Mastering Property-Based Testing with `test.check` in Clojure

15.3.2 Using test.check

As experienced Java developers, you’re likely familiar with unit testing frameworks such as JUnit, which focus on example-based testing. In contrast, property-based testing is a powerful paradigm that allows you to define general properties that your code should satisfy, and then automatically generate a wide range of test cases to verify those properties. Clojure’s test.check library is a robust tool for implementing property-based testing, enabling you to test your code more thoroughly and uncover edge cases that example-based tests might miss.

Understanding Property-Based Testing

Property-based testing shifts the focus from specific test cases to general properties of the code. Instead of writing individual tests for each scenario, you define properties that should hold true for a wide range of inputs. The testing framework then generates numerous random inputs to validate these properties.

Key Concepts:

• Properties: Assertions about the behavior of your code that should hold true for all inputs.
• Generators: Functions that produce random test data to be used in testing properties.

Getting Started with test.check

To begin using test.check, you’ll need to add it to your Clojure project. If you’re using Leiningen, add the following dependency to your project.clj file:

:dependencies [[org.clojure/test.check "1.1.0"]]

Defining Generators

Generators are at the heart of property-based testing. They produce random data that is used to test the properties of your code. test.check provides a variety of built-in generators, and you can also create custom generators to suit your needs.

Built-in Generators

test.check includes a range of built-in generators for common data types:

• Integers: gen/int
• Booleans: gen/boolean
• Strings: gen/string
• Collections: gen/vector, gen/list, gen/map

Here’s a simple example of using a generator to produce random integers:

(require '[clojure.test.check.generators :as gen])

(def int-gen (gen/int))

Custom Generators

You can create custom generators using the gen/fmap and gen/bind functions to transform existing generators. For example, to create a generator that produces even integers, you can use:

(def even-int-gen
  (gen/fmap #(* 2 %) (gen/int)))

Defining Properties

Properties are the core of property-based testing. They describe the expected behavior of your code for a wide range of inputs. In test.check, properties are defined using the prop/for-all macro.

Example: Testing a Simple Function

Let’s consider a simple function that reverses a list. We want to test the property that reversing a list twice should yield the original list:

(defn reverse-twice [lst]
  (reverse (reverse lst)))

(require '[clojure.test.check.properties :as prop])

(def reverse-twice-property
  (prop/for-all [lst (gen/vector gen/int)]
    (= lst (reverse-twice lst))))

Running Property-Based Tests

To run property-based tests, use the clojure.test.check/quick-check function, which takes a property and the number of tests to run:

(require '[clojure.test.check :as tc])

(tc/quick-check 100 reverse-twice-property)

This will run the property 100 times with different random inputs. If a failure is found, test.check will attempt to shrink the input to find the smallest failing case.

Comparing with Java

In Java, property-based testing can be achieved using libraries like JUnit-Quickcheck. However, Clojure’s test.check offers a more seamless integration with functional programming paradigms, allowing for more expressive and concise test definitions.

Java Example

Here’s a similar test in Java using JUnit-Quickcheck:

import com.pholser.junit.quickcheck.Property;
import com.pholser.junit.quickcheck.runner.JUnitQuickcheck;
import org.junit.runner.RunWith;
import static org.junit.Assert.*;

@RunWith(JUnitQuickcheck.class)
public class ReverseTest {
    @Property
    public void reverseTwice(int[] array) {
        assertArrayEquals(array, reverse(reverse(array)));
    }

    private int[] reverse(int[] array) {
        int[] reversed = new int[array.length];
        for (int i = 0; i < array.length; i++) {
            reversed[i] = array[array.length - 1 - i];
        }
        return reversed;
    }
}

Advanced Features of test.check

Shrinking

Shrinking is the process of reducing a failing test case to its simplest form. test.check automatically attempts to shrink inputs when a property fails, helping you identify the root cause of the failure.

Composing Generators

You can compose generators to create complex data structures. For example, to generate a map with string keys and integer values:

(def map-gen
  (gen/map gen/string gen/int))

Conditional Properties

You can use the prop/for-all macro to define conditional properties, which only apply to certain inputs:

(def conditional-property
  (prop/for-all [x gen/int]
    (when (pos? x)
      (> (* x x) x))))

Try It Yourself

Experiment with the following exercises to deepen your understanding of test.check:

1. Modify the reverse-twice property to test a different function, such as sorting a list.
2. Create a custom generator for generating valid email addresses and test a property of an email validation function.
3. Explore shrinking by intentionally introducing a bug in the reverse-twice function and observing how test.check shrinks the failing input.

Diagrams and Visualizations

Below is a diagram illustrating the flow of data through a property-based test using test.check:

    graph TD;
	    A[Define Property] --> B[Generate Random Inputs];
	    B --> C[Run Tests];
	    C --> D{Property Holds?};
	    D -->|Yes| E[Success];
	    D -->|No| F[Shrink Inputs];
	    F --> C;

Diagram: The flow of data through a property-based test using test.check.

Further Reading

For more information on property-based testing and test.check, consider exploring the following resources:

• Official Clojure test.check Documentation
• ClojureDocs test.check Examples
• Property-Based Testing in Java with JUnit-Quickcheck

Exercises

1. Define a property for a function that calculates the factorial of a number, ensuring it satisfies the mathematical properties of factorials.
2. Create a generator for binary trees and define properties that test tree traversal algorithms.
3. Implement a property-based test for a function that checks if a string is a palindrome.

Key Takeaways

• Property-based testing allows you to define general properties of your code and test them against a wide range of inputs.
• Generators are used to produce random test data, and test.check provides a variety of built-in and custom generators.
• Shrinking helps identify the simplest failing test case, making debugging easier.
• Clojure’s test.check offers a powerful and expressive way to implement property-based testing, leveraging the strengths of functional programming.

Now that we’ve explored the fundamentals of property-based testing with test.check, you’re equipped to enhance your testing strategy and improve the robustness of your Clojure applications.

Quiz: Mastering Property-Based Testing with test.check