Processing XML With Clojure

Although XML is nice in theory, I have always hated dealing with it. It requires so much boiler plate code just to parse or create a simple XML file. Recently I needed to do some XML processing and I still can't believe how easy it was to create and parse XML in clojure.

Clojure contrib includes a library for creating XML called prxml. Vectors become XML tags. Such as,

(prxml [:p {:class "greet"} [:i "Ladies & gentlemen"]])
; => <p class="greet"><i>Ladies &amp; gentlemen</i></p>

First let's define some data to turn in to XML.

(def data #{{:title "Clojure" :link "http://clojure.org"  
	     :description "Clojure Homepage"}

	    {:title "Java"    :link "http://java.sun.com" 
	     :description "JVM Homepage"}

	    {:title "Debian"  :link "http://debian.org"   
	     :description "Debian Homepage"}})

By default prxml function outputs to the screen if you want to output to a string use prxml in combination with with-out-str.

(defn articles []
  (reduce 
   (fn [feed v]
     (conj feed 
	   [:item 
	    [:title (:title v)] 
	    [:url (:url v)] 
	    [:description (:description v)]]))
   () data))

We build a list of vectors for every node in the XML.

[:item 
  [:title "Clojure"] 
  [:url "clojure.org"] 
  [:description "Clojure Homepage"]]

[:item 
  [:title "Java"] 
  [:url "java.sun.com"] 
  [:description "JVM Homepage"]]

If you wrap everything, it takes less than 20 lines of code to produce an RSS feed.

(defn xml-data []
  (with-out-str
   (prxml [:decl! {:version "1.0"}] 
	  [:rss {:version "2.0"} 
	   [:channel 
	    [:title "The Site"]
	    [:link "http://site.com"]
	    [:description "The Site"]
	    (articles)]])))

Parsing XML is even easier, clojure core has built in support for XML processing. clojure.xml/parse can take a File, InputStream or String naming a URI and return a tree of the xml/element struct-map. You can then treat it like any other sequence.

Such as to iterate over all the titles in the XML file,

(let [input-stream (ByteArrayInputStream. (.getBytes (xml-data) "UTF-8"))]
  (for [x (xml-seq (parse input-stream))
	:when (= :title (:tag x))]
    (:content x)))

;;rss=> (["site-title"] ["Clojure"] ["Java"] ["Debian"])