rack_lazy.rb

class HashWithLazy < Hash
  attr_reader :lazy

  def initialize(*args, &block)
    @lazy = {}
    super(*args, &block)
  end

  def [](key)
    if @lazy.key?(key)
      self[key] = @lazy.call
      @lazy.delete(key)
    end
    super(key)
  end
end

class RackLazy
  def initialize(app)
    @app = app
  end

  def call(env)
    @app.call(HashWithLazy.new.update(env))
  end
end

class XmlDocumentParser
  def initialize(app)
    @app = app
  end

  def call(env)
    if env['Content-Type'] == 'text/xml'
      env.lazy[:doc] = lambda { Nokogiri::XML.parse(env[:input]) }
    end
    @app.call(env)
  end
end