Example of using the Null Object Pattern for Lists

null_list.rb

class EmptyList
  def first
    nil
  end

  def rest
    self
  end

  def append(item)
    NonEmptyList.new(item, self)
  end

  def render
    "<p>No results were found.</p>"
  end

  def each
  end

end

class NonEmptyList
  def initialize(item, rest)
    @item = item
    @rest = rest
  end

  def first
    @item
  end

  def rest
    @rest
  end

  def append(item)
    NonEmptyList.new(item, self)
  end

  def each(&block)
    block.call(first)
    rest.each(&block)
  end

  def render
    items = []  # cheating here: convert to 
                # array to use its join method
    each do |item|
      items.push(
        "  <li>\n" + 
        %Q(    <a href="#{item.url}">#{item.title}</a>\n) + 
        "  </li>")
    end
    %Q(<ul>\n#{items.join("\n")}\n</ul>)
  end
end

Result = Struct.new(:title, :url)
results = EmptyList.new
  .append(Result.new("Null Object Pattern", "http://en.wikipedia.org/wiki/Null_Object_pattern"))
  .append(Result.new("Introduce Null Object", "http://refactoring.com/catalog/introduceNullObject.html"))
  .append(Result.new("Q & A: is nothing something?", "https://van.physics.illinois.edu/qa/listing.php?id=1235"))

puts "<h1>Search Results</h1>"
puts results.render
puts
puts "<h1>Search Results</h1>"
puts EmptyList.new.render

output.txt

<h1>Search Results</h1>
<ul>
  <li>
    <a href="https://van.physics.illinois.edu/qa/listing.php?id=1235">Q & A: is nothing something?</a>
  </li>
  <li>
    <a href="http://refactoring.com/catalog/introduceNullObject.html">Introduce Null Object</a>
  </li>
  <li>
    <a href="http://en.wikipedia.org/wiki/Null_Object_pattern">Null Object Pattern</a>
  </li>
</ul>

<h1>Search Results</h1>
<p>No results were found.</p>