I can't repeat this enough: add as much type safety to your codebases as the maintaining team feels comfortable where changing remains ergonomic AND add good meaningful tests: both property-based and edge/boundary case tests.
Types and tests aren't mutually exclusive. Have both.
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Are types not tests?
2
Are successful test runs not existence proofs? 🙃
Is a stopped watch not right twice a day?
("Well, what if those are the only times that matter?" he asked, screaming while exiting....)