simple list

class List
  attr_reader :head, :tail

  def self.from_ar ar
    if h = ar.shift
      self.new h, self.from_ar(ar)
    end
  end

  def initialize head, tail = nil
    @head = head
    @tail = tail
  end

  def each &block
    block.call head
    if tail
      tail.each &block
    end
  end
end