Simple (incomplete) stack with a command pop

stack.rb

#!/usr/bin/env ruby

class Stack
  def initialize(container_class=Array)
    @container = container_class.new
  end

  def empty?
    @container.empty?
  end

  def push(value)
    @container.push value
    return
  end

  def pop
    value = @container.pop
    yield value if block_given?
    return
  end
end

require 'minitest/autorun'

describe Stack do
  before do
    @stack = Stack.new
  end

  describe "when created" do
    it "must be empty" do
      assert_empty @stack
    end
  end

  describe "when a value is pushed onto the stack" do
    it "must not be empty" do
      @stack.push 'foo'
      refute_empty @stack
    end

    it "must be able to pop a value off the stack" do
      pushed_value = "Foo"
      @stack.push pushed_value
      @stack.pop { |popped_value|
        assert_same pushed_value, popped_value
      }
      assert_empty @stack
    end
  end
end