calc.rb

class Calculator
	attr_reader :mode, :last

	def initialize(mode=:real)
		@mode = mode
    @last = 0 # XXX why this? without @last == nil
	end

	def mode=(mode)
		@mode = mode
		@last = 0
	end

	def add(a, b)
		@last = a + b
	end

end

class CalculatorTest < MiniTest::Test
	def test_store_last_result
		calc = Calculator.new
		assert_equal 0, calc.last
		calc.add 1,2
		assert_equal 3, calc.last
	end
end

=begin
  1) Failure:
CalculatorTest#test_store_last_result [calc.rb:22]:
Expected: 0
  Actual: nil
=end