[QuackingTheQ]

It's possible to hack interface verification into place at test-time, but that has a couple of problems:

1. Running the whole testing framework to determine if you implemented an interface is a high overhead when you're developing

2. You have a lot of tests to write to really check every error. Perhaps a package which defines an interface could provide a tester for this purpose

3. Interfaces should be attached to the types, and that should be sufficient for verifying the interface

I would settle for something like checking for the implementation of methods a la BinaryTraits.jl over what we have now, which is nothing. A huge step would be documentation and automated testing that proper interface methods are implemented, not even verifying if they're "correct". This drastically reduces the surface area you need to write and check to confirm compatibility with outside code.

This simple interface specification does produce design issues of its own, but correctness is much easier to handle if you know what needs to be correct in the first place.