From: Hal Fulton <hal9000@...>
Date: 2004-03-29T10:39:30+09:00
Subject: YAML and initializers

Here's a thought question for you. If you read the subject line, you
already know the answer.

A user-defined class has an initializer. When/how would you ever create
an instance of that class without calling the initializer?

Hmm.

The answer, of course, is when you load it from a marshaled format such
as YAML.

This brings me to my question.

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).

Here's some code in case it clarifies a little.

   require "yaml"

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

   foo = MyClass.new(3,4)

   y1 = foo.to_yaml
   y2 = y1.split("\n").find_all {|x| x !~ /gamma/ }.join("\n")
   # y2 has no gamma

   f2 = YAML.load(y2)
   p f2
   # I'd like f2 to have a '5' for @gamma


Is this doable with add_domain_type or something? This is beyond my
current knowledge of YAML.

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.


Thanks,
Hal