csv.rb

module ActAsCsv
  def self.included(base)
    base.extend ClassMethods
  end

  module ClassMethods
    def act_as_csv
      include InstanceMethods
    end
  end

  module InstanceMethods
    def read
      @csv_contents = []
      filename = self.class.to_s.downcase + '.txt'
      file = File.new(filename)
      @headers = file.gets.chomp.split(', ')

      file.each do |row|
        @csv_contents << CsvRow.new(self, row.chomp.split(', '))
      end
    end

    def each
      @csv_contents.each {|row| yield row}
    end


    attr_accessor :headers, :csv_contents

    def initialize
      read
    end
  end

  class CsvRow
    def method_missing(name, *args)
      @row[@instance.headers.index(name.to_s)]
    end

    def initialize(instance, row)
      @instance = instance
      @row = row
    end
  end
end

class RubyCsv
  include ActAsCsv
  act_as_csv
end

csv = RubyCsv.new
csv.each {|row| puts row.one}

grep.rb

while line = ARGF.gets
  puts line if line =~ /ARGF/
end

tree.rb

class Tree
  attr_accessor :children, :node_name

  def initialize(name, hash)
    @node_name = name
    @children = hash.map do |name, h|
      Tree.new(name, h)
    end
  end

  def visit_all(&block)
    visit &block
    children.each {|c| c.visit_all &block}
  end

  def visit(&block)
    block.call self
  end
end

ruby_tree = Tree.new('grandpa', {'dad' => {'child 1' => [], 'child 2' => []},
                                 'uncle' => {'child 3' => [], 'child 4' => []}})
puts "Visiting a node"
ruby_tree.visit {|node| puts node.node_name}
puts

puts "Visiting entire tree"
ruby_tree.visit_all {|node| puts node.node_name}