REPL Driven development and Unit testing in Clojure

In Clojure the unit under test is the function. Unit test coverage should test all public function that form the API of their respective namespace.

clojure.test namespace provides a unit testing framework and is included in the Clojure library, so is available in all Clojure projects.

Test runners are used to run one or more tests in a project.

Simple principles for writing unit tests

  • One test namespace for each src namespace
  • One deftest function for each function under test
  • Multiple is assertions for one function
  • Group assertions in testing and provide a meaningful description of that grouping, adding more information when reviewing test failures especially for larger code bases.
  • `are for testing similar functionality with different data sets
  • Test private functions (or don't use them) through public functions of each namespace (minimize test churn and time to run all tests)
  • Use generative testing to create less code and yet test with more extensive range of data
  • Use test selectors to organize tests and optimize speed of test runs

Test with Development REPL and Command Line

There is value in running unit tests via the REPL used to develop the code as well as using a test runner via the command line (optionally using a watcher).

Clojure unit testing approach - editor and command line

Using the develop-time REPL provides fast selection of tests using editor commands and shows instant feedback within the editor itself, helping maintain focus.

Using the develop-time REPL to run tests does require evaluation of both test and source code to ensure changes are loaded into the REPL. Stale definitions, especially stale tests should be removed from the REPL when using this approach, either undef-ing a deftest before renaming or re-evaluating other changes to a deftest expression.

Running tests via a command like tool (i.e. koacha, Cognitect Labs runner) ensures tests are run from a known state, as all changes are captured in the source code files and a clean REPL state is established each time the tests. This clearly defined state is especially valuable for running integration tests.

Using a watch process with a command line tool can also give fast feedback, especially if the test runner can be configure to run only selective tests (i.e kaocha)

Run all tests (including integration tests) via the command line before pushing commits to ensure all changes to the code have been tested.

If tests are not running in the REPL or are returning unexpected errors, a command line test runner is a useful way to diagnose if it is the test code or test tools causing the error.

The CLI approach is also more robust for longer running tests than running within an editor.

Project structure with tests

By convention, separate src and test directories are used to hold the source code and the code that tests that source code.

For each source code file in src there should be a corresponding file in test with the same name and _test postfix.

For example, code to test the src/codewars/rock_paper_scissors.clj is saved in the file src/codewars/rock_paper_scissors_test.clj file.

Clojure project structure - src and test branches

Example project: CodeWars: Rock Paper Scissors

Source and Test Namespaces

As with file names, the namespaces for each test code file is the same as the source code it is testing, with a -test postfix.

codewars/rock-paper-scissors source code namespace will have a matching codewars/rock-paper-scissors-test namespace.

Create Projects from templates

Templates typically include a parallel test and src directory structure. The clj-new tool has build it templates (app, lib) and will create src and test directories in the projects it creates.

clojure -M:clj-new app practicalli/rock-paper-scissors-lizard-spock

Project Examples: Code challenges with unit tests


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