glados 0.0.2

🍰 A property-based testing framework that tries to break your invariances.

Testing is tedious! At least that's what I thought before I stumbled over property-based testing – a simple approach that allows you to write less tests yet gain more confidence in your code.

Instead of defining concrete inputs and testing whether they result in the desired output, you define certain conditions that are always true (also called invariants). In mathematics, there's the ∀ operator for that. In Dart, there's glados.

dev_dependencies:
  test: ...
  glados: ...

Getting started

Suppose you write a function that tries to find the maximum in a list. I know – that's pretty basic – but it's enough to get you started. Here's an obviously wrong implementation:

/// If the list is empty, return null, otherwise the biggest item.
int max(List<int> input) => null;

To be sure that the function does the right thing, you might want to write some tests. Here's how those would look like in traditional unit testing:

test('maximum of empty list', () {
  expect(max([]), equals(null));
});
test('maximum of non-empty list', () {
  expect(max([40, 2, 10]), equals(40));
});

If you're not familiar with the syntax of the test package, you should read their documentation first.

Executing pub run test path/to/tests.dart should show that the second test fails.

In property-based testing, you look for invariants – conditions that should be true for any input. For example, if max produces null, the list should be empty:

glados<List<int>>('maximum is only null if the list is empty', (list) {
  if (max(list) == null) {
    expect(list, isEmpty);
  }
});

You can use the glados function whereever you would use the test function. glados then tests your code with a variety of inputs and all of them need to succeed. The glados function also takes a generic type parameter describing which values to generate – in this case, List<int>.

Running the test should produce something like this:

Tested 1 inputs, shrunk 25 times.
Failing for input: [0]
...

glados discovered that the a list containing 0 breaks the condition!

Let's modify our max function to pass this test:

int max(List<int> input) => 42;

We need to add another invariant to reject this function as well. Arguably the most obvious invariant for max is that the result should be greater than or equal to all items of the list:

glados<List<int>>('maximum is >= all items', (list) {
  var maximum = max(list);
  if (maximum != null) {
    for (var item in list) {
      expect(maximum, greaterThanOrEqualTo(item));
    }
  }
});

Running the tests produces the following result:

Tested 35 inputs, shrunk 117 times.
Failing for input: [43]

glados detected that the invariant breaks if the input is a list containing only 43.

Let's actually add a more reasonable implementation for max:

int max(List<int> input) {
  if (input.isEmpty) {
    return null;
  }
  var max = 0;
  for (var item in input) {
    if (item > max) {
      max = item;
    }
  }
  return max;
}

This fixes the tests, but still doesn't work for lists containing only negative values. So, let's add a final test:

glados<List<int>>('maximum is in the list', (list) {
  var maximum = max(list);
  if (maxmium != null) {
    expect(list, contains(maximum));
  }
});

I'll leave implementing the function correctly to you, the reader.

But whatever solution you come up with, it'll be correct: Our tests aren't merely some arbitrary examples that we came up with anymore. Rather, they correspond to the actual mathematical definition of max.

How does it work?

glados works in two phases:

• The exploration phase: glados generates increasingly complex, random inputs until one breaks the invariant or the maximum number of tries is reached.
• The shrinking phase: This phase only happens if glados found an input that breaks the invariant. In this case, the input is gradually simplified and the smallest input that's still breaking the invariant is returned.

Both phases internally use the Arbitrary<T> class, which has two methods:

• T generate(Random random, int size) generates a new value of type T, using the random generator for random values. The size argument should be used as a rough estimate on how big or complex the returned value should be. For example, for a given size n, the intArbitrary produces ints from -n to n.
• Iterable<T> shrink(T input) takes a value and returns an Iterable containing similar, but smaller values. Smaller means that calling shrink repeatedly on the smaller values and their children etc., the program should eventually terminate (aka the transitive hull with regard to shrink should be finite).

glados looks for a fitting arbitrary in the global variable gladosArbitraries and then uses that to generate and shrink values.

Creating a custom Arbitrary

The basic types all have corresponding Arbitrarys implemented. If you want to use a custom type, you need to create a custom arbitrary and register it:

// Assuming User consists of name (String) and age (int).
class UserArbitrary extends Arbitrary<User> {
  UserArbitrary(this.nameArbitrary, this.ageArbitray);
  
  final Arbitrary<String> nameArbitrary;
  final Arbitrary<int> ageArbitrary;

  @override
  List<T> generate(Random random, int size) {
    return User(
      name: nameArbitrary.generate(random, size),
      age: ageArbitrary.generate(random, size),
    );
  }

  @override
  Iterable<List<T>> shrink(List<T> value) sync* {
    yield User(
      name: nameArbitrary.shrink(value.name),
      age: value.age,
    );
    yield User(
      name: value.name,
      age: ageArbitrary.shrink(value.age),
    );
  }
}

final userArbitrary = UserArbitrary(stringArbitrary, intArbitrary);

// in the main method
gladosArbitraries.add(userArbitrary);

It's best practice to make arbitraries that are internally used in another arbitrary configurable so that you can customize them if needed.

What's up with the name?

GLaDOS is a very nice robot in the Portal game series. She's the head of the Aperture Science Laboratory facilities, where she spends the rest of her days testing. So I thought that's quite a fitting name. 🍰

Further info

• Here's the talk that got me into property-based testing.
• This article covers the topic in more detail.