Example of using eval in a safe way to do something cool and useful.

cache_helper.rb

module CacheHelper

  # Caches the block given, and prefixes the cache key with the name of the controller and action.
  #
  # In JBuilder, the +json+ object is a local variable and to access it, we must pass it to this method.
  # Fortunately, this method requires a block, from which we can get the binding, and use that to gain
  # access to the json variable. Now we don't have to pass the json variable anymore.
  def json_cache(keys, &block)
    json = eval("json", block.send(:binding))
    json.cache! [params[:controller], params[:action]] + Array.wrap(keys), &block
  end

end