RackUnit is designed to allow tests to evolve in step with the evolution of the program under testing. RackUnit scales from the unstructured checks suitable for simple programs to the complex structure necessary for large projects.
Simple programs, such as those in How to Design Programs, are generally purely functional with no setup required to obtain a context in which the function may operate. Therefore the tests for these programs are extremely simple: the test expressions are single checks, usually for equality, and there are no dependencies between expressions. For example, a HtDP student may be writing simple list functions such as length, and the properties they are checking are of the form:
RackUnit directly supports this style of testing. A check on its own is a valid test. So the above examples may be written in RackUnit as:
(check-equal? (length null) 0) (check-equal? (length '(a)) 1) (check-equal? (length '(a b)) 2)
Simple programs now get all the benefits of RackUnit with very little overhead.
There are limitations to this style of testing that more complex programs will expose. For example, there might be dependencies between expressions, caused by state, so that it does not make sense to evaluate some expressions if earlier ones have failed. This type of program needs a way to group expressions so that a failure in one group causes evaluation of that group to stop and immediately proceed to the next group. In RackUnit all that is required is to wrap a test-begin expression around a group of expressions:
(test-begin (setup-some-state!) (check-equal? (foo! 1) 'expected-value-1) (check-equal? (foo! 2) 'expected-value-2))
Now if any expression within the test-begin expression fails no further expressions in that group will be evaluated.
Notice that all the previous tests written in the simple style are still valid. Introducing grouping is a local change only. This is a key feature of RackUnit’s support for the evolution of the program.
The programmer may wish to name a group of tests. This is done using the test-case expression, a simple variant on test-begin:
Most programs will stick with this style. However, programmers writing very complex programs may wish to maintain separate groups of tests for different parts of the program, or run their tests in different ways to the normal RackUnit manner (for example, test results may be logged for the purpose of improving software quality, or they may be displayed on a website to indicate service quality). For these programmers it is necessary to delay the execution of tests so they can be processed in the programmer’s chosen manner. To do this, the programmer simply wraps a test-suite around their tests:
(test-suite "Suite name" (check ...) (test-begin ...) (test-case ...))
The tests now change from expressions that are immediately evaluated to objects that may be programmatically manipulated. Note again this is a local change. Tests outside the suite continue to evaluate as before.
Most testing frameworks, including earlier versions of RackUnit, support only the final form of testing. This is likely due to the influence of the SUnit testing framework, which is the ancestor of RackUnit and the most widely used frameworks in Java, .Net, Python, and Ruby, and many other languages. That this is insufficient for all users is apparent if one considers the proliferation of “simpler” testing frameworks in Scheme such as SRFI-78, or the practice of beginner programmers. Unfortunately these simpler methods are inadequate for testing larger systems. To the best of my knowledge RackUnit is the only testing framework that makes a conscious effort to support the testing style of all levels of programmer.