Sample rspec implementation from DestroyAllSoftware

rspec.rb

require 'test/unit'

def describe(description, &block)
  ExampleGroup.new(block).evaluate!
end

class ExampleGroup
  def initialize(block)
    @block = block
  end

  def evaluate!
    instance_eval(&@block)
  end

  def it(description, &block)
    block.call
  end
end

class Object
  def should
    DelayedAssertion.new(self)
  end
end

class DelayedAssertion
  def initialize(subject)
    @subject = subject
  end

  def ==(other)
    raise AssertionError unless @subject == other
  end
end

class AssertionError < Exception
end


class TestDescribe < Test::Unit::TestCase
  def test_that_it_can_pass
    describe 'some thing' do
      it 'has some property' do
      end
    end
  end

  def test_that_it_can_fail
    assert_raise(IndexError) do
      describe 'some failing thing' do
        it 'fails' do
          raise IndexError
        end
      end
    end
  end
end

class TestAssertion < Test::Unit::TestCase
  def test_it_should_pass
    2.should == 2
  end

  def test_it_can_fail
    assert_raise(AssertionError) do
      1.should == 2
    end
  end

end