From: Matthias Reitinger <reitinge+usenet@...>
Date: 2009-07-20T12:05:07+09:00
Subject: Re: [QUIZ] Prototype-Based Inheritance (#214)

Daniel Moore wrote:
> ## Prototype-Based Inheritance (#214)

I decided to give it a shot:

  require 'ostruct'
  
  class Prototype < OpenStruct
    attr_reader :prototype
  
    def prototype=(new_prototype)
      if new_prototype.prototypes.include?(self)
        raise ArgumentError, "circular prototype chain"
      end
      @prototype = new_prototype
    end
  
    def prototypes
      if @prototype.nil?
        []
      else
        [@prototype] + @prototype.prototypes
      end
    end
  
    def delete_slot(name)
      name = name.to_sym
      if @table.has_key?(name)
        meta = class << self; self; end
        meta.send(:remove_method, name)
        meta.send(:remove_method, :"#{name}=")
      end
      delete_field(name)
    end
  
    def method_missing(mid, *args)
      if @prototype.nil? || mid.id2name =~ /=$/
        super
      else
        @prototype.send(mid, *args)
      end
    end
  end

Most of the functionality is inherited from OpenStruct.  The trip down 
the prototype chain happens in method_missing.  A short IRB session 
should explain the usage:

  >> require 'prototype'
  true
  >> starbucks = Prototype.new(:name => "Starbucks", :drink_size => 3)
  => #<Prototype name="Starbucks", drink_size=3>
  >> new_coffee_shop = Prototype.new(:name => "New")
  => #<Prototype name="New">
  >> new_coffee_shop.prototype = starbucks
  => #<Prototype name="Starbucks", drink_size=3>
  >> new_coffee_shop.drink_size
  => 3
  >> new_coffee_shop.drink_size += 1
  => 4
  >> new_coffee_shop.drink_size
  => 4
  >> new_coffee_shop.delete_slot(:drink_size)
  => 4
  >> new_coffee_shop.drink_size
  => 3
  >> another_coffee_shop = Prototype.new(:name => "Another")
  => #<Prototype name="Another">
  >> another_coffee_shop.prototype = new_coffee_shop
  => #<Prototype name="New">
  >> another_coffee_shop.drink_size
  => 3
  >> another_coffee_shop.prototypes
  => [#<Prototype name="New">, #<Prototype name="Starbucks", 
drink_size=3>]
  >> starbucks.prototype = another_coffee_shop
  ArgumentError: circular prototype chain
    from ./prototype.rb:8:in `prototype='
    from (irb):23
  >>

-Matthias