Modules and parametric modules, subtyping (polymorphic variants [1] and objects), proper records, classes making bindings to OOP APIs natural, strict evaluation, ppx extensions [2].

[1] http://keleshev.com/composable-error-handling-in-ocaml

[2] https://victor.darvariu.me/jekyll/update/2018/06/19/ppx-tuto...