no.rb

class FooController < ApplicationController
  def an_action
    do_thing
  end

  private

  def do_thing
    @side_effect = a_value
  end
end

yes.rb

class FooController < ApplicationController
  def an_action
    @side_effect = do_thing
  end
  
  private
  
  def do_thing
    return a_value
  end
end