What is Liskov Principle?

In layman’s terms Liskov Substitution Principle says that if class Foo inherits from class Bar, then you should be able to use (substitute) derived class in every place that the base class is used. For a better definition and further references check out The Liskov Substitution Principle by Uncle Bob.

Testing LSP with MiniTest

MiniTest has a really simple design. A test case is a class and an example is a method of that class. After requiring minitest/autorun every subclass of MiniTest::Unit::TestCase is instantiated and test methods are executed one by one.

One very nice result of this design, which is kind of obvious when you think about it, is that you can not only inherit helper methods (eg. you subclass ActionController::TestCase to have get, post etc) but you may as well inherit whole examples! This is a perfect way to test LSP because, again, you should be able to substitute base class with a derived class.

Example

Let’s re-implement Ruby’s built-in Set class. I’ll write a test first:

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require 'minitest/autorun'

class SetTest < MiniTest::Unit::TestCase
  def setup
    @set = Set.new
  end

  def test_size
    assert_equal 0, @set.size
    @set.add 42
    assert_equal 1, @set.size
  end

  def test_include?
    refute @set.include? 42
    @set.add 42
    assert @set.include? 42
  end

  def test_add
    @set.add 13
    @set.add 13
    assert_equal 1, @set.size
  end

  def test_to_a
    @set.add 1
    @set.add 4
    @set.add 2

    ary = @set.to_a

    assert_equal 3, ary.size
    assert ary.include? 1
    assert ary.include? 2
    assert ary.include? 4
  end
end

Note I didn’t write the exact result of Set#to_a because a cannonical set is unordered. A Ruby 1.9 built-in Set is actually ordered, it simply preserves the order of insertion.

A basic implementation is very easy using Hash like this:

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class Set
  include Enumerable

  def initialize
    @hash = {}
  end

  def size
    @hash.size
  end

  def add(obj)
    @hash[obj] = true
  end

  def include?(obj)
    @hash.include? obj
  end

  def each(&block)
    @hash.keys.each(&block)
  end
end

Let’s run it:

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~% ruby set.rb
Run options: --seed 59316

# Running tests:

....

Finished tests in 0.000589s, 6791.1715 tests/s, 15280.1358 assertions/s.

4 tests, 9 assertions, 0 failures, 0 errors, 0 skips

Now, let’s write a SortedSet that will keep values sorted. Again let’s write a test and run it first:

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class SortedSetTest < SetTest
end

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~% ruby set.rb
Run options: --seed 54235

# Running tests:

........

Finished tests in 0.000944s, 8474.5763 tests/s, 19067.7966 assertions/s.

8 tests, 18 assertions, 0 failures, 0 errors, 0 skips

We now have exactly twice assertions because all test methods have been inherited. Let’s now build a simple SortedSet class and adjust the test, so that we actually use the derived class:

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class SortedSetTest < SetTest
  def setup
    @set = SortedSet.new
  end
end

class SortedSet < Set
  def each(&block)
    @hash.keys.sort.each(&block)
  end
end

Sure enough all tests passes and we’re now certain that a Set object can be substituted with a SortedSet object.

Let’s also test the unique behaviour of the SortedSet. We won’t just define test_to_a method, because we would overwrite assertions from the base test. We’ll pick a different name instead:

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class SortedSetTest < SetTest
  def setup
    @set = SortedSet.new
  end

  def test_to_a_sorted
    @set.add 1
    @set.add 4
    @set.add 2

    assert_equal [1, 2, 4], @set.to_a
  end
end

Now, we could stop it right here, but you propably noticed some duplication between test_to_a and test_to_a_sorted. Again, because we’re using just classes and methods, we can actually write:

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class SortedSetTest < SetTest
  def setup
    @set = SortedSet.new
  end

  def test_to_a
    super
    assert_equal [1, 2, 4], @set.to_a
  end
end

I’m not sure if it’s that useful and you should use it, but you must agree it’s pretty neat!