yaml_hash.rb

require 'yaml'

class YAMLHash
  
  def reload
    raise 'File not found' if !File.exists?(@filename)
    @data = YAML::load(File.open(@filename))
  end

  def initialize(*filename)
    if filename.size==1
      @filename = filename[0]
      reload
    else
      @data = Hash.new
    end
  end
  
  def save(filename)
    @filename = filename if filename
    File.open(@filename, 'w+') { |out| YAML::dump(@data, out) } 
  end

  def method_missing(m, *args)
    case args.size
      when 0
        return @data[m.to_s]
      when 1
        m = m.to_s.chop if m[-1]=='='      
        @data[m] = args[0]
      else
        raise "Too many arguments provided to #{self.class}."
    end
  end
end

square=YAMLHash.new()
square.length = 5
square.width = 5
square.save('square.yaml')
puts "The area of the square is #{square.length * square.width}"

rectangle=YAMLHash.new('square.yaml')
rectangle.length = 10
puts "The area of the rectangle is #{rectangle.length * rectangle.width}"