A stack implementation from http://codon.com/refactoring-ruby-with-monads

stack.rb

# API
#
# + push
# + top
# + pop
# + empty
# + empty?

ArrayStack = Struct.new(:values) do
  def push(value)
    ArrayStack.new([value] + values)
  end

  def top
    values.first
  end

  def pop
    ArrayStack.new(values.drop(1))
  end

  def ==(other)
    return false unless top == other.top
    return true if empty? && other.empty?

    self.pop == other.pop
  end

  def empty?
    values.empty?
  end

  def self.empty
    new([])
  end

end

LinkedListStack = Struct.new(:top, :pop) do
  def push(value)
    LinkedListStack.new(value, self)
  end

  def empty?
    top.nil?
  end

  def ==(other)
    return false unless top == other.top
    return true if empty? && other.empty?

    self.pop == other.pop
  end

  def self.empty
    new(nil, nil)
  end

end

require 'minitest/autorun'

module StackTest
  def test_top
    @stack.push('a').top == 'a'
  end

  def test_pop
    @stack.push('a').pop == @stack
  end

  def test_empty?
    refute @stack.push('a').empty?
  end

  def test_empty
    assert @stack.class.empty.empty?
  end
end

class ArrayStackTest < MiniTest::Unit::TestCase
  include StackTest

  def setup
    @stack = ArrayStack.new([1,2,3])
  end
end

class LinkedListStackTest < MiniTest::Unit::TestCase
  include StackTest

  def setup
    @stack = LinkedListStack.new([1,2,3])
  end
end