ocaml-cohttp

Here's a common example. Some systems provide blocking operations: e.g. read_line waits for a line of text and then returns it as a string. Others use promises: read_line returns a promise of a string. What if you want to write a library that works with either blocking calls or asynchronous promises? The cohttp HTTP library is written that way. For example, the Transfer_io module[1] (supporting both chunking and non-chunking HTTP transfers) takes as an argument another module, IO[2], that provides a read_line function of type "ic -> string option t", where the type t is abstract and higher-kinded (and "ic" = input channel). You can instantiate the module with a blocking IO module (where a "string t" is just the same as a "string" and read_line blocks) or with a non-blocking one (where a "string t" is a promise for a "t" and read_line retu

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

FWIW, the current preferred web server at JS seems to be https://github.com/mirage/ocaml-cohttp (which supports both Async and Lwt). I wouldn't go so far as to call it a framework, though.