How do you make a web application in Clojure?

There's also "Noir" (http://www.webnoir.org/), which is a new Clojure web framework (so new the docs aren't there yet). Coming from Django/Rails, I dig the simple, straightforward syntax and it's pretty lean.

Compojure is no longer a complete framework for developing web applications. Since the 0.4 release, compojure has been broken off into several projects.

Ring provides the foundation by abstracting away the HTTP request and response process. Ring will parse the incoming request and generate a map containing all of the parts of the request such as uri, server-name and request-method. The application will then handle the request and based on the request generate a response. A response is represented as a map containing the following keys: status, headers, and body. So a simple application would look like:

(def app [req]
  (if (= "/home" (:uri req))
    {:status 200
     :body "<h3>Welcome Home</h3>"}
    {:status 200 
     :body "<a href='/home'>Go Home!</a>"}))

One other part of Ring is the concept of middle-ware. This is code that sits between the handler and the incoming request and/or the outgoing response. Some built in middle-ware include sessions and stacktrace. The session middle-ware will add a :session key to the request map that contains all of the session info for the user making the request. If the :session key is present in the response map, it will be stored for the next request made by the current user. While the stack trace middle-ware will capture any exceptions that occur while processing the request and generate a stack trace that is sent back as the response if any exceptions do occur.

Working directly with Ring can be tedious, so Compojure is built on top of Ring abstracting away the details. The application can now be expressed in terms of routing so you can have something like this:

(defroutes my-routes
  (GET "/" [] "<h1>Hello all!</h1>")
  (GET "/user/:id" [id] (str "<h1>Hello " id "</h1>")))

Compojure is still working with the request/response maps so you can always access them if needed:

(defroutes my-routes
  (GET "*" {uri :uri} 
           {:staus 200 :body (str "The uri of the current page is: " uri)}))

In this case the {uri :uri} part accesses the :uri key in the request map and sets uri to that value.

The last component is Hiccup which makes generating the html easier. The various html tags are represented as vectors with the first element representing the tag name and the rest being the body of the tag. "<h2>A header</h2>" becomes [:h2 "A Header"]. The attributes of a tag are in an optional map. "<a href='/login'>Log In Page</a>" becomes [:a {:href "/login"} "Log In Page"]. Here is a small example using a template to generate the html.

(defn layout [title & body]
  (html
    [:head [:title title]]
    [:body [:h1.header title] body])) 

(defn say-hello [name]
  (layout "Welcome Page" [:h3 (str "Hello " name)]))

(defn hiccup-routes
  (GET "/user/:name" [name] (say-hello name)))

Here is a link to a rough draft of some documentation currently being written by the author of compojure that you might find helpful: Compojure Doc

By far the best Clojure web framework I have yet encountered is Compojure: http://github.com/weavejester/compojure/tree/master

It's small but powerful, and has beautifully elegant syntax. (It uses Jetty under the hood, but it hides the Servlet API from you unless you want it, which won't be often). Go look at the README at that URL, then download a snapshot and start playing.

Consider the Luminus web framework. I have no affiliation but have heard good things from friends I respect.