I think Julia (mostly meaning pervasive multiple dispatch, which is unprecedented at that scale Julia has it) unlocks a new way to organize programs. Some form

I think Julia (mostly meaning pervasive multiple dispatch, which is unprecedented at that scale Julia has it) unlocks a new way to organize programs. Some formalized notion of "interface" is certainly important, but it seems no clear formulation has emerged. I think it's fairly consistent with the dynamic nature of Julia and fairly inconsistent with the static nature of Julia. I don't know what a good solution to the interface problem is. I think one can get pretty far with writing tests to check interfaces. If you are a library that expects user-defined types, you can expose an interface to these tests so that a user can check if they've implemented everything. This is a very generic approach, and aside from the key limitation of only giving results at runtime, is much more powerful.