stack.rb

# Corey's challenge is to make the second test pass without ever making the first test fail.
# run ruby stack.rb to run the tests
# For more information see http://tooky.co.uk/kickstart-academy-podcast-with-corey-haines-and-sandi-metz/

class Stack
  def empty?
    true
  end
  def push(element)
    @empty = false
  end
end

def assert(expected, actual, msg="FAIL!")
  if expected != actual
    raise "Failed test. #{expected} should have been #{actual}"
  end
end

# a new stack is empty
s = Stack.new
assert(true, s.empty?)

# a stack is not empty after I push
s = Stack.new
s.push 5
if false != s.empty?
  raise "Stack should not have been empty"
end

I forgot to mention one little constraint on this. You can ONLY ADD code on red and ONLY REMOVE code on green. And you must run your tests after each change, Katrina Owen-style. Otherwise, it is too straightforward.
My little rhyme that I use is

"Add code on red, remove code on green"

(Even though I'm told that doesn't rhyme)

One effect of this is that it is much harder to mutate the existing code, relying instead on adding and removing.

I'd also start without lines 9-11 (the push method). Start with a very clean slate.

Shouldn't the msg argument to #assert be used somewhere?

HAHA! Yeah, that would be good.