sample.rb

helpers do
  def format
    (params[:format] || :json).to_sym
  end
  
  def render_to(format, results)
    case format
    when :json
      results.to_json
    when :xml
      results.to_xml
    end
  end
end

before do
  not_found unless params.key?(:query)
end

not_found do
  render_to(format, {:error => "Not Found", :status => 404})
end

get '/api/v1/noun.:format' do
  render_to(format, params[:query])
end