From: Luca Pireddu <ilveroluca@...>
Date: 2005-06-28T08:15:40+09:00
Subject: Re: how to add YAML serialization support to a class?

Joel VanderWerf wrote:

> 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.

Thanks for your reply Joel.  I think I understand the basic idea.  To emit,
define a method #to_yaml in your class.  To enable YAML to load your class
register the type with YAML using one of the add_x_type methods, and
associate a block that knows how to recreate your data structure from the
YAML parse tree.

For anyone who may be interested, I've found helpful documentation at this
site:
http://yaml4r.sourceforge.net/doc/

Luca