Creating XML with Clojure

2012-09-03 ·

In this post I show how to easily create a XML file in Clojure. We will use clojure.data.xml library. It can transform XML documents directly into and from Clojure data structures. The library is lazy which in practice means that it can operate on large XML documents, i.e. files that don't fit into memory.

Setup

Let's start by generating a new project with lein

lein new app clj-xml

clojure.data.xml replaces clojure.contrib.lazy-xml which is now deprecated. If you are using Java 1.5, you will need to put StAX inside project.clj as a dependency.

:dependencies [...
               [stax "1.2.0"]]

For Java 1.6 and higher no such change is needed.

Building XML

Open core.clj and require data.xml as xml.

(ns clj-xml.core
  (:require [clojure.data.xml :as xml]))

For the purpose of this article, we will build an XML representing a list of books. XML nodes are created using element function and then emitted on display as a string with emit-str function.

(def bookshelf
  (xml/element :books {}
               (xml/element :book {:author "Stuart Halloway"}
                            "Programming Clojure")
               (xml/element :book {:author "Christian Queinnec"}
                            "Lisp in Small Pieces")
               (xml/element :book {:author "Harold Abelson, Gerald Jay Sussman"}
                            "Structure and Interpretation of Computer Programs")))

(defn -main []
  (println (xml/emit-str bookshelf))

Once we run it, we should get

λ lein run
Compiling clj-xml.core
<?xml version="1.0" encoding="UTF-8"?><books><book author="Stuart Halloway">Programming
Clojure</book><book author="Christian Queinnec">Lisp in Small Pieces</book><book
author="Harold Abelson, Gerald Jay Sussman">Structure and Interpretation of Computer
Programs</book></books>

Let's now check if it's possible to transform emitted string back to previous Clojure representation.

(= bookshelf
  (let [input-xml (java.io.StringReader. (xml/emit-str bookshelf))]
    (xml/parse input-xml)))

It is also possible to represent XML elements with vectors (similar to Hiccup-like expressions).

(def bookshelf-as-sexp
  (xml/sexp-as-element
    [:books
     [:book {:author "Stuart Halloway"} "Programming Clojure"]
     [:book {:author "Christian Queinnec"} "Lisp in Small Pieces"]
     [:book {:author "Harold Abelson, Gerald Jay Sussman"}
       "Structure and Interpretation of Computer Programs"]]))
=> (= bookshelf bookshelf-as-sexp)
true

Writing to File

XML representations can be also directly written to a file as string using emit function.

(with-open [out-file (java.io.OutputStreamWriter.
                        (java.io.FileOutputStream. "/tmp/output.xml") "UTF-8")]
  (xml/emit bookshelf out-file))

I'm not using FileWriter here as it doesn't allow to specify file encoding. OutputStreamWriter/FileOutputSteram provide more general and flexible solution.

XML from Data Structures

Existing data structures can be easily transformed into corresponding data.xml representations. For example, let's say we have a following set of private-books:

(def private-books #{
  {:author "Stuart Halloway", :title "Programming Clojure"}
  {:author "Christian Queinnec", :title "Lisp in Small Pieces"}
  {:author "Harold Abelson, Gerald Jay Sussman",
   :title "Structure and Interpretation of Computer Programs"}})

We can easily create another bookshelf from it.

(defn bookshelf-from [data]
  (xml/element :books {}
               (reduce
                 (fn [books b]
                   (conj books (xml/element :book {:author (:author b)} (:title b))))
                 () data)))

Summary

Creating XML documents with Clojure is both easy and convenient. This article shows basics to get you started with XML in Clojure. Those two languages, however, are much more « related » that it may seem on the surface. If you are interested in that underlaying relation, be sure to check Nature of Lisp.

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