There are many ways to look at unit testing, but one of my favorites is to see it as an attempt to have a REPL in languages that don't have one.

When you have a REPL, you can call a function and learn how it works interactively. You get immediate feedback—you can have a conversation with it. In dynamically-typed languages this is easy. Most of them have a REPL. In statically-typed languages it is harder. The compiler needs to perform its checking. In the worst cases it has to check across a tr...