stack.rb

class Stack
  attr_reader :stack
  def initialize
    @stack = []
  end
  def peek(obj)
    @stack.push(obj)
  end
  def pop
    @stack.pop
  end
end

stack_spec.rb

describe Stack do
  before(:each) do
   @stack = Stack.new
   @stack.push :item
 end
  describe "#peek" do
    it "should return the top element" do
      @stack.peek.should == :item
    end
    it "should not remove the top element" do
      @stack.peek
      @stack.size.should == 1
    end
  end
  describe "#pop" do
    it "should return the top element" do
      @stack.pop.should == :item
    end
    it "should remove the top element" do
      @stack.pop
      @stack.size.should == 0
    end
  end
end