From: Matthew Westcott <gasman@...>
Date: 2005-05-25T07:20:18+09:00
Subject: Re: Doing fixups after deserializing YAML

Logan Capaldo wrote:
> On 5/20/05, Matthew Westcott <gasman@raww.org> wrote:
> 
>>class Squarer
>>        attr_reader :num, :num_squared
>>
>>        def initialize(num)
>>                @num = num
>>                @num_squared = num * num
>>        end
>>end

[serialising / deserialising a Squarer object to YAML without writing 
the @num_squared variable]

> Well this solution doesn't involve yaml but...
> 
> class Squarer
>          def num_squared
>               @num_squared ||= @num * @num
>          end
> end
> 
> So it will calculate it only the first time you ask for it.

Hi,
Thanks very much for the suggestion - as you might expect, though, the 
example I gave was drastically simplified from the real-life problem I 
was stuck on. In that situation, it just wasn't feasible to eliminate 
all redundant internal state from the object itself - but I rather 
wanted to eliminate it from the object's YAML representation, at least.

In the end I found the bit of documentation I was after 
<http://yaml4r.sourceforge.net/doc/page/type_families.htm> , and arrived 
at this solution:

require 'yaml'

class Squarer
	attr_reader :num, :num_squared

	def initialize(num)
		@num = num
		@num_squared = num * num
	end

	def to_yaml_type
		# register this as a custom type
		"!west.co.tt,2005-05-24/squarer"
	end

	def to_yaml_properties
		%w{ @num }
	end
end

# tell the YAML parser how to deal with an object of this type
YAML.add_domain_type( "west.co.tt,2005-05-24", "squarer" ) { |type, val|
	# val is a hash of all the fields we serialised (i.e. just @num)
	Squarer.new(val['num'])

	# here we'd do any more initialisation of the object
	# if necessary
}