console.rb

irb(main):001:0> module Foo
irb(main):002:1> def the_var(var)
irb(main):003:2> @var ||= var
irb(main):004:2> end
irb(main):005:1> end
=> nil
irb(main):006:0> class Bar
irb(main):007:1> end
=> nil
irb(main):008:0> Bar.extend(Foo).the_var("something")
=> "something"
irb(main):009:0> Bar.extend(Foo).the_var("blah")
=> "something"