From: Joel VanderWerf <vjoel@...>
Date: 2002-11-12T06:30:54+09:00
Subject: Re: More fun with modules

Eric Schwartz wrote:
> Joel VanderWerf <vjoel@PATH.Berkeley.EDU> writes:
> 
>>Method lookup doesn't go thru enclosing modules, just included modules
>>(and superclasses, of course).
>>
>>However, constant lookup *does* look thru enclosing modules, in
>>addition to included modules and superclasses:
> 
> 
> Ah, that explains it!  I was using this facility to define a few
> module-level global variables that I didn't want to be /real/
> globals, and it seems to me that the PoLS would indicate that what
> works for constants works for other things, too.  Why does it work
> this way?

One answer is: because methods can access the internal state of an 
object. You want to be able to nest a class within another class, 
without sharing state. For example:

class Space
   class Point
     attr_accessor :x, :y
     def initialize x, y
       @x = x; @y = y
     end
   end

   attr_accessor :top_left, :bottom_right

   def initialize x0, y0, x1, y1
     @top_left = Point.new x0, y0
     @bottom_right = Point.new x1, y1
   end
end

space = Space.new 100, 100, 500, 200

tl = space.top_left

p tl.x         # ==> 100

# if method lookup for Point fell through to Space, we'd
# be able to do this, which is senseless:
#p tl.bottom_right