I think I agree with your claim that writing tests would be a super set in terms of covering the use cases of interfaces. But 1) Testing is such a PAIN in Juli

I think I agree with your claim that writing tests would be a super set in terms of covering the use cases of interfaces. But 1) Testing is such a PAIN in Julia. You HAVE to run all the tests every time. You HAVE to write tests in a separate folder. Multiple dispatch and composibility prevent having confidence that your tests cover all the cases. 2) In a lot of cases, interfaces improve readability of the code. Being able to just look at code in a large code base and know which interfaces are implemented + need to be implemented is such an advantage 3) Static analysis tooling can provide linting. This doesn't even have to be implemented by the core team at the moment. The lack of interface limits any kind of tooling to be developed. All in all, when I write Julia code, I know I have to write EXTENSIVE tests. Even more tests than I have to write with Python (with mypy). Almost an order of magnitude more tests than I have to write with Rust. Sometimes it feels like I spend more time writing / running tests than adding features to our packages. And that is honestly just not fun for me, let alone for other scientists and researchers on my team.