Inspired by http://bugs.python.org/issue12345 and of course The Tau Manifesto, here is a patch for ruby to define tau, the true circle constant. Of course we don't need to really need to patch ruby to get this functionality (but I had fun doing it anyway).
Here is tau.rb so you can just require 'tau'. (Maybe this should be a gem...) Ps, I just learned about module_function.
Update 1: I made this ticket which has been mostly ignored.
Update 2: Ticket has been noticed and discussed! :) Ticket has been categorised as joke! :(
Update 3: Some active debate is going on now and the ticket is now 'assigned'! :) See the details.
Update 4: (28-Jun-2014) Made a pull request in Github.