I see. I wasn't aware that Nix was a build system and probably should have investigated a bit more before responding. Truthfully I don't use OCaml professionally but just in a hobby capacity. I believe you could roll your own using Make but the compiler invocations (which you can get dune to spit out if you want) become more complex over time the more features you add like PPXs, or `ctypes`, and unit testing, the more time you will save by using dune. tl;dr running ocamlopt directly works but will become cumbersome the bigger the project becomes