Chapter 7. XML Primer

In the original days of computers, electronic text was simple. There were a few character sets like ASCII or EBCDIC, but that was about it. Text was text, and what you saw really was what you got. No frills, no formatting, no intelligence.

Inevitably, this was not enough. When text is in a machine-usable format, machines are expected to be able to use and manipulate it intelligently. Authors want to indicate that certain phrases should be emphasized, or added to a glossary, or made into hyperlinks. Filenames could be shown in a “typewriter” style font for viewing on screen, but as “italics” when printed, or any of a myriad of other options for presentation.

It was once hoped that Artificial Intelligence (AI) would make this easy. The computer would read the document and automatically identify key phrases, filenames, text that the reader should type in, examples, and more. Unfortunately, real life has not happened quite like that, and computers still require assistance before they can meaningfully process text.

More precisely, they need help identifying what is what. Consider this text:

It is easy to see which parts are filenames, which are commands to be typed in, which parts are references to manual pages, and so on. But the computer processing the document cannot. For this we need markup.

“Markup” is commonly used to describe “adding value” or “increasing cost”. The term takes on both these meanings when applied to text. Markup is additional text included in the document, distinguished from the document's content in some way, so that programs that process the document can read the markup and use it when making decisions about the document. Editors can hide the markup from the user, so the user is not distracted by it.

The extra information stored in the markup adds value to the document. Adding the markup to the document must typically be done by a person—after all, if computers could recognize the text sufficiently well to add the markup then there would be no need to add it in the first place. This increases the cost (the effort required) to create the document.

The previous example is actually represented in this document like this:

<para>To remove <filename>/tmp/foo</filename>, use &man.rm.1;.</para>

<screen>&prompt.user; <userinput>rm /tmp/foo</userinput></screen>

The markup is clearly separate from the content.

Markup languages define what the markup means and how it should be interpreted.

Of course, one markup language might not be enough. A markup language for technical documentation has very different requirements than a markup language that is intended for cookery recipes. This, in turn, would be very different from a markup language used to describe poetry. What is really needed is a first language used to write these other markup languages. A meta markup language.

This is exactly what the eXtensible Markup Language (XML) is. Many markup languages have been written in XML, including the two most used by the FDP, XHTML and DocBook.

Each language definition is more properly called a grammar, vocabulary, schema or Document Type Definition (DTD). There are various languages to specify an XML grammar, or schema.

A schema is a complete specification of all the elements that are allowed to appear, the order in which they should appear, which elements are mandatory, which are optional, and so forth. This makes it possible to write an XML parser which reads in both the schema and a document which claims to conform to the schema. The parser can then confirm whether or not all the elements required by the vocabulary are in the document in the right order, and whether there are any errors in the markup. This is normally referred to as “validating the document”.

Most contributions to the Documentation Project will be content marked up in either XHTML or DocBook, rather than alterations to the schemas. For this reason, this book will not touch on how to write a vocabulary.