Karl Kedrovsky

Clojure, Emacs, nREPL and Leiningen

UPDATE: - I had a couple of mistakes in the original post below and a thing or two has changed since I wrote it. I’ve updated the post to hopefully clear those things up.

I’ve just started learning Clojure and for whatever reason I’ve had a hard time finding the “right” way of just setting up an environment to play with the language and try things out. I think a lot of my problems stem from the fact that clojure is still very new and the speed at which things change is pretty high. Googling for solutions to my problem leads to all sorts of solutions but trying to figure out which approach is the latest/greatest/best is all but impossible. Below is the solution that I’ve found that works great for me (at least for now).


When I first (naively) approached clojure the first thing I did was create a directory with a file test.clj, stuck some clojure code in it and then tried to figure out how to run the “clojure interpreter” to execute my shiny new code. That was not a good idea. Clojure runs in the JVM and while there is a way to run a command that will read your clojure source file and run it (or to run a REPL) that’s not the way it’s typically done and it becomes a real pain as soon as you get past the simplest things. In a nutshell, here’s what you do instead:


Update Note: In the original post I had instructions here for installing clojure. As Arthur points out in the comments below you don’t need to do this and furthermore it can cause problems if you do.


I’m using leiningen 2 so I did the install manually as installing using my different package managers got me the old version. Since the leiningen command is just a bash script it’s as easy as the following.

cd ~/bin
wget https://raw.github.com/technomancy/leiningen/preview/bin/lein
chmod 755 lein

{: .language-bash }

You can put it in any directory that’s on your PATH, I just happen to like to stuff it in the bin directory of my home directory.

Now we’re finally ready to try something. To create a project and test it by creating a REPL do:

cd ~/workspace # or wherever you like to put projects
lein new testclj
cd testclj
lein repl

{: .language-bash }

This will create a project structure, download all the dependencies, start the JVM and leave you at the REPL prompt where you can try some simple clojure commands but like I said earlier it doesn’t work all that well, what we really want is to be able to start and interact with the REPL from emacs. To do that just exit out of the REPL you started above and move on to the next section.

nREPL and Emacs

The first thing you’ll want to do is add the clojure-mode and nrepl emacs packages, I’m using emacs24 and the package manager to get both. You can just look at my emacs init.el file to see how I do this, the important part is below.

(require 'package)
(add-to-list 'package-archives
         '("marmalade" . "http://marmalade-repo.org/packages/"))
(when (not package-archive-contents)
(defvar my-packages '(clojure-mode
(dolist (p my-packages)
  (when (not (package-installed-p p))
    (package-install p)))

{: .language-bash }

What this does is check the package archives (including marmalade) for all the packages in “my-packages” and installs them if they aren’t there when I start emacs.

Update Note: In the original post I had instructions at this point for adding org.clojure/tools.nrepl to the project.clj file. That doesn’t appear to be necessary to get nrepl running in emacs for the project and honestly I’m not sure it ever was.

Now run

M-x nrepl-jack-in

{: .language-bash }

That will start a new JVM and REPL, connect emacs to it and open a new emacs buffer/window (named *nrepl*) with the REPL in it. To try it out edit the ~/workspace/testclj/src/testclj/core.clj, which should look something like this.

(ns testclj.core)

(defn foo
  "I don't do a whole lot."
  (println x "Hello, World!"))

{: .language-bash }

Put your cursor at the end of the new function “foo” and do

C-x C-e

{: .language-bash }

That will send the function definition to the REPL and evaluate it. To see it in action just move your focus to the REPL window and run

(foo "bar")

{: .language-bash }

You should see something like this:

; nREPL 0.1.4-preview
user> (foo "bar")
bar Hello, World!

{: .language-bash }

At this point you’re ready to go and have an environment that’s very efficient and effective at running clojure code. There are other ways to accomplish much the same way, such as using swank in emacs to connect to a running REPL but this is a bit simpler and seems to work great for me.

Here are a few links that can give you more details about all of the above.


Hopefully this helps a couple of folks and let me know if you run into any problems with it or know of A Better Way.