example.rb

module Tags
  def tag name, contents = nil, attrs = {}, &block
    contents, attrs = nil, contents if contents.is_a? Hash
    create_tag name, contents, attrs, &block
  end
  
  def create_tag name, contents, attrs, &block
    "<#{name} #{attrs.map { |k,v| %(#{k}="#{v}") }.join(' ') }>#{contents}</#{name}>\n"
  end
end

module Buffer
  class Base
    def initialize
      @buffer = ''
    end

    def create_tag *args, &block
      @buffer << Buffer.create_tag(*args, &block)
    end
    
    def to_s
      @buffer
    end
  end

  def create_tag name, contents, attrs, &block
    if block
      contents = contents.to_s << capture(&block).to_s
    end
    super
  end
  
  def capture &block
    buffer = Base.new
    buffer.instance_eval &block
    buffer
  end
end

include Tags
include Buffer

def menu attrs = {}, &block
  tag :ul, attrs, &block
end

def menu_item name, attrs = {}
  tag :li, tag(:a, :href => '/' + name.downcase), attrs
end

markup = menu do
  menu_item 'Users'
  menu_item 'Reports'
  menu_item 'Content'
end
puts markup