Testing the Halting Problem blog post: http://www.cloudspace.com/blog/2009/09/29/testing-the-halting-problem/

RecursiveCall.rb

def reader(record, previous_ids = []) 
  raise "loop" if previous_ids.include?(record.id)
  ...
  return reader(next_record, previous_ids + [record.id])
end

TestTheHaltingProblem.rb

it "doesn't loop forever" do
  # setup test and create loop
  puts "This is a test of the halting problem. If you see this message for more than a few seconds, it means the test has failed."
  # run test on the loop
end