Clojure data zip xml composition

Question

Here's an example to illustrate what I wanted to do:

(ns sample
  (:require [clojure.zip :as zip]
            [clojure.data.zip.xml :refer [attr text xml-> xml1->]]
            [clojure.data.xml :as xml]))

;; From https://github.com/clojure/data.zip/blob/ca5a2efcc1c865baa25f904d7d9f027809b8f738/src/test/clojure/clojure/data/zip/xml_test.clj
(def atom1 (xml/parse-str "<?xml version='1.0' encoding='UTF-8'?>
<feed xmlns='http://www.w3.org/2005/Atom'>
  <id>tag:blogger.com,1999:blog-28403206</id>
  <updated>2008-02-14T08:00:58.567-08:00</updated>
  <title type='text'>n01senet</title>
  <link rel='alternate' type='text/html' href='http://n01senet.blogspot.com/'/>
  <entry>
    <id>1</id>
    <published>2008-02-13</published>
    <title type='text'>clojure is the best lisp yet</title>
    <author><name>Chouser</name></author>
  </entry>
  <entry>
    <id>2</id>
    <published>2008-02-07</published>
    <title type='text'>experimenting with vnc</title>
    <author><name>agriffis</name></author>
  </entry>
</feed>
"))

(def atom1z (zip/xml-zip atom1))

(defn get-entries-titles [z]
  (xml-> z :entry :title text))

(defn get-entries [z]
  (xml-> z :entry))

(defn get-titles [z]
  (xml-> z :title))

(defn f1 []
  (-> atom1z get-entries-titles))

(defn f2 []
  (-> atom1z get-entries get-titles text))

Running f1 produces an expected result:

("clojure is the best lisp yet" "experimenting with vnc")                                                                                                                                    

Running f2 throws an exception:

ClassCastException clojure.lang.LazySeq cannot be cast to clojure.lang.IFn  clojure.zip/node (zip.clj:67)

My goal was to split processing into steps:

• Get the xml
• Get entries from xml
• Get title from entries

That way I can split things into separate methods. For example, I might need to have different attributes of elements that belong to different parts of the XML picked up, resulting in an output collection that is flat (eg take all the <id> elements from above atom1, resulting in a vector of IDs).

I want to have methods that process each type of node (in the above example, get the ID from feed and get the ID from entry ) and then chain them as above. Ie descend from the top, pick things from each of the levels, if needed call a method that further processes the children in the same fashion (using zippers).

To put it another way - I want to:

1. Create a zipper
2. Forward that zipper to one of processing methods
3. Move the zipper to a specific location
4. Process that location
5. Process children in the same fashion (steps 2. - 5.), using the location set in step 3

However, looks like it doesn't work that way, based on the exception in f2 . How can this be done? If this is not how one should use clojure.data.zip.xml, what would be the recommended one, having decomposition in mind?

Answer 1

I ran into the same problem. There is a very simple reason why you can't call two xml-> operators in succession. As already mentioned by Alex xml-> returns a seq. There are two answers to your question. One way to idiomatically process a tree (or XML document) is to process each level of the tree:

(map (fn [entry] (xml-> entry :title text))
 (get-entries atom1z))

If what you actually want is to combine zippers, then you would have to write a macro to construct a final zipper like the one in get-entries-titles. However, you should use a macro only if it actually helps you. Think carefully. What is it that you are missing from clojure.data.zip.xml for the purposes of processing XML?