Welcome to the Mutant module. This module's primary concern is the concept of testing.
Why testing, you might ask? Typically compiler features can eliminate many classes of bugs, but they cannot solve the halting problem. The only true way to know if your code will work is to run the code. In Ruby, however, there aren't any error-checking compiler features such as a strong type system and therefore testing is sort of a big deal.
Here enters a problem, and the main focus of this module; in order for the tests to verify that the code is correct, the tests themselves must be correct, and moreover meaningful. The question then is, how do we verify the correctness of tests we've written? "We test our tests!" you say, and clearly this recurses to infinity and exhuasts your stack.
Thus we turn to mutation testing. This form of testing ensures that test cover all edge-cases of your developed code. Not only does mutation testing test the current state of your code, but it also changes the code under review to ensure that errors in the code result in failed tests as is expected. An example of this would be changing an == to !=, and similar concepts. Using a mutation testing tool will ensure that all edge cases are covered, and furthermore mutation testing typically gives us a more reliable metric of test coverage as opposed to something like counting LoC coverage.
Generally, it's bad practice to write tests which are aware of the implementation, especially when using behavioral testing tools such as Rspec. However, mutation testing often pushes you to write implementation-aware tests. The power of mutation testing thus lies in the number of techniques used to mutate the code under test.
Mutant is a Ruby library to perform (you guessed it) mutation testing. To use it, you simply feed it your Rspec tests and a module or class name, and it Magically™ tests as many edge cases as it knows how to.
As you may have already guessed, the workshop for this section makes heavy use of both Ruby's Mutant and Rspec libraries. If you have never used either of these you may want to consider running through the below material before starting the workshop: