When you create a project description for your project (for the package manager) you can define a library, executables and test suites. In the test suite you w

When you create a project description for your project (for the package manager) you can define a library, executables and test suites. In the test suite you write your tests against the library. You can do that at any stage, even before writing a line of library code. But yes, Haskell also has a REPL that can be used to experiment with code.