OCaml has a typesystem in which modules are first-class citizens, just like functions. They have a clear separation between interface (they call it "signatures"

OCaml has a typesystem in which modules are first-class citizens, just like functions. They have a clear separation between interface (they call it "signatures") and implementation. The compiler enforces that you can only write against the interface, making modules with the same interface really exchangeable. The modules are also parametrizable (they call such modules "functors"). http://caml.inria.fr/pub/docs/manual-ocaml/moduleexamples.html http://caml.inria.fr/pub/docs/manual-ocaml/moduleexamples.ht... https://realworldocaml.org/v1/en/html/functors.html https://realworldocaml.org/v1/en/html/functors.html https://realworldocaml.org/v1/en/html/first-class-modules.html https://realworldocaml.org/v1/en/html/first-class-modules.ht...