xml2.rb

class XMLGrammar
  def initialize(out)
    @out = out
  end

  def self.generate(out, &block)
    new(out).instance_eval(&block)
  end

  def self.element(tagname, attributes={})
    @allowed_attributes ||= {}
    @allowed_attributes[tagname] = attributes
    class_eval %Q{
      def #{tagname}(attributes={}, &block)
        tag(:#{tagname},attributes,&block)
      end
    }
  end

  OPT = :opt
  REQ = :req
  BOOL = :bool

  def self.allowed_attributes
    @allowed_attributes
  end

  def content(text)
    @out << text.to_s
    nil
  end

  def comment(text)
    @out << "<!-- #{text} -->"
    nil
  end

  def tag(tagname, attributes={})
    @out << "<#{tagname}"
    allowed = self.class.allowed_attributes[tagname]
    attributes.each_pair do |key,value|
      raise "unknown attribute: #{key}" unless allowed.include? key
      @out << " #{key}='#{value}'"
    end
    allowed.each_pair do |key,value|
      next if attributes.has_key? key
      if value == REQ
        raise "required attribute '#{key}' missing in <#{tagname}>"
      elsif value.is_a? String
        @out << " #{key}='#{value}'"
      end
    end
    if block_given?
      @out << "\n"
      content = yield
      if content
        @out << content.to_s
      end
      @out << "</#{tagname}>" 
    else
      @out << "/>\n"
    end
    nil
  end
end


class HTMLForm < XMLGrammar
  element :form,
    :action => REQ,
    :method => "GET",
    :enctype => "application/x-www-form-urlencoded",
    :name => OPT

  element :input,
    :type => "text",
    :name => OPT,
    :value => OPT,
    :maxlength => OPT,
    :size => OPT,
    :src => OPT,
    :checked => BOOL,
    :disabled => BOOL,
    :readonly => BOOL

  element :textarea,
    :rows => REQ,
    :cols => REQ,
    :name => OPT,
    :disabled => BOOL,
    :readonly => BOOL

  element :button,
    :name => OPT,
    :value => OPT,
    :type => "submit",
    :disabled => OPT
end

puts '--------------------'
p XMLGrammar.instance_variables
p HTMLForm.instance_variables
puts '--------------------'








HTMLForm.generate(STDOUT) do
  comment "This is a simple HTML form"
  form :name => "registration", :action => "http://www.example.com/register.cgi" do
    content "Name:"
    input :name => "name"
    content "Address:"
    textarea :name => "address", :rows=>6, :cols=>40 do
      "Please enter your mailing address here"
    end
    button do
      "Submit"
    end
  end
end