Clojure and Emacs for Lispers

According to my e-mail archives I’ve been interested in Clojure for about three and a half years now. During that period I would occasionally spend an evening trying to pick it up, only to give up after getting stuck on some installation or configuration issue. With a little bit of pushing from Brian, and the fact that this installation and configuration is now trivial, I finally broke that losing streak last week.

I’m Damn Picky

Personally, there’s a high barrier in place to learn new programming languages. It’s entirely my own fault. I’m really picky about my development environment. If I’m going to write code in a language I need Emacs to support a comfortable workflow around it. Otherwise progress feels agonizingly sluggish. If at all possible this means live interaction with the runtime (Lisp, JavaScript). If not, then I need to be able to invoke builds and run tests from within Emacs (C, Java). Basically, I want to leave the Emacs window as infrequently possible.

I also need a major mode with decent indentation support. This tends to be the hardest part to create. Automatic indentation in Emacs is considered a black magic. Fortunately, it’s unusual to come across a language that doesn’t already have a major mode written for it. It’s only happened once for me and that’s because it was a custom language for a computer languages course. To remedy this, I ended up writing my own major mode, including in-Emacs evaluation.

Unsatisfied with JDEE, I did the same for Java, growing my own extensions to support my development for the couple of years when Java was my primary programming language. The dread of having to switch back and forth between Emacs and my browser kept me away from web development for years. That changed this past October when I wrote skewer-mode to support interactive JavaScript development. JavaScript is now one of my favorite programming languages.

I’ve wasted enough time in my life configuring and installing software. I hate sinking time into doing so without capturing that work in source control, so that I never need to spend time on that particular thing again. I don’t mean the installation itself but the configuration — the difference from the defaults. (And the better the defaults, the smaller my configuration needs to be.) With my dotfiles repository and Debian, I can go from a computer with no operating system to a fully productive development environment inside of about one hour. Almost all of that time is just waiting on Debian to install all its packages. Any new language development workflow needs to be compatible with this.

Clojure Installation

Until last year sometime the standard way to interact with Clojure from Emacs was through swank-clojure with SLIME. Well, installing SLIME itself can be a pain. Quicklisp now makes this part trivial but it’s specific to Common Lisp. This is also a conflict with Common Lisp, so I’d basically need to choose one language or the other.

SLIME doesn’t have any official stable releases. On top of this, the SWANK protocol is undocumented and subject to change at any time. As a result, SWANK backends are generally tied to a very specific version of SLIME and it’s not unusual for something to break when upgrading one or the other. I know because I wrote my own SWANK backend for BrianScheme. Thanks to Quicklisp, today this isn’t an issue for Common Lisp users, but it’s not as much help for Clojure.

The good news is that swank-clojure is now depreciated. The replacement is a similar, but entirely independent, library called nREPL. (I’d link to it but there doesn’t seem to be a website.) Additionally, there’s an excellent Emacs interface to it: nrepl.el. It’s available on MELPA, so installation is trivial.

There’s also a clojure-mode package on MELPA, so install that, too.

That covers the Emacs side of things, so what about Clojure itself? The Clojure community is a fast-moving target and the Debian packages can’t quite keep up. At the time of this writing they’re too old to use nREPL. The good news is that there’s an alternative that’s just as good, if not better: Leiningen.

Leiningen is the standard Clojure build tool and dependency manager. Here, “dependencies” includes Clojure itself. If you have Leiningen you have Clojure. Installing Leiningen is as simple as placing a single shell script in your $PATH. Since I always have ~/bin in my $PATH, all I need to do is wget/curl the script there and chmod +x it. The first time it runs it pulls down all of its own dependencies automatically. Right now the biggest downside seems to be that it’s really slow to start. I think the JVM warmup time is to blame.

Let’s review. To install a working Emacs live-interaction Clojure development environment,

With this all in place, do M-x nrepl-jack-in in Emacs and any clojure-mode buffer will be ready to evaluate code as expected. It’s wonderful.

Further Extending Emacs

I made some tweaks to further increase my comfort. Perhaps nREPL’s biggest annoyance is not focusing the error buffer, like all the other interactive modes. Once I’m done glancing at it I’ll dismiss it with q. This advice fixes that.

(defadvice nrepl-default-err-handler (after nrepl-focus-errors activate)
  "Focus the error buffer after errors, like Emacs normally does."
  (select-window (get-buffer-window "*nrepl-error*")))

I also like having expressions flash when I evaluate them. Both SLIME and Skewer do this. This uses slime-flash-region to do so when available.

(defadvice nrepl-eval-last-expression (after nrepl-flash-last activate)
  (if (fboundp 'slime-flash-region)
      (slime-flash-region (save-excursion (backward-sexp) (point)) (point))))

(defadvice nrepl-eval-expression-at-point (after nrepl-flash-at activate)
  (if (fboundp 'slime-flash-region)
      (apply #'slime-flash-region (nrepl-region-for-expression-at-point))))

For Lisp modes I use parenface to de-emphasize parenthesis. Reading Lisp is more about indentation than parenthesis. Clojure uses square brackets ([]) and curly braces ({}) heavily, so these now also get special highlighting. See my .emacs.d for that. Here’s what it looks like,

Learning Clojure

The next step is actually learning Clojure. I already know Common Lisp very well. It has a lot in common with Clojure so I didn’t want to start from a pure introductory text. More importantly, I needed to know upfront which of my pre-conceptions were wrong. This was an issue I had, and still have, with JavaScript. Nearly all the introductory texts for JavaScript are aimed at beginner programmers. It’s a lot of text for very little new information.

More good news! There’s a very thorough Clojure introductory guide that starts at a reasonable level of knowledge.

A few hours going through that while experimenting in a *clojure* scratch buffer and I was already feeling pretty comfortable. With a few months of studying the API, learning the idioms, and practicing, I expect to be a fluent speaker.

I think it’s ultimately a good thing I didn’t get into Clojure a couple of years ago. That gave me time to build up — as a sort of rite of passage — needed knowledge and experience with Java, which deliberately, through the interop, plays a significant role in Clojure.

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Chris Wellons (PGP)
~skeeto/ (view)