Defining custom XPath functions in Nokogiri

custom_functions.rb

require 'nokogiri'
require 'singleton'

class CustomFunctions
  include Singleton

  # Implements fn:string-join of XPath 2.0
  def string_join(nodeset, separator)
    nodeset.map(&:text).join(separator)
  end

  alias_method :'string-join', :string_join

  module SetDefaultHandler
    private

    def extract_params(args)
      params = super
      params[1] ||= CustomFunctions.instance
      params
    end

    Nokogiri::XML::Node.prepend self
    Nokogiri::XML::NodeSet.prepend self
  end
end

xml = Nokogiri::XML(ARGF.read)
xml.root.add_namespace('fn', 'http://www.w3.org/2005/xpath-functions')

puts xml.xpath('normalize-space(fn:string-join(.//text(), ":"))')