Recently I have been working on a new service. It has a single rest endpoint that triggers some work.
The tests for this use Hammox to mock out the external services that it talks to. This ensures that the tests always match the declared typespecs for the services.
This week I added ex_coveralls to the project to see how complete the test coverage was. It revealled several parts of the code that had missing tests. Adding those revealled a few small bugs (one of which had been found in production while I was writing the tests!).
The test coverage was approaching 100% in the main processing area. Combined with a suite of tests that produced all of the possible output states I was confident about performing some refactoring.
The refactors were to replace some tuples with structs and them to remove some pointless wrapping error tuples. Coveralls helped to reveal dead code paths that once removed achieved the 100% coverage.
The tests also helped when working out what could be changed. By adding a log statement at a key points I could see all possible inputs to a function. This helped in finding what to change when adapting.
Now I have simpler code that with the tests I have confidence in.
Developer Rants 