Install Node and psc-package dependencies, including PureScript itself and PureScript build tools:
npm install
Run the following command to compile and run the exercise. This will also re-compile the code and re-run the exercise when the code changes.
npm start
This should compile successfully and display one failing test in test/Exercise4.purs. Open that file in your editor and write code to make that test pass.
There are other tests or test suites within the same file prefixed by skip, as in skipTest and skipSuite. Once the first test has been passed, remove the skip portion from the next test, watch the test fail, and then write the code to make that pass, too. Repeat until you run out of tests.
When you are ready, you can move on to Exercise 5.