The is macro in the clojure.test namespace can be used to write assertions about the code we want to test. Usually we provide a predicate function as argument to the is macro. The prediction function will call our code under test and return a boolean value. If the value is true the assertion passes, if it is false the assertion fails. But we can also provide a custom assertion function to the is macro. In the clojure.test package there are already some customer assertions like thrown? and instance?. The assertions are implemented by defining a method for the assert-expr multimethod that is used by the is macro. The assert-expr multimethod is defined in the clojure.test namespace. In our own code base we can define new methods for the assert-expr multimethod and provide our own custom assertions. This can be useful to make tests more readable and we can use a language in our tests that is close to the domain or naming we use in our code.

The clojure.test namespace has the are macro that allows us to combine multiple test cases for the code we want to test, without having to write multiple assertions. We can provide multiple values for a function we want to test together with the expected values. Then then macro will expand this to multiple expressions with the is macro where the real assertion happens. Besides providing the data we must also provide the predicate where we assert our code under test. There is a downside of the are macro and that is that in case of assertion failures the line numbers in the error message could be off.