gistfile1.rb

>> module M                                                                        
>>   def foo                                                                       
>>     self.class::FOO                                                             
>>     end                                                                         
>>   end                                                                           
=> nil                                                                             
>> class A; FOO = 1; extend M; end                                                 
=> A                                                                               
>> class B; FOO = 42; extend M; end                                                
=> B                                                                               
>> A.foo                                                                           
NameError: uninitialized constant Class::FOO                                       
        from (irb):3:in `foo'                                                      
        from (irb):9                                                               
        from /Users/brianstanwyck/.rvm/rubies/ruby-1.9.3-p0/bin/irb:16:in `<main>'