Defining Properties and Generators in Clojure: A Deep Dive into Property-Based Testing

14.3.2 Defining Properties and Generators

Property-based testing is a powerful methodology that allows developers to specify the behavior of their code in a more general and abstract way compared to traditional example-based testing. In Clojure, the test.check library facilitates property-based testing by providing tools to define properties and generate random test data. This section will guide you through the process of defining properties and generators, focusing on how to apply these concepts to ensure the robustness of your Clojure applications.

Understanding Property-Based Testing

Before diving into the specifics of defining properties and generators, it’s essential to understand the core idea behind property-based testing. Unlike traditional unit tests, which check for specific inputs and expected outputs, property-based tests define general properties that should hold true for a wide range of inputs. This approach helps uncover edge cases and unexpected behaviors that might not be evident through example-based testing.

For instance, consider a function that reverses a list. A property-based test would assert that reversing a list twice should yield the original list, regardless of the list’s content. This property is more general and powerful than testing the function with a few specific lists.

Defining Generators for Custom Data Structures

Generators are at the heart of property-based testing. They produce random data that is used to test the properties of your functions. Clojure’s test.check library provides a variety of built-in generators for primitive types, but you’ll often need to define custom generators for complex data structures.

Creating Basic Generators

Let’s start with a simple example. Suppose you want to test a function that operates on a pair of integers. You can define a generator for this pair using gen/tuple:

1(require '[clojure.test.check.generators :as gen])
2
3(def int-pair-gen
4  (gen/tuple gen/int gen/int))

This generator will produce random pairs of integers. You can use this generator to test properties of functions that take such pairs as input.

Custom Generators for Complex Structures

For more complex data structures, you can compose existing generators or create entirely new ones. Consider a scenario where you need to generate a map with specific keys and values. Here’s how you can define a generator for a map with :name and :age keys:

1(def person-gen
2  (gen/let [name gen/string-alphanumeric
3            age  (gen/choose 0 120)]
4    {:name name
5     :age  age}))

The gen/let construct allows you to bind generated values to names and then use those values to construct more complex data structures.

Recursive Generators

Sometimes, you need to generate recursive data structures, such as trees or nested lists. test.check supports recursive generators through the gen/recursive-gen function. Here’s an example of a generator for a simple binary tree:

1(defn tree-gen [leaf-gen]
2  (gen/recursive-gen
3    (fn [inner-gen]
4      (gen/one-of
5        [leaf-gen
6         (gen/tuple inner-gen inner-gen)]))
7    leaf-gen))

This generator produces either a leaf node (using leaf-gen) or a tuple representing a binary tree with two subtrees.

Writing Properties with defspec

Once you have defined your generators, the next step is to write properties that your functions should satisfy. In test.check, properties are defined using the defspec macro, which combines a property with a generator.

Basic Property Definition

Let’s revisit the list reversal example. You can define a property that checks if reversing a list twice returns the original list:

1(require '[clojure.test.check.properties :as prop]
2         '[clojure.test.check.clojure-test :refer [defspec]])
3
4(defspec reverse-twice-is-identity
5  100 ;; Number of tests
6  (prop/for-all [v (gen/vector gen/int)]
7    (= v (reverse (reverse v)))))

In this example, prop/for-all is used to specify that the property should hold for all vectors of integers generated by gen/vector gen/int.

Properties for Custom Functions

Consider a function that calculates the sum of integers in a list. A meaningful property might be that the sum of a list is equal to the sum of its elements when the list is split into two parts:

1(defspec sum-of-parts-equals-whole
2  100
3  (prop/for-all [v (gen/vector gen/int)]
4    (let [split-pos (rand-int (count v))
5          [left right] (split-at split-pos v)]
6      (= (reduce + v)
7         (+ (reduce + left) (reduce + right))))))

This property ensures that the sum function behaves correctly when the list is partitioned.

Best Practices for Defining Properties and Generators

Defining effective properties and generators requires careful consideration of the domain and behavior of your functions. Here are some best practices to keep in mind:

1. Start Simple: Begin with simple properties and gradually increase complexity. Ensure that basic properties hold before testing more intricate behaviors.

2. Use Descriptive Names: Name your properties and generators descriptively to convey their purpose and the aspect of the function they test.

3. Test Invariants: Focus on invariants—properties that should always hold true regardless of input. These are often the most powerful and revealing tests.

4. Consider Edge Cases: Think about edge cases and how your generators can produce inputs that test these scenarios. For example, test empty lists, large numbers, or boundary values.

5. Leverage Shrinking: test.check automatically shrinks failing inputs to find the minimal failing case. Ensure your generators support shrinking to make debugging easier.

6. Iterate and Refine: Continuously refine your properties and generators as you gain insights into your code’s behavior and potential edge cases.

Advanced Techniques and Optimization Tips

Combining Generators

You can combine multiple generators to create more complex test scenarios. For instance, if you’re testing a function that operates on both strings and numbers, you can combine generators using gen/one-of or gen/frequency:

1(def string-or-int-gen
2  (gen/one-of [gen/string-alphanumeric gen/int]))

Parameterized Properties

Properties can be parameterized to test different aspects of a function. For example, you might want to test a sorting function with different comparison functions:

1(defspec sorting-property
2  100
3  (prop/for-all [v (gen/vector gen/int)
4                 cmp (gen/elements [<= >=])]
5    (let [sorted (sort cmp v)]
6      (or (empty? sorted)
7          (apply cmp sorted)))))

Performance Considerations

Property-based testing can be computationally intensive due to the large number of test cases. Optimize performance by:

• Limiting the size of generated data.
• Reducing the number of test cases during development and increasing it for final testing.
• Profiling and optimizing your generators if they become a bottleneck.

Conclusion

Defining properties and generators is a crucial skill for leveraging the full power of property-based testing in Clojure. By carefully crafting generators and properties, you can uncover subtle bugs and ensure your code behaves correctly across a wide range of inputs. As you integrate these techniques into your testing strategy, you’ll gain confidence in the reliability and robustness of your Clojure applications.

Further Reading and Resources

• Clojure test.check Documentation
• Property-Based Testing with PropEr, Erlang, and Elixir
• QuickCheck: A Lightweight Tool for Random Testing of Haskell Programs

Quiz Time!