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XML Transformation with RSLT

The problem

One of the major tasks in Lonely Planet's switch to producing guidebooks from our shiny new content management system was writing an engine to convert data in our internal "bundle" format to fit templates in the ContentXML format required by our automated layout system Typefi.

This seemed like a simple enough XML-to-XML conversion problem, so we initially tackled this with XSLT, but it soon became clear that the mapping was way more complicated than one-to-one. Attempt two was with SAX, which at least allowed us to leverage Ruby fully, but as the rule set grew in complexity, our code turned into untestable if-else spaghetti that looked kind of like this:

If this tag is a heading
  immediately following the start of a section
    within the Welcome to X chapter
      within a City guide
        then output "Heading 3"
  not immediately following the start of a section
    within the Welcome to X chapter
      within a City guide
        then output "Heading 7"

We soon grew to realize that, given the scope of the problem (we're presently at 1867 rules and counting), this was not going to scale. What to do?

The solution: RSLT

Enter our saviour: RSLT. Hacked up by Dan Heath in a moment of inspiration, it reduced the monster above to this:

class CityGuide::Stylesheet
  within '.welcome-to-x' do
    within '.section' do
      render('> heading') { heading(3) }
      render('heading')   { heading(7) }

The basic idea is simple. An RSLT stylesheet consists of an ordered list of CSS matchers, followed by an action. Call stylesheet.transform(@xml) on an XML document, and it walks the document tag by tag, testing each tag against each rule until it finds a match. When a match is found, it executes the action with a handy Builder hook available for attaching your content, and then moves on to the next tag. Repeat until the source document ends.


An example is worth a thousand words:

require 'rslt'

class MyStylesheet < RSLT::Stylesheet
  def rules
    render('parent > child') { builder.p(:style => "child")  { child_content }  }
    render('parent')         { builder.p(:style => "parent") { child_content }  }
    render('text()')         { add element.to_xml.upcase }

puts MyStylesheet.transform <<-XML
  <child>Use RSLT</child>
-> <p style="parent"><p style="child">USE RSLT</p></p>

The child_content bit there simply means "keep processing my children". And since repeating that and builder.p(...) bit is not very DRY, we can wring out the moisture by abstracting them out into a paragraph method:

def paragraph(style, &block)
  @builder.p(:style => style) { child_content }

class MyStylesheet < RSLT::Stylesheet
  def rules
    render('parent > child') { paragraph('child') }
    render('parent')         { paragraph('parent') }
    render('text()')         { add element.to_xml.upcase }

Pretty elegant, no? And easily testable, too, since we can write unit tests for the individual methods, plus integration tests that check that input A converts correctly into expected output B. In this way, we quickly built up a library of reusable bits (paragraphs, headings, sections, points of interest etc), so the layout of an entire chapter could be reduced to:

within '.understand.title-page' do
  render('> section')           { section('title page')}
  render '> section > heading', :with => :intro_main_heading
  render 'heading',             :with => :title_page_heading
  render('p')                   { paragraph("List C") }

With the really magical bit being that, unless explicitly overridden, the subsequent default rules handle anything else found in the chapter.

Tricks of the trade

Since RSLT's documentation is prudent enough not to overwhelm the novice with verbosity, here are some nifty underdocumented features.

  • There are two ways for an action to call a method.
  1. Pass in a block that explicitly calls the method, as in { heading(7) }. (This requires parens around the match.)
  2. Supply in a hash like :with => :heading, :level => 7. This calls heading() and passes in the rest of the hash. (This does not require parens.)
  • You can use helper ModuleName do ... end blocks within your stylesheet to load methods from a module. Scoping can get a bit hairy though; in particular, watch out for name conflicts, as having a method of the same name in multiple modules in the same stylesheet can lead to unexpected behaviour.

  • The XML tag currently being matched is accessible as element, which is a Nokogiri::XML::Element with a huge slew of methods for digging into the tag's context and through it the whole source document.

  • To automatically process a tag's children by default, add this line into your methods:

    def sample(&block)
      block ||= lambda { |builder| child_content }
  • child_content() can be called multiple times, and it can take a CSS matcher of its own. This lets you reorder children in the output:

    def ploughs_before_hoes
  • Stylesheets can extend other stylesheets, just subclass the parent and call super at the end.

    class CityGuide::Stylesheet < Compilers::Typefi::Stylesheet
      def rules