From: David Masover <ninja@...>
Date: 2008-08-24T10:25:56+09:00
Subject: Re: Passing parameters into a singleton class def?

On Saturday 23 August 2008 20:09:52 Doug Glidden wrote:
> class MyClass
>   ...
> 
>   def add_fields(*field_list)
>     class << self
>       attr accessor *field_list
>     end
>   end
> end
[snip]
> Both throw an error because field_list is not defined within the
> singleton class definition?  Am I making a simple mistake, or should I
> be doing this differently?

Well, if you actually have fields called :field1, :field2, and :field3, I 
might suggest an array... Assuming that's not the case.

I don't even remember the actual Ruby way to do this, but _why's metaid gem 
would let you do it like this:

def add_fields(*field_list)
  meta_eval do
    attr_accessor *field_list
  end
end

The key, however, is that it's some sort of _eval method -- that it's taking a 
block, which means you get to inherit the parent scope. In fact, if it's 
actually a singleton (as in, only one instance of this class will be 
defined), you can do this:

def add_fields(*field_list)
  self.class.class_eval do
    attr_accessor *field_list
  end
end

In this case, self.class refers to the actual class that the current object is 
an instance of. On the other hand, self.metaclass or meta_eval (both from 
metaid) refer to the current object's metaclass, or eigenclass.

Given that you were doing "class << foo" stuff, I'm assuming you wanted the 
metaclass.

One more hack before I'm through:

def add_fields(*field_list)
  self.metaclass.send :attr_accessor, *field_list
end

I haven't tested any of this. Let me know how it works for you.