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Org-babel support for Clojure


Clojure is a dynamic LISP dialect programming language which is built on the Java Virtual Machine (and also CLR and Javascript).

org-babel-clojure allows Clojure code to be executed directly within embedded code blocks in Org-mode documents. These code blocks and their results can be included in an exported document.

The following covers details on how to implement Clojure source code blocks in Org Mode documents.


A functional org-mode Clojure code block system requires several technologies:

Instructions for Installation

The following page is a recommended source of instructions for installing a working emacs/Clojure system. These instructions assume you already have a working Java installation.

Clojure with Emacs

After successful installation using the instructions from the above linked page, the remaining task is to install the latest org-mode which includes support for CIDER. Follow the procedure described on this page to install the development version:

Download and compile the org-mode development version

Next, add this line to your .emacs file:

(add-to-list 'load-path "/full-path-to/org-mode/lisp")

The above code will load the development version of org-mode upon emacs invocation.

Configure Org-mode

Add these lines to your .emacs file to configure org-mode for Clojure code blocks. Note that emacs lisp code blocks are turned on by default in org.

(require 'org)
(require 'ob-clojure)

Configure CIDER

Add these lines to your .emacs file to configure CIDER:

(setq org-babel-clojure-backend 'cider)
(require 'cider)

Create a Clojure Project with Leiningen

Create a Leiningen project directory tree:

lein new clojure-examples

You should be able to descend into the newly create directory clojure-examples. You should be able to see a Clojure project tree. This would normally be used as an outline of a Clojure project. For the purposes of demonstrating Clojure source code in org-mode, this project is only used to allow Leiningen to deal with Java class paths and resolve dependencies. In this case, it will download the Clojure core into your machine. If other dependencies are required, they should be added to the project.clj file. Please see the Leiningen tutorial for further details. For the purposes of this demonstration, simply creating the project is sufficient to proceed.

An Introduction to Org Code Block Evaluation

This is a simple example which evaluates emacs LISP code. From within the Leiningen project directory, begin editing an org file:

emacs clojure-examples.org

Copy the following into the Org buffer:

#+BEGIN_SRC emacs-lisp
  (message "Yeah!")

Notice that a ``code block'' is delimited by the lines #+BEGIN_SRC emacs-lisp and #+END_SRC.

To see how a code block is evaluated, hit C-c C-c anywhere within the code block.

The emacs minibuffer displays the output: yeah! This is the simplest possible example. There are numerous options available for output and format of the result of code evaluation. Options are added to the #+BEGIN_SRC line. Please refer to the org-mode manual for usage of these options. Another source for information on options is this page at Worg: Header arguments and result types in Org Babel

Next, a similar process for executing code will be used with Clojure.

Connect to the REPL

To compile and run Clojure code, you will need to connect to a REPL (Read Evaluation Print Loop). To connect the current Org buffer to a REPL:

M-x cider-jack-in RET

… and wait until you get a confirmation message in the minibuffer. A second buffer (window) should open, and a CIDER REPL prompt should appear.


The following are Clojure code block examples.

Basic Functionality

Let's start really simple. We will test the evaluation of a simple Clojure form. Insert the following into the org file:

#+begin_src clojure :results silent
  (+ 1 4)

Now place the cursor in the code block and enter the command:

C-c C-c

This should evaluate the Clojure form, and echo the results: "5" in the mini-buffer.

Now let's insert the results into the buffer immediately after the Clojure code block. Insert the following into your org file:

#+begin_src clojure :results value
  [ 1 2 3 4]

Execute as before:

C-c C-c

Now, immediately following the code block, the following results block will be inserted:

[ 1 2 3 4]

The result of the last form evaluated will be inserted into the results block.

Here is another simple example, with the results of evaluation included:

#+begin_src clojure :results value
  (def small-map {:a 2 :b 4 :c 8})
  (:b small-map)

: 4

A More Complicated Example- Make a Graph and Insert It into the Document

The next example will use an interesting Clojure based library called Incanter. The code will demonstrate the creation of a basic x-y line plot using the Incanter xy-plot function. There is a preliminary step which is required to download the Incanter library into your machine. In the Leiningen project, there is a file called project.clj which must have the Incanter library dependency added to it as follows:

(defproject clojure-examples "0.1.0-SNAPSHOT"
  :description "FIXME: write description"
  :url "http://example.com/FIXME"
  :license {:name "Eclipse Public License"
            :url "http://www.eclipse.org/legal/epl-v10.html"}
  :dependencies [[org.clojure/clojure "1.5.0"]
                 [incanter "1.5.4"]])

After the project.clj file is modified, issue the command lein deps at the command line. You must have an internet connection for this to successfully download the dependencies into your local machine.

The following code block shows how the Incanter library is used to create an x-y line plot. The view function will display the plot. The plot is also saved to both PDF and PNG format image files.

#+begin_src clojure
  (use '(incanter core charts pdf))
  ;;; Create the x and y data:
  (def x-data [0.0 1.0 2.0 3.0 4.0 5.0])
  (def y-data [2.3 9.0 2.6 3.1 8.1 4.5])
  (def xy-line (xy-plot x-data y-data))
  (view xy-line)
  (save-pdf xy-line "incanter-xy-line.pdf")
  (save xy-line "incanter-xy-line.png")

To insert the image into the exported document, add this code:

#+CAPTION: A basic x-y line plot
#+NAME: fig:xy-line

Note that the file will be saved to the highest level of the Leiningen project. Depending on where you created the org file, the path to the file may have to be different than shown.

Export to LaTeX or HTML

To export to LaTeX, C-c C-e l l. To export to HTML, C-c C-e h h.

Note that the exported HTML will hyperlink the PDF file; to embed the image in the HTML, switch to the PNG image file. The exported LaTeX (.tex) file will embed either the PDF or PNG file. Graphical quality will be superior with the PDF file.

Session Evaluation

By default, each individual code block will execute by starting a Clojure process. Each code block will execute in isolation from the others. Due to the start-up time of a Java virtual machine, this can make execution of many blocks slow. The :session option will allow control of the process assigned to each code block. Please refer to the org manual for details on :session usage.

Specify namespace

By default, each individual code block will execute under CIDER connection session's current namespace. But you can specify a namespace like bellowing:

#+begin_src clojure :results pp
(ns kk.test)
(def he "hi")

: #'kk.test/he

You can see the second code block still us individual namespace.

#+begin_src clojure :results value :ns kk2
(def he "hi")

: nil#'kk2/he#namespace[kk2

Change Clojure cwd in Org-mode way

As this blog post Clojure Plotting to Org inline image in ob-clojure mentioned.

Because Clojure is based on JVM, can JVM does not allow to dynamically change current working directory (cwd). So if you use ob-clojure in Org-mode, and need to interact with path, directory, file etc. You need to figure other ways. Here is an Org-mode way, use header argument :var to pass current buffer's path into Clojure, then use this dir variable in Clojure to construct a correct path.

#+begin_src clojure :results graphics :dir "data/images" :file "ob-clojure-literate.png" :var dir=(concat (file-name-directory (buffer-file-name)) "data/images/")
(use '(incanter core stats datasets charts io pdf))
(def hist (histogram (sample-normal 1000)))
(save hist (str dir "ob-clojure-literate.png"))

Additional Examples

The above set-up and examples were intended for the beginner to achieve success with Clojure code blocks in org mode documents.

Please refer to the documentation for emacs, Clojure-mode, and CIDER which are referenced in the requirements session for details on how to enhance the system beyond the basics described in this tutorial.

More examples of Clojure code blocks can be found at these sites:

Documentation from the orgmode.org/worg/ website (either in its HTML format or in its Org format) is licensed under the GNU Free Documentation License version 1.3 or later. The code examples and css stylesheets are licensed under the GNU General Public License v3 or later.