ruby

http://www.railstips.org/blog/archives/2006/11/18/class-and-instance-variables-in-ruby/
http://thoughts.codegram.com/understanding-class-instance-variables-in-ruby/
http://www.devalot.com/articles/2008/09/ruby-singleton
http://rubymonk.com/learning/books/4-ruby-primer-ascent/chapters/45-more-classes/lessons/110-instance-variables

https://ruby-china.org/wiki/coding-style
http://www.alanmacdougall.com/blog/2012/06/08/interactive-debugging-with-pry/