Understanding XInclude

I’m often surprised when people don’t know about XInclude, but I suppose not everybody eats and breathes XML the way I do. XInclude is a way to include other files (or portions of other files) into a single XML file. We actually use them throughout GNOME documentation, though few people realize it. XInclude isn’t tied to any particular XML vocabulary like Mallard or DocBook. It’s an XML feature defined by the W3C, and you can use it in any XML file, as long as your processing tools support it.

If you’ve used SYSTEM entities in XML before, it’s important to understand a key difference. (If you haven’t, skip this paragraph.) SYSTEM entities are a pre-parse text slurp. The text of the included file are inserted, byte-for-byte, at the inclusion point, and the resultant run of characters is then parsed. With XInclude, the included file is parsed, and its infoset is merged into the inclusion point.

Basic XInclude

The simplest use of XInclude is to include the entirety of an external XML file. We use this in many of our Mallard and DocBook documents to include common legal information. In gnome-help, for example, we have a file called legal.xml that looks like this:

<license xmlns="http://projectmallard.org/1.0/">
  <p>Creative Commons Share Alike 3.0</p>

Then, in the info element of every page file, we use this:

<include href="legal.xml" xmlns="http://www.w3.org/2001/XInclude"/>

When the file is parsed, the entirety of the license element from legal.xml is inserted in place of the include element.

Text XInclude

By default, XInclude expects the included file to be well-formed XML. You can tell it to treat the file as text instead. This is useful if you want to show the text contents of a file, such as inside a Mallard code element.

Just add parse=”text” to the XInclude element, like so:

<include href="somefile.txt" parse="text"

I use this on the source pages of the tutorials on projectmallard.org. Look at the Ten Minute Tour Source page, for example. This shows the entire XML source of the Ten Minute Tour inside a Mallard code block.

The XML markup for the Ten Minute Tour Source page uses a text XInclude. The nice thing about this is that you don’t have to worry about escaping characters in the included file. So if you’re writing a lot of code examples with angle brackets, text XIncludes can be a convenient alternative to escaping or using CDATA blocks.

It’s important to note that an XInclude processor does not care what the file extension or reported MIME type of the included file is. The file is either parsed as XML or as text, and this depends solely on the parse attribute.

Parts of Documents

In the first example, we included a single standard element. You might wonder if you can include lots of boilerplate elements. If all the pages in a document share the same authors, you might want to put them all in one file and XInclude them in.

<credit><name>Shaun McCance</name></credit>
<credit><name>Phil Bull</name></credit>
<credit><name>Jim Campbell</name></credit>

If you put this into a file and try to XInclude it, you’ll get an error. Any file you XInclude (with the XML parse type) must be fully well-formed XML. Among other things, that means there must be a single root element. The example above has three root elements. So just wrap them with another element:

<info xmlns="http://projectmallard.org/1.0/">
  <credit><name>Shaun McCance</name></credit>
  <credit><name>Phil Bull</name></credit>
  <credit><name>Jim Campbell</name></credit>

Notice also that you do need the xmlns declaration to use namespaces. This will now parse, and XInclude works. But you’ll have an extra info element nested in the including document’s info element. That’s not right.

You can include only a portion of the included XML using XPointer. XPointer is a W3C syntax for pointing to pieces of documents. There are different schemes you can use with it to select data in different ways, but we’ll just stick to the xpointer() scheme, which uses XPath. The basic syntax looks like this:

<include href="credits.xml" xpointer="xpointer(/info/credit)"

This won’t work, however, because we’re using XML namespaces. You need to declare a namespace prefix and use it in your XPath. To do that, use the xmlns() XPointer scheme:

<include href="credits.xml"

This does exactly what we need. We wrap the credit elements with an info element to make the included file well-formed. (It also gives us a convenient single place to declare namespaces.) Then we select only the credit elements to XInclude with an XPath expression. I won’t go into all the details of what XPath can do, but for simple cases like this, it looks basically like a directory path.

This does require you to keep and distribute an extra file. Instead of doing that, you could keep the information in one of your page files, then XInclude portions of that file in every other page file. Since every Mallard document has an index page, you could do this for index.page:

<page  xmlns="http://projectmallard.org/1.0/">
  <credit><name>Shaun McCance</name></credit>
  <credit><name>Phil Bull</name></credit>
  <credit><name>Jim Campbell</name></credit>
<title>My Index Page</title>

Then in every page except index.page, use this in the info element:

<include href="index.page"

You might wonder if you can use XPointer to include only a portion of a file included with parse=”text”, such as a certain range of lines. XPointer allows extension schemes to be defined. In fact, there are a couple dozen schemes registered with the W3C. One of them allows you to select a string range from an XML file, although there is no registered scheme to select a range from a text file.

You don’t need a registered scheme, though. All you need is for your XML processor to understand the scheme you’re using. Unfortunately, libxml2 only supports the xpointer(), xmlns(), and element() schemes at this time. But if you really need this kind of functionality, you can probably hire an expert to implement it for you.

Mallard Glossaries

Mallard has been successful as a software help format in large part because it doesn’t include every feature under the sun. It provides a strong core language for dynamic, topic-oriented documents, and that’s what most people need most of the time. Sometimes you need some extra bells and whistles, though. So Mallard was designed to be extended, allowing you to add features without bloating the core language.

I’ve been working on a few extensions over the last few months. The one that seems to be in the most demand is the Mallard Glossaries extension. Right now, the story for glossaries is that you should use a term list on a dedicated page. And that really is enough for a simple, static list of terms. But there are disadvantages:

  • Term lists are static, and that’s not a very Mallard thing to do.
  • Term lists are manually sorted, which is a pain to begin with, but an even bigger pain for translations.
  • You can’t link to individual terms. The payload of a page is basically opaque to the linking system.
  • There’s no potential for more dynamic presentation, such as showing a short definition when you hover over a term on a topic page.

With the Glossaries extension, any page can declare a term and provide a definition in its info element. So to provide a definition for “Notifications”:

  <p><em>Notifications</em> are messages that pop up at the bottom of
  the screen, telling you that something just happened. For example,
  when someone chatting with you sends a message, a message will pop
  up to tell you. If you don't want to deal with a message right now,
  it is hidden in your messaging tray. Move your mouse to the
  bottom-right corner to see your messaging tray.</p>

This gets put in shell-notifications.page, which is the page that talks about notifications. The glossary page then collects terms from different pages and shows them, together with a link to the pages that defined them.

Since this automatically provides links to defining pages, it also serves as a sort of index. (Professional indexers might get upset with me right now. Relax, I said “sort of index”.) Pages can even declare glossary terms without providing definitions. Just don’t include any block content other than the title. Then the entry on the glossary page will link back to the right pages.

Multiple pages can even provide full definitions. The glossary page will then show all definitions, collating the links to keep them next to their definitions. Here’s a very contrived example:

Note that the first definition doesn’t have an associated link. That’s because I defined the term on the glossary page itself. There’s little point in having the glossary link to itself.

This is very basic right now. Plans and goals include:

  • Linking to individual terms from anywhere in any page
  • Showing short definitions of terms when hovering over those links
  • A tag-based selection system, so you could have glossary pages that only display a subset of the terms (e.g. symbols that were new in 3.0)
This is potent stuff, and it removes a lot of the headaches of maintaining glossaries and (sort-of) indexes in topic-oriented documents. Mallard is so far proving to be a very solid design to base extensions off of.
Creative Commons Attribution 3.0 United States
This work by Shaun McCance is licensed under a Creative Commons Attribution 3.0 United States.