As the Dijkstrian expression goes, testing shows the presence of bugs, not their absence. Unit tests can show that a bug exists, but it cannot show you that the

As the Dijkstrian expression goes, testing shows the presence of bugs, not their absence. Unit tests can show that a bug exists, but it cannot show you that there are no bugs, save for the particular cases tested and even then, only in a behaviorist sort of way (meaning, a your buggy code may still produce the expected output for tested cases). For that, you need to be able to prove your code possesses certain properties. Type systems and various forms of static analysis are going to increasingly shape the future of software development, I think. Large software systems especially become practically impossible to work with and impossible to verify and test without types.