Do unit tests or type systems make sense when the product changes from week to week? Unit tests and type systems are a formal method (technically; usually they

Do unit tests or type systems make sense when the product changes from week to week? Unit tests and type systems are a formal method (technically; usually they usually do not fall under the academic discipline of formal methods). Formal methods cover a very wide range of tools, that drastically differ in cost and effort. Also, if you have code, then you have a spec, because the code is itself a formal specification. It's just not a very convenient one because it is a very low-level spec.