This idea is the basis for one of the popular HTTP libraries in OCaml, cohttp [1]. The core library (see the lib/ directory in the source code) implements the H

This idea is the basis for one of the popular HTTP libraries in OCaml, cohttp [1]. The core library (see the lib/ directory in the source code) implements the HTTP protocol, but makes no attempt to perform IO. The file lib/s.mli describes the interface that may be used to perform the IO for the library, making use of OCaml's module system to work generically over multiple interfaces. What's interesting about this approach is that it almost "generates the code" for IO once you plug it in to the implementation code (i.e. all you have to do is provide a handful of very basic functions and types and it will make use of those to construct the rest of the library). [1] - https://github.com/mirage/ocaml-cohttp https://github.com/mirage/ocaml-cohttp