xpath_dsl.rb

module XPath
  extend self
  
  def self.generate
    yield(Expression::Self.new)
  end
  
  class Expression
    include XPath

    def current
      self
    end
    ...
  end
  
  class Self < Expression
    def to_xpath(predicate=nil)
      '.'
    end
  end

  class Descendant < Expression
    def initialize(left, right)
      @left = wrap_xpath(left)
      @right = wrap_xpath(right)
    end

    def to_xpath(predicate=nil)
      "#{@left.to_xpath(predicate)}//#{@right.first.to_xpath(predicate)}"
    end
  end
  
  def current
    Expression::Self.new
  end

  def descendant(*expressions)
    Expression::Descendant.new(current, expressions)
  end
end

XPath.descendant('foo')                                                                   # => .//foo
XPath.descendant('foo').descendant('bar')                                                 # => .//foo//bar
XPath.descendant('foo').descendant('bar')[XPath.descendant('baz').attr('id') == 'bar']    # => .//foo//bar[.//baz/@id = 'bar']

XPath.generate { |x| x.descendant('foo').descendant('bar')[x.descendant('baz').attr('id') == 'bar']

module MyModule
  include XPath
  
  def link(locator)
    descendant(:a)[text == locator]
  end
end