The problem with that style of testing at the repl is it proves your code worked for those tests at that time, but unless you turn that into test cases that wil

The problem with that style of testing at the repl is it proves your code worked for those tests at that time, but unless you turn that into test cases that will keep on being run then there's a good chance that you'll forget one of those test cases in the future. Trust me, a large, old, code base like that is terrifying to work with. Don't obsess over code coverage though, concentrate on areas of risk and scenarios, I've written code with 100% coverage that still had an exciting range of bugs, and I've seen extremely robust code that had relatively low coverage overall but had tests focusing on the really tricky areas.