Sounds like Nim. Nim doesn't have objects but it has something it calls universal function call syntax/uniform call syntax which lets you prefix a function call with the first argument instead of putting it in the argument list. This makes it look like object oriented coding without having to put the structure definition in the same file as the methods that act on it. This means that new methods can be added without having to edit the source of the structure or inherit from it.

If I’m writing a hash map module that’s generic over what it accepts, and later someone else is going to want to use it with Foo keys, how am I able to write my hash map such that the hash function for Foos are in scope in my hash map module?