· clojure

Clojure: Creating XML document with namespaces

As I mentioned in an earlier post we’ve been parsing XML documents with the Clojure zip-filter API and the next thing we needed to do was create a new XML document containing elements which needed to be inside a namespace.

We wanted to end up with a document which looked something like this:

<mynamespace:foo xmlns:mynamespace="http://www.magicalurlfornamespace.com">

We can make use of lazy-xml/emit to output an XML string from some sort of input? by wrapping it inside with-out-str like so:

(require '[clojure.contrib.lazy-xml :as lxml])
(defn xml-string [xml-zip] (with-out-str (lxml/emit xml-zip)))

I was initially confused about how we’d be able to create a map representing name spaced elements to pass to xml-string but it turned out to be reasonably simple.

To create a non namespaced XML string we might pass xml-string the following map:

(xml-string {:tag :root :content [{:tag :foo :content [{:tag :bar :content ["baz"]}]}]})

Which gives us this:

"<?xml version=\"1.0\" encoding=\"UTF-8\"?>

Ideally I wanted to prepend :foo and :bar with ':mynamespace" but I thought that wouldn’t work since that type of syntax would be invalid in Ruby and I thought it’d be the same in Clojure.

mneedham@Administrators-MacBook-Pro-5.local ~$ irb
>> { :mynamespace:foo "bar" }
SyntaxError: compile error
(irb):1: odd number list for Hash
{ :mynamespace:foo "bar" }
(irb):1: syntax error, unexpected ':', expecting '}'
{ :mynamespace:foo "bar" }
(irb):1: syntax error, unexpected '}', expecting $end
	from (irb):1

In fact it isn’t so we can just do this:

(xml-string {:tag :root
  :content [{:tag :mynamespace:foo :attrs {:xmlns:meta "http://www.magicalurlfornamespace.com"}
              :content [{:tag :mynamespace:bar :content ["baz"]}]}]})
"<?xml version=\"1.0\" encoding=\"UTF-8\"?>
<mynamespace:foo xmlns:meta=\"http://www.magicalurlfornamespace.com">

As a refactoring step, since I had to append the namespace to a lot of tags, I was able to make use of the keyword function to do so:

(defn tag [name value] {:tag (keyword (str "mynamespace" name)) :content [value]})
> (tag :Foo "hello")
{:tag :mynamespace:Foo, :content ["hello"]}
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