Installing Clojure: Linux, Emacs, Swank - Step by Step

(This was posted in 2012. Don't count on it being up to date.)

I had to format my computer recently, so now I also have to go through the entire setup process again.

When I did it the first time, it was an epic pain in the arse, but this time I got through with barely a problem, so I decided I might want to share my fairly simple method. Also, because it annoys me when I find handwritten guides and am too agitated to check the post date for outdatedness, I am writing this post on the 21 April 2012. And minor explanations why I do certain things, because that's another thing I don't like about handwritten guides: Magic. ;P


I'm using Emacs + leiningen under Linux Mint to serve all my needs (any linux distro will do, I suppose. I'm not an expert on distros, or Linux in general, quite the opposite). Leiningen is a truly wonderful tool that is surprisingly comfortable to use, provided the command line does not scare you, of course. Anyways, leiningen provides much useful functionality for handling projects and is fairly configurable. It also, and that is the most important thing, I think, it handles dependencies for your projects. Emacs is of course the editor we'll use.

So, without further ado:

1) Get all the things you need (via "sudo apt-get install [name]"). After a fresh install of Mint 12, that was:
  a) emacs23
  b) leiningen
Any other dependencies should be handled by apt-get.

2) Install the swank plugin for leiningen.

> lein plugin install swank-clojure 1.4.2

A very useful thing. Really, very useful. Very short version: Get reasonably feature-loaded REPL within Emacs with minimal hassle. Long version: Slime. Slime is a REPL for lisps (it literally stands for "Superior Lisp Interaction Mode for Emacs"), which consists of two parts, the server and the client. This easily allows Emacs to interact with it; the client is integrated into Emacs, with it not caring where the server really is. lein-swank provides the server part, customised for Clojure.

3) Launch emacs once. I usually use emacs -nw, so it runs in the console, but some people might prefer the more interface-y version. This spawns the .emacs.d/ folder in your home directory.

4) Get clojure-mode (and paredit).

First, create and enter ~/.emacs.d/site-lisp   **
Paredit is a minor mode for Emacs that makes working with LISPs sane, and also infinitely more pleasurable. It has a few quirks you need to get used to, but saying it is "recommended" when working with a LISP (like Clojure) is like saying that breathing is "recommended" when living.
Obtain it via

> wget http://mumble.net/~campbell/emacs/paredit.el

** Small plugins for Emacs, like paredit, I usually put into a common site-lisp directory, whereas larger ones, that span multiple files, go into their own directories. You don't have to do it like that, of course, you''ll just have to adjust the load paths (will come up soon) accordingly. It's not hard.

Now, Clojure Mode. I am sad to say my git-fu failed me, and I downloaded it from github via the ZIP button and unzipped it into ~/.emacs.d/clojure-mode/. Oh well, one day I shall be capable of everything from within the command line!
Anyways, Clojure mode. Major mode for Emacs. It provides syntax highlighting, intendation, and other stuff for Clojure.

5) Configure the crap out of Emacs.

This is pretty much the final step. Go to your home directory, and open the file called .emacs - this file will be executed when Emacs is being started up and can be used to configure it.
We'll be using a few commands in Emacs Lisp, a language integrated into Emacs, here.


  a) I usually disable the menu bars and stuff. If you want, too, put:

(menu-bar-mode nil)

This is of course entirely optional, but those one or two lines of extra space are more valuable to me than those pesky menu bars. :P


  b) Add site-lisp to the load path.

(add-to-list 'load-path "~/.emacs.d/site-lisp")

That way, Emacs knows where to look when you want to load a file. Actually, there are a multitude of directories on the load path already (to check which ones, type "load-path" into the file and press C-x C-e with the curser directly after what you just typed. Just remember to remove that line afterwards :3), and you theoretically could use those, but they are shared, and it's always a good idea to keep personal configurations bound to your account - thus we use the appropriate directories in your home folder.


  c) Load paredit

(require 'paredit)

Fairly self-explanatory - we load the paredit.el file. This does not activate it yet, but it supplies the function paredit-mode, which can be called to enable (and disable) paredit. We will use it in the next steps. Emacs knows where to find paredit.el thanks to our adding the site-lisp directory to the load path, of course.


  d)  Load and configure clojure-mode

(add-to-list 'load-path "~/.emacs.d/clojure-mode")

(require 'clojure-mode)

Two fairly self-explanatory steps, again. We basically do the same as with paredit. Again, this does not yet activate clojure-mode, it merely loads the file.

(add-hook 'clojure-mode-hook

          (lambda ()

            (paredit-mode 1)))

 This is the by far trickiest part, but by no means complicated. add-hook takes two arguments, the hook (I'm not sure what exactly they are, but they are usually [or always?] called [mode-name]-hook), and a function. Then, whenever the mode is enabled, that function is called. Due to this snippet of code, whenever clojure-mode is enabled, paredit goes along with it.

Almost done.

clojure-mode gets enabled automatically when you open a .clj file, though you can manually enable it via M-x clojure-mode any time.

 Once you have clojure-mode loaded, you can use M-x clojure-jack-in to open a slime session/REPL (after typing that, wait for a few seconds - booting it up takes some times).

Actually, you could add clojure-jack-in  to the function that is hooked onto clojure-mode, so that the REPL is automatically launched, though since clojure-mode is started everytime you open a new .clj file, it tries connecting each time you do so (and succeeds, but I'm not sure if there are any side effects - better to just do it manually, or do some slightly more advanced hacking that checks whether there is a clojure repl buffer and doesn't start a new one if there is).

And a final thing about clojure-jack-in: It is intimatly bound with leiningen. It will only work within leiningen projects, which isn't much of limitation actually. Simply learn to use leiningen - it is worth it.

Aaaaand now really a final thing:

When using swank-clojure 1.4.2, I got an error message about Emacs not being able to start a swank server - it STILL WORKS for some reason. If it doesn't, or the message annoys you, use swank-clojure 1.3.3.

I hope I helped.