Clojure [1] made its first appearance in the software landscape in 2007. The functional programming language combines Lisp-like syntax with an implementation on the Java Virtual Machine (JVM). Within a few years, a Clojure community with user groups [2] and conferences (e.g., Clojure West [3] and Clojure/conj [4]) has been established.

An extensive ecosystem of libraries has emerged for the language, which is released under the Eclipse Public License. Clojure benefits from its Java origins, which lets it share the VM with Java and JRuby, and impresses with abstraction over concurrency [5] on multiprocessor systems. The name evokes the closure programming concept, with the "j" coming from Java.

Clojure advocates are enthusiastic about the improved productivity this expression-rich language permits. I strongly encourage you to take a look at an essay by Lisp advocate and entrepreneur Paul Graham [6] that describes the implications of the advantages Lisp programming confers, leading some Internet startups to focus on the language they use in large web applications. In this article, I show you how to use Clojure to implement a simple chat for the web browser (Figure 1).

Figure 1: This chat application can be implemented with Clojure and JavaScript.

Building with Leiningen

Most Clojure users rely on the Leiningen build system [7], which also takes care of installing the required libraries. If your package manager does not include it, you can follow the README file that comes with the source code for the install. The lein -v command outputs the Leiningen version; when first called, the build system takes a few moments to download a few libraries. I used version 2.3.4 for this article, along with the most recent stable release of Clojure (1.5.1), with OpenJDK 7 as the Java implementation.

The project directory for the webchat is set by the

lein new webapp

command, which also sets up a skeleton for the application. Figure 2 shows the contents of webapp: The project.clj file contains information about the project and the components it needs – at present, only the current Clojure release.

Figure 2: The Leiningen build system produces the skeleton for the application.

Additionally, the src/ directory contains the first namespace of the webapp.core application. Namespaces are the Clojure programmer's primary means of separating modules with different tasks. Each namespace is allowed to reference all objects or individual objects in other namespaces. However, it is possible to make certain objects private.

Interactive: REPL

The src/webapp/core.clj file already contains a function. It can be called at the interactive command prompt, with which Clojure developers typically work, as well as in the editor. As usual in the Lisp family, the prompt is named REPL (Read Eval Print Loop). In the terminal, the developer types CD to change to the project directory and then calls lein repl.

The following REPL session uses the require function and the in-ns function to change into webapp.core. Finally, it calls the foo function with the argument "It works!":

user=> (require 'webapp.core)
nil
user=> (in-ns 'webapp.core)
#<Namespace webapp.core>
webapp.core=> (foo "It works!")
It works! Hello, World!
nil

A simple change to the source file demonstrates how REPL responds to changed code. In Listing 1 [8], the order of arguments in the println function is reversed. After saving in the editor, reload the namespace in the same REPL to see the changed output:

webapp.core=> (require 'webapp.core :reload)
nil
webapp.core=> (foo "It works!")
Hello, World! It works!
nil

This ability of REPL allows the developer to test code changes continuously. Editors like vim or emacs can integrate a REPL.

Bundled Libraries

The Clojure world is scarcely aware of large, monolithic frameworks. Most applications use a collection of small libraries, each of which serves a clearly limited purpose. Typical Clojure web applications rely on the Ring library, which represents an abstraction of the HTTP protocol. The sample application does not directly use Ring, however, but Compojure [9], which is based on Ring. On top of this, the project.clj file defines some other libraries as dependencies (Listing 2).

Adding a new library is one of the few occasions the developer needs to restart the REPL. Although special tools can do this, entering Ctrl+D then lein repl does the trick, and Leiningen automatically downloads the required files. This is followed by more changes to the core.clj file in the editor. The following lines import functions from the Compojure library into the current namespace:

(ns webapp.core (:require [compojure.core :refer [defroutes GET]]))

This imports defroutes and GET from compojure.core and makes them available for a first simple application:

(defroutes app (GET "/" [] "<h1>Hello World</h1>"))

Here, app defines a Ring handler. It welcomes web surfers who access the / address using HTTP-GET with a friendly "Hello World".

For this to work, the application requires an HTTP server, which is supplied by Httpkit [10] and is added to the required namespace in core.clj:

(ns webapp.core (:require [compojure.core :refer [defroutes GET]][org.httpkit.server :as http]))

Now all functions can access org.httpkit.server under the http/ prefix. This means you can try out the web server in the REPL:

user=> (require 'webapp.core :reload)
nil
user=> (ns webapp.core)
nil
webapp.core=> (def srv (http/run-server app {:port 8080}))
#'webapp.core/srv

The http/run-server function starts the application; def binds to the return value to srv. The browser application is now accessible under http://localhost:8080. The function for launching the web server in turn returns a function. This can be checked in the REPL with the fn? predicate. If you call the function itself, the server shuts down:

webapp.core=> (fn? srv)
true
webapp.core=> (srv)
nil

The server can be restarted using (http/run-server app {:port 8080})) without restarting the REPL, and you can even run multiple instances on different ports.