https://twitter.com/execat/status/404173120375754752

test_associativity.rb

#!/usr/bin/env ruby -w

puts "hello" << "meow" << "rofl"
class String
  def <<(arg)
    puts "#{self.to_s} called with #{arg.inspect}"
    "#{self}#{arg}"
  end
end
puts "hello" << "meow" << "rofl"

# Result with ruby 2.0.0p247 (2013-06-27 revision 41674) [x86_64-darwin12.4.0] =>
#
# hellomeowrofl
# test_associativity.rb:5: warning: method redefined; discarding old <<
# hello called with "meow"
# hellomeow called with "rofl"
# hellomeowrofl