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OCaml promises and concurrent I/O

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Lwt is a concurrent programming library for OCaml. It provides a single data type: the promise, which is a value that will become determined in the future. Creating a promise spawns a computation. When that computation is I/O, Lwt runs it in parallel with your OCaml code.

OCaml code, including creating and waiting on promises, is run in a single thread by default, so you don't have to worry about locking or preemption. You can detach code to be run in separate threads on an opt-in basis.

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

open Lwt.Syntax

let () = let request = let* addresses = Lwt_unix.getaddrinfo "" "80" [] in let google = Lwt_unix.((List.hd addresses).ai_addr) in

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


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

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

(* ocamlfind opt -package lwt.unix -linkpkg && ./a.out *)

In the program, functions such as

create promises. The
let* ... in
construct is used to wait for a promise to become determined; the code after
is scheduled to run in a "callback."
races promises against each other, and behaves as the first one to complete.
forces the whole promise-computation network to be executed. All the visible OCaml code is run in a single thread, but Lwt internally uses a combination of worker threads and non-blocking file descriptors to resolve in parallel the promises that do I/O.


Lwt compiles to native code on Linux, macOS, Windows, and other systems. It's also routinely compiled to JavaScript for the front end and Node by jsofocaml.

In Lwt,

  • The core library
    provides promises...
  • ...and a few pure-OCaml helpers, such as promise-friendly mutexes, condition variables, and mvars.
  • There is a big Unix binding,
    that binds almost every Unix system call. A higher-level module
    provides nice I/O channels.
  • Lwt_process
    is for subprocess handling.
  • Lwt_preemptive
    spawns system threads.
  • The PPX syntax allows using all of the above without going crazy!
  • There are also some other helpers, such as
    for reactive programming. See the table of contents on the linked manual pages!


  1. Use your system package manager to install a development libev package. It is often called
  2. opam install conf-libev lwt


We are currently working on improving the Lwt documentation (drastically; we are rewriting the manual). In the meantime:

  • The current manual can be found here.
  • Mirage has a nicely-written Lwt tutorial.
  • An example of a simple server written in Lwt.
  • Concurrent Programming with Lwt is a nice source of Lwt examples. They are translations of code from the excellent Real World OCaml, but are just as useful if you are not reading the book.

Note: much of the current manual refers to

'a Lwt.t
as "lightweight threads" or just "threads." This will be fixed in the new manual.
'a Lwt.t
is a promise, and has nothing to do with system or preemptive threads.


Open an issue, visit Discord chat, ask on, or on Stack Overflow.

Release announcements are made in /r/ocaml, and on Watching the repo for "Releases only" is also an option.


    contains tips for working on the code, such as how to check the code out, how review works, etc. There is also a high-level outline of the code base.
  • Ask us anything, whether it's about working on Lwt, or any question at all about it :)
  • The documentation always needs proofreading and fixes.
  • You are welcome to pick up any other issue, review a PR, add your opinion, etc.
  • Any feedback is welcome, including how to make contributing easier!

Libraries to use with Lwt

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