From: Caleb Clausen <vikkous@...>
Date: 2010-06-07T22:52:13+09:00
Subject: Re: Singleton classes and Namespce

On 6/7/10, Robert Klemme <shortcutter@googlemail.com> wrote:
> 2010/6/6 Intransition <transfire@gmail.com>:
>> Do singleton classes not get the same namespace treatment as normal
>> classes?
>
> I think so.
>
>>  module M
>>    class X; end
>>  end
>>
>>  o = Object.new
>>  => #<Object:0x7fc61f1a1e58>
>>
>>  (class << o; self; end).class_eval{ include M }
>>  => #<Object:0x7fc61f1a1e58>
>>
>>  def o.x; X; end
>
> This cannot work since X is not statically in scope.

Ah, but constant lookup is supposed to check the ancestors if the
constant is not found in the scope. M should be among the ancestors of
o's singleton class, shouldn't it? I would have expected this to work.
OTOH, constant lookup is confusing.

This way works:

 module M
   class X; end
 end

 o = Object.new  # => #<Object:0x7fc61f1a1e58>

 (class << o; self; end).class_eval{ include M } # => #<Object:0x7fc61f1a1e58>
 #or o.extend M as well, presumably

  class<<o
    def x; X end
  end

 o.x   #=>M::X

See http://ruby.runpaint.org/variables#constants in which the lookup
rules are summarized as:
  (Module.nesting + container.ancestors + Object.ancestors).uniq
#slightly paraphrased

I guess the key point to understand is that for singleton methods,
container is the (static) module of class enclosing the singleton
method, NOT the singleton class that they affect.