This is a short followup to my first post about experimenting with the Lego EV3’s and leJOS. Shortly after I wrote that post, I went on the Clojure IRC channel and talked to Phil Hagelberg and some other helpful folks about my issues. They steered me away from attempting to install Leiningen on the EV3 itself, which was a relief since I think that way lay madness. Instead, they suggested that I have a small launcher program that would just setup a REPL. Then I could run my robot-controlling Clojure code from there.

This advice led to the creation of ev3-nrepl. It’s basically a small skeleton Clojure project to provide the basis for running Clojure code on the EV3. It’s a minimalist setup, so it should provide a good starting point for other people to continue experimenting. I tried to explain how to use it fairly clearly, as well as add all the links to the necessary documentation to get leJOS running on the EV3.

Unfortunately, despite this seemingly great start, I never ended up actually doing anything interesting with Clojure on the EV3. There are two related reasons for this. First, the robotics class I was in started having actual coding assignments (thanks to the fact that we could now run leJOS). Second, it turns out that leJOS is designed from an incredibly stateful and imperative point of view. This makes it very hard to develop clean Clojure code that makes use of their API. Specifically, leJOS makes the assumption that any code making use of it is going to start-up, do robot stuff, and then die. A Clojure REPL obviously breaks this assumption rather badly. As it turned out, this was the most painful when I was trying to figure out how to program and build my robot at the same time.

What I wanted to do was be able to open a Clojure REPL connect to it, and then leave it running for a long time. This is my normal modus operandi, but the EV3 makes it even more critical that this work. Because of the limited hardware, it takes several minutes for nrepl to actually boot up.

I think that using Clojure as a prototyping tool for building robots with the EV3 could be incredibly powerful. All of the arguments for why Clojure is a great language for exploring Java API’s are applicable to working with the EV3 API’s. Possibly even more so, since there is something really, really cool about typing some commands at the REPL and having your robot start moving around and doing stuff. Unfortunately, because of the current architecture of leJOS and the limited processing power of the EV3’s it’s not quite there yet.