MKDoc::XML::TreeBuilder(3) Builds a parsed tree from XML data


my @top_nodes = MKDoc::XML::TreeBuilder->process_data ($some_xml);


MKDoc::XML::TreeBuilder uses MKDoc::XML::Tokenizer to turn XML data into a parsed tree. Basically it smells like an XML parser, looks like an XML parser, and awfully overlaps with XML parsers.

But it's not an XML parser.

XML parsers are required to die if the XML data is not well formed. MKDoc::XML::TreeBuilder doesn't give a rip: it'll parse whatever as long as it's good enough for it to parse.

XML parsers expand entities. MKDoc::XML::TreeBuilder doesn't. At least not yet.

XML parsers generally support namespaces. MKDoc::XML::TreeBuilder doesn't - and probably won't.


This module does low level XML manipulation. It will somehow parse even broken XML and try to do something with it. Do not use it unless you know what you're doing.


my @top_nodes = MKDoc::XML::Tokenizer->process_data ($some_xml);

Returns all the top nodes of the $some_xml parsed tree.

Although the XML spec says that there can be only one top element in an XML file, you have to take two things into account:

1. Pseudo-elements such as XML declarations, processing instructions, and comments.

2. MKDoc::XML::TreeBuilder is not an XML parser, it's not its job to care about the XML specification, so having multiple top elements is just fine.

my $tokens = MKDoc::XML::Tokenizer->process_data ('/some/file.xml');

Same as MKDoc::XML::TreeBuilder->process_data ($some_xml), except that it reads $some_xml from '/some/file.xml'.

Returned parsed tree - data structure

I have tried to make MKDoc::XML::TreeBuilder look enormously like HTML::TreeBuilder. So most of this section is stolen and slightly adapted from the HTML::Element man page.


It may occur to you to wonder what exactly a ``tree'' is, and how it's represented in memory. Consider this HTML document:

  <html lang='en-US'>
      <meta name='author' content='Jojo' />
     <h1>I like potatoes!</h1>

Building a syntax tree out of it makes a tree-structure in memory that could be diagrammed as:

                     html (lang='en-US')
                      / \
                    /     \
                  /         \
                head        body
               /\               \
             /    \               \
           /        \               \
         title     meta              h1
          |       (name='author',     |
       "Stuff"    content='Jojo')    "I like potatoes"

This is the traditional way to diagram a tree, with the ``root'' at the top, and it's this kind of diagram that people have in mind when they say, for example, that ``the meta element is under the head element instead of under the body element''. (The same is also said with ``inside'' instead of ``under'' --- the use of ``inside'' makes more sense when you're looking at the HTML source.)

Another way to represent the above tree is with indenting:

  html (attributes: lang='en-US')
      meta (attributes: name='author' content='Jojo')
        "I like potatoes"

Incidentally, diagramming with indenting works much better for very large trees, and is easier for a program to generate. The $tree->dump method uses indentation just that way.

However you diagram the tree, it's stored the same in memory --- it's a network of objects, each of which has attributes like so:

  element #1:  _tag: 'html'
               _parent: none
               _content: [element #2, element #5]
               lang: 'en-US'
  element #2:  _tag: 'head'
               _parent: element #1
               _content: [element #3, element #4]
  element #3:  _tag: 'title'
               _parent: element #2
               _content: [text segment "Stuff"]
  element #4   _tag: 'meta'
               _parent: element #2
               _content: none
               name: author
               content: Jojo
  element #5   _tag: 'body'
               _parent: element #1
               _content: [element #6]
  element #6   _tag: 'h1'
               _parent: element #5
               _content: [text segment "I like potatoes"]

The ``treeness'' of the tree-structure that these elements comprise is not an aspect of any particular object, but is emergent from the relatedness attributes (_parent and _content) of these element-objects and from how you use them to get from element to element.


This is pretty much the kind of data structure MKDoc::XML::TreeBuilder returns. More information on different nodes and their type is available in MKDoc::XML::Token.


Did I mention that MKDoc::XML::TreeBuilder is NOT an XML parser?


Copyright 2003 - MKDoc Holdings Ltd.

Author: Jean-Michel Hiver

This module is free software and is distributed under the same license as Perl itself. Use it at your own risk.