app.rb

require "sinatra"
require "sinatra/rendering"

render_with :haml

get "/" do
  render :homepage
end

get "/some-ajax-action", :provides => "application/javascript" do
  partial :section
end

use_inline_templates!

__END__

@@layout
%body
  = yield

@@homepage
#section= partial :section
%a{ :href => "#" } Update via ajax

@@section
hi!

rendering.rb

module Sinatra
  module Rendering
    module Helpers
      def render(view, options={})
        send self.class.rendering_engine || :erb, view, options
      end

      def partial(view, options={})
        render view, { :layout => false }.merge(options)
      end
    end

    module Extensions
      attr_reader :rendering_engine

      def render_with(engine)
        @rendering_engine=engine
      end
    end
  end

  helpers  Rendering::Helpers
  register Rendering::Extensions
end