojurescript. Good question. People who were doing things like building HTML5 games seemed to see the potential and advantage much more. This is an area I hadn’t really considered before but it does make a lot of sense as regular Clojure has already had a lot of success in implementing animation and complex state machines.

For me the alternating between high-level Clojure and low-level DOM APIs was painful. I’m going to be more interested in having wrappers that allow high-level programming consistently in a project. And I am going to be thinking about games more!

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