Implementing Linked List with structs

linked_list.rb

Node = Struct.new(:val, :next_node) do

  def length
    return 1 if next_node.nil?
    1 + next_node.length
  end

  def to_s
    self.to_a.join(", ")
  end

  def to_a
    return [val] if next_node.nil?
    [val] + self.next_node.to_a
  end

  def add value
    if self.next_node.nil?
      self.next_node = Node.new(value, nil)
    else
      self.next_node.add value
    end
  end

  def index_of value, i = 0
    return i if val == value
    return -1 if next_node.nil?
    next_node.index_of value, i + 1
  end

end

n1 = Node.new(1, nil) # => #<struct Node val=1, next_node=nil>
n1.to_a # => [1]
n1.add(2) # => #<struct Node val=2, next_node=nil>
n1.to_a # => [1, 2]
n1.add(3)
n1.to_a # => [1, 2, 3]
n1.add("foo")
n1.add("bar")
n1.to_a # => [1, 2, 3, "foo", "bar"]
n1.length  # => 5
n1.index_of "foo" # => 3
n1.to_s # => "1, 2, 3, foo, bar"
n1.index_of 4 # => -1