From: Assaph Mehr <assaph@...>
Date: 2005-01-07T08:41:29+09:00
Subject: Re: Ruby design question: lazy construction of object graph containing forward references

Here's an implementation of lazy forward references. Notice that if you
assign to a variable before reading it, the previous lazy
initialization will be overwritten.

__BEGIN CODE__
class Object
def singleton_class
class << self; self; end
end
end

module LazyForwardRef
def forward_ref(meth, &bl)
self.singleton_class.send :define_method, meth, lambda{
self.singleton_class.send :define_method, meth, lambda {
self.singleton_class.send :attr_accessor, meth
instance_variable_set "@#{meth}", bl.call
}
return send(meth)
}
self.singleton_class.send :define_method, "#{meth}=", lambda{
|args|
self.singleton_class.send :define_method, meth, lambda {
self.singleton_class.send :attr_accessor, meth
instance_variable_set "@#{meth}", *args
}
return send(meth)
}
end
end


require 'pp'

x = Object.new
x.extend LazyForwardRef

begin
p x.foo
rescue NoMethodError => detail
pp detail
pp [x, x.methods.sort - Object.instance_methods]
end


x.forward_ref(:foo) { Hash[1,2,3,4] }
pp [x, x.methods.sort - Object.instance_methods]

p x.foo
pp [x, x.methods.sort - Object.instance_methods]

x.foo = 'Something else'
pp x

p x.foo

puts "\n~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\n\n"

y = Object.new
y.extend LazyForwardRef

y.forward_ref(:bar) { [1,2,3,4] }
pp [y, y.methods.sort - Object.instance_methods]
y.bar= 10
pp [y, y.methods.sort - Object.instance_methods]
p y.bar