From: Joel VanderWerf <vjoel@...>
Date: 2005-06-26T13:08:18+09:00
Subject: Re: how to add YAML serialization support to a class?

Luca Pireddu wrote:
> Hello.  Can someone point me to some documentation describing how to add YAML
> support to a class?  I have a class that contains an RGL graph.  I'd like to
> be able to save it and restore it, but 
> 
> YAML::dump(RGL::DirectedAdjacencyGraph.new)
> 
> throws an exception
> 
> TypeError: can't dump anonymous class Class

Looks like the problem is that somewhere in the graph of objects 
connected to your instance is a Class object:

$ irb -r yaml
irb(main):001:0> YAML.dump(Class)
TypeError: can't dump anonymous class Class
...

(I don't see what's anonymous about Class...)

Anyway, there's nothing shameful about wanting to yaml a class! It's 
easy to convince YAML to let you do it:

require 'yaml'

class Module
   def is_complex_yaml?
     false
   end
   def to_yaml( opts = {} )
     YAML::quick_emit( nil, opts ) { |out|
       out << "!ruby/module "
       self.name.to_yaml( :Emitter => out )
     }
   end
end
YAML.add_ruby_type(/^module/) do |type, val|
   subtype, subclass = YAML.read_type_class(type, Module)
   val.split(/::/).inject(Object) { |p, n| p.const_get(n)}
end

class Class
   def to_yaml( opts = {} )
     YAML::quick_emit( nil, opts ) { |out|
       out << "!ruby/class "
       self.name.to_yaml( :Emitter => out )
     }
   end
end
YAML.add_ruby_type(/^class/) do |type, val|
   subtype, subclass = YAML.read_type_class(type, Class)
   val.split(/::/).inject(Object) { |p, n| p.const_get(n)}
end

p YAML.dump(Class) # ==> "--- !ruby/class Class"


So, really, all that is getting written is a reference to the class. Any 
state in the class itself (class vars, class instance vars) is not stored.