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:
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)))
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.writecreate promises. The
let* ... inconstruct is used to wait for a promise to become determined; the code after
inis scheduled to run in a "callback."
Lwt.pickraces promises against each other, and behaves as the first one to complete.
Lwt_main.runforces 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_unixthat binds almost every Unix system call. A higher-level module
Lwt_ioprovides nice I/O channels.
Lwt_processis for subprocess handling.
Lwt_preemptivespawns system threads.
Lwt_reactfor reactive programming. See the table of contents on the linked manual pages!
opam install conf-libev lwt
We are currently working on improving the Lwt documentation (drastically; we are rewriting the manual). In the meantime:
Note: much of the current manual refers to
'a Lwt.tas "lightweight threads" or just "threads." This will be fixed in the new manual.
'a Lwt.tis 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.