Enter code in the right pane. Click on the status bar or hit Ctrl + Enter to compile the code and run tests automatically.
If your code compiles correctly, these instructions will be replaced by an debugging window where you can set breakpoints, run functions, and step through execution of your code. (This is available even if some of your tests fail.)
Tests are built into the language to encourage good testing practices. Each time you compile your code, the tests you define are run against it. A where block at the end of a function contains tests like name(1, 2, 3) is 2, 3. The left-hand side must be a function call and the right-hand side can be a list of integers or another function call.
For additional documentation, refer to the README.