[MoreQARespect]

I dont think tests and types are the same "thing" per se - they work vastly better in conjunction with each other than alone and are weirdly symmetrical in the way that theyre bad substitutes for each other.

However, Im convinced that theyre both part of the same class of thing, and that "TDD" or red/green/refactor or whatever you call it works on that class, not specifically just on tests.

Documentation is a funny one too - I use my types to generate API and other sorts of reference docs and tests to generate how-to docs. There is a seemingly inextricable connection between types and reference docs, tests and how-to docs.

[eyelidlessness]

Types are a kind of test. Specifically they’re a way to assert certain characteristics about the interactions between different parts of the code. They’re frequently assertions you’d want to make another way, if you didn’t have the benefit of a compiler to run that set of assertions for you. And like all tests, they’re a means to gain or reinforce confidence in claims you could make about the code’s behavior. (Which is their symmetry with documentation.)