OCaml has higher-kinded types in the guise of modules, and so does SML with common extensions. I'm not completely sure what you're implying, but type classes ar

OCaml has higher-kinded types in the guise of modules, and so does SML with common extensions. I'm not completely sure what you're implying, but type classes aren't required for higher kinds. OCaml has a powerful module system, labeled and optional function parameters, row polymorphism, equirecursive types, and GADTs. Most of these features would be fairly difficult to add to Rust. OCaml also has a better track record with soundness issues than Rust, and more interesting fragments of the language have been formalized.