12 Testing Functional Programs
In this chapter you will learn how to
- test pure functions by providing examples
- test pure functions by providing properties
- test side-effects without using any mocking libraries
- develop new functionalities in a test-driven way
Beware of bugs in the above code;
I have only proved it correct, not tried it.
— Donald Knuth
12.1 Do you have tests for that?
The last chapter of the book is devoted to one of the most important software engineering activities: testing. Tests are one of the major methods of writing maintainable software. They can be used to make sure the program behaves according to the requirements, it doesn’t have bugs we discovered earlier and that it integrates with external APIs, services or databases properly.
| On top of all that, we can also use tests to document the application: to help understand the responsibilities and inner working to other team members. This is the best kind of documentation because it can’t get outdated as easily as normal text-based documentation usually does—assuming that the test itself is easy to understand. That all makes tests a very important element of a well-developed, readable and maintainable software. |
Having an application that consists mostly of pure functions that operate on immutable values makes writing test a very enjoyable endeavor. The tests in this chapter are small, readable and they cover a lot of production code. |
