But you have to define the types for them to work. That's exactly the problem and that's what Julia has already solved.

What are user defined types that don’t require being defined?

Generic functions in Julia can use types which have no reference to their definition using dependent compilation.

http://www.stochasticlifestyle.com/why-numba-and-cython-are-...

So you can work with types even when you've never seen their definition given how the compilation will occur with all of the pieces together instead of separately.

I usually profit from type definitions during debugging and memory layout work, so perhaps i don’t see this is a feature.

It's probably us that are using types in a weird way so it's my lack of explanation that's the issue. Types in Julia undergo multiple dispatch, so by passing a type into a generic function you can make the same generic algorithm run in different ways. So I use types to parallelize my code, calculate derivatives, propagate parameter uncertainties, and things like that. This talk from JuliaCon discusses a lot of the things for free that were developed by taking a type from one package and putting it into another:

https://www.youtube.com/watch?v=dmWQtI3DFFo

thank you