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 "google.com" "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))) in let timeout = let* () = 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 (* ocamlfind opt -package lwt.unix -linkpkg example.ml && ./a.out *)
In the program, functions such as
Lwt_io.write create promises. The
let* ... in construct is used to wait for a promise to become determined; the code after
in is scheduled to run in a "callback."
Lwt.pick races promises against each other, and behaves as the first one to complete.
Lwt_main.run 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.
The core library
Lwt_processis for subprocess handling.
Lwt_preemptivespawns system threads.
The PPX syntax allows using all of the above without going crazy!
There are also some other helpers, such as
Lwt_reactfor reactive programming. See the table of contents on the linked manual pages!
Use your system package manager to install a development libev package. It is often called
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.
CONTRIBUTING.mdcontains 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
alcotest — unit testing
angstrom — parser combinators
cohttp — HTTP client and server
cstruct — interop with C-like structures
ezjsonm — JSON parsing and output
faraday — serialization combinators
logs — logging
lwt-parallel — distributed computing
mwt — preemptive (system) thread pools
opium — web framework