instance_eval_and_method_missing.rb

module TagBuilder
  def method_missing(sym, *args, &block)
    properties_map = args[0]
    child_tag = 
        create_and_initialize_tag(block,sym,properties_map)
    child_tag
  end

  def p(*args, &block)
    create_and_initialize_tag(block, :p, args[0])
  end

  def create_tag(sym, properties_map)
    child_tag = Tag.new(sym, properties_map)
    child_tag
  end

  def create_and_initialize_tag(block, sym, properties_map)
    child_tag = create_tag(sym, properties_map)
    if (!block.nil?)
      child_tag.instance_eval &block
    end
    child_tag
  end
end

class Tag
  include TagBuilder
  def initialize(name, properties_map)
    @name = name
    @children = []
    @properties_map = properties_map
  end

  def create_tag(sym, properties_map)
    a_tag = super
    @children.push(a_tag)
    a_tag
  end

  def formatted_properties_map
    if(@properties_map.nil?)
      ""
    else
      " " + @properties_map.entries.map{ |anEntry|
        "#{anEntry[0]}=\"#{anEntry[1]}\""
      }.join(" ")
    end
  end

  def to_xml
    "<#{@name}#{formatted_properties_map}>
#{@children.map{ |aChild| aChild.to_xml}.join("\n")}
</#{@name}>"
  end
end

module Ejemplo
  extend TagBuilder

  tag = tagnuevoysupergrosso {
    span(:style => "color: blue") {
      div {
        p {
          b {
            sarlompa
          }
        }
      }
    }
    span
    cualquier_cosa
  }

  puts tag.to_xml #"<div><span><div><div></span></div>"
end