[fulafel]

Type systems are of course not the only possible mechanism that can enforce these kinds of invariants. In dynamic languages, schema style solutions (eg malli or spec in Clojure) have advantages: they have more expressivity than typical type systems, you can manipulate them as data, you can use in contexts that are not statically verifiable, like at the data interfaces of yuor app.

[mejutoco]

I agree it has more flexibility, but more expressiveness is a double edged sword. Type systems are often not Turing-complete, or at least they are limited in some ways. A language being very expressive means it cannot be run at compile time, sine we cannot know if it terminates.

If we cannot run it at runtime now we need tests to hit that path or to test it manually to actually know if it works.