myapp.rb

require "sinatra"
require "partials"

helpers do
  include Sinatra::Partials
end

partials.rb

module Sinatra
  module Partials
    def partial(template, *args)
      options = args.extract_options!
      options.merge!(:layout => false)
      if collection = options.delete(:collection) then
        collection.inject([]) do |buffer, member|
          buffer << erb(template, options.merge(:layout =>
          false, :locals => {template.to_sym => member}))
      end.join("\n")
      else
        erb(template, options)
      end
    end
  end
end