Lwt provides lightweight (a.k.a. cooperative or green) threads for OCaml. Normally-blocking operations can be run concurrently in a single OCaml process, without managing system threads or locking. Lwt threads are type-disciplined and composable – Lwt is a monad.

Here is a simplistic program which requests the Google search page, and fails if the request is not completed in five seconds:

let () =
  let request =
    let%lwt addresses = Lwt_unix.getaddrinfo "google.com" "80" [] in
    let server = (List.hd addresses).Lwt_unix.ai_addr in

    Lwt_io.(with_connection server (fun (incoming, outgoing) ->
      let%lwt () = write outgoing "GET / HTTP/1.1\r\n" in
      let%lwt () = write outgoing "Connection: close\r\n\r\n" in
      let%lwt response = read incoming in
      Lwt.return (Some response)))
  in

  let timeout =
    let%lwt () = Lwt_unix.sleep 5. in
    Lwt.return None
  in

  match Lwt_main.run (Lwt.pick [request; timeout]) with
  | Some response -> print_string response
  | None -> prerr_endline "Request timed out"; exit 1

The above program can be compiled and run with

ocamlfind opt -package lwt.unix -package lwt.ppx -linkpkg -o request example.ml
./request

opam install lwt

Documentation can be found here. There are also some examples available in doc/examples.

Open an issue, visit Gitter chat, email the maintainer, or ask in #ocaml. If you think enough people will be interested in the answer, it is also possible to ask on Stack Overflow.

Lwt is a very mature library, but there is considerable room for improvement. Contributions are welcome. To clone the source and install a development version,

opam source --dev-repo --pin lwt

This will also install the development dependency OASIS.

A list of project suggestions and a roadmap can be found on the wiki.

Lwt is released under the LGPL, with the OpenSSL linking exception. See COPYING.