From: why the lucky stiff <ruby-talk@...>
Date: 2004-03-29T19:11:52+09:00
Subject: Re: YAML and initializers

Good discussion, Hal.  I've been really rethinking the YAML+Ruby typing 
mechanism, so this is a perfect time for this.  Right on.

Hal Fulton wrote:
> 
> When entities have default values in the constructor, we can omit them
> when calling #new.
> 
> I'd like to be able to omit these in YAML also. (I can omit them now,
> of course, but they get nil values).
> 

So, if I understand you correctly, you use default values in the 
constructor to enforce default values for object properties.  In the 
case below:

> 
>   require "yaml"
> 
>   class MyClass
>     def initialize(a,b,c=5)
>       @alpha, @beta, @gamma = a,b,c
>     end
>   end
> 

...the use of c=5 is a way of ensuring that 5 is the default value for 
@gamma.

> 
> A part of me wishes that YAML could somehow be smart enough to call
> the constructor, maybe with a special call like 'construct' instead
> of 'load' -- but there are likely ramifications I haven't thought of.
> 

In the case of the above constructor, it would be impossible for YAML to 
know in what order to pass the three parameters and I don't have any 
ideas for making the YAML loader work with the constructor.

I think the best solution for this would be akin to 
Object#initialize_copy.  In fact, it seems like YAML could call 
Object#initialize_copy with the new object, should an 
Object#initialize_copy method exist.  Yeh?

Yes, you can also do this with a typing callback:

   YAML::add_domain_type( "hypermetrics.com,2004", "MyClass" ) do
       |type, val|
       val['gamma'] ||= 5
       YAML::object_maker( MyClass, val )
   end

   YAML::load( <<EOY )
   --- !hypermetrics.com,2004/MyClass
   alpha: 10
   beta: 20
   EOY
   #=> #<MyClass:0x82418c0 @alpha=10, @gamma=5, @beta=30>


_why