I'm one of the people who believe that TDD and safety-by-design (which is based on strong, static typing) are actually two sides of the same coin. In a sufficiently powerful type system, they could possibly even be two implementations of the same thing.

- Start with spec. - Write the specs as something that the compiler/test harness will check for you. - Write the code.

The main differences being, of course, that you'll typically get your compiler error messages much faster than your test error messages (typically while you're writing the code), which makes them easier to fix - on the other hand, in pretty much all languages, the type system isn't expressive enough to represent all the specs that can be represented as tests.

In other words, in the current state of the world, we need both.