gistfile1.rb

def assert_difference(string, expected_difference = 1, &block)
  before = eval(string, block.binding)
  block.call
  after = eval(string, block.binding)
  fail "Expected a difference of #{expected_difference}" unless after - before == expected_difference
end

a = 1
assert_difference("a", 1) do
  a += 1
end