package lwt

  1. Overview
  2. Docs
Promises and event-driven I/O

Install 

dune-project
 Dependency

github.com Readme MIT License Edit opam file Versions (29)

Authors

  1. J
    Jérôme Vouillon
  2. J
    Jérémie Dimino

Maintainers

  1. R
    Raphaël Proust <code@bnwr.net>
  2. Anton Bachin

Sources

6.0.0-alpha00.tar.gz
md5=8a88c5d5fcb01d179bbb12e6b6888dca
sha512=bfecd3fa0a3461a3368bd7724e2fce7a751ba3899d590d5b08bc4257b058ca34ec25de1a3ead6585528afe77c6ca6db1c3e2d2421d6a773c845ef091472d2ed9

Description

A promise is a value that may become determined in the future.

Lwt provides typed, composable promises. Promises that are resolved by I/O are resolved by Lwt in parallel.

Meanwhile, OCaml code, including code creating and waiting on promises, runs in a single thread by default. This reduces the need for locks or other synchronization primitives. Code can be run in parallel on an opt-in basis.

Published: 24 Jul 2025

README

Lwt

version GitHub Actions status

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.


Overview

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 js_of_ocaml.

In Lwt,

  • The core library Lwt provides promises...
  • ...and a few pure-OCaml helpers, such as promise-friendly mutexes, condition variables, and mvars.
  • There is a big Unix binding, Lwt_unix that binds almost every Unix system call. A higher-level module Lwt_io 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 Lwt_react for reactive programming. See the table of contents on the linked manual pages!

Installing

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

Documentation

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.


Contact

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

Release announcements are made on discuss.ocaml.org. Watching the repo for "Releases only" is also an option.


Contributing

  • CONTRIBUTING.md 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

Dependencies (5)

  1. ocplib-endian
  2. dune-configurator
  3. cppo build & >= "1.1.0"
  4. ocaml >= "4.08"
  5. dune >= "2.7"

Dev Dependencies (3)

  1. bisect_ppx with-test
  2. odoc with-doc & >= "2.3.0"
  3. ocamlfind with-dev-setup & >= "1.7.3-1"

  1. 0install >= "2.15.1"
  2. aches-lwt
  3. activitypub
  4. albatross
  5. alcotest-lwt
  6. alcotest-mirage
  7. ambient-context-lwt
  8. amqp-client >= "1.1.0"
  9. amqp-client-lwt
  10. angstrom-lwt-unix >= "0.11.0"
  11. anthill
  12. anycache-lwt
  13. archi-lwt
  14. arp
  15. awa-mirage
  16. aws-lwt
  17. aws-s3-lwt < "4.4.0" | >= "4.8.1"
  18. awsm-lwt
  19. azure-cosmos-db
  20. balancer
  21. bastet_lwt
  22. bistro
  23. brisk-reconciler
  24. brozip
  25. builder
  26. builder-web
  27. bun >= "0.3.3"
  28. cachet-lwt
  29. calculon
  30. caldav
  31. camltc
  32. canary
  33. capnp-rpc-lwt < "2.0"
  34. capnp-rpc-unix < "2.1"
  35. caqti-mirage
  36. carton < "1.0.0"
  37. carton-git
  38. carton-lwt
  39. catala-format >= "0.2.0"
  40. cf-lwt
  41. chamelon
  42. chamelon-unix
  43. chamo
  44. charrua-client
  45. charrua-unix
  46. chess_com_api
  47. clz
  48. cmdtui-lambda-term
  49. coap
  50. coap-server-lwt
  51. cohttp-curl-lwt
  52. cohttp-lwt
  53. cohttp-lwt-jsoo
  54. cohttp-lwt-unix
  55. cohttp-mirage
  56. cohttp-server-lwt-unix
  57. comby
  58. comby-semantic
  59. conan-lwt
  60. conduit-lwt
  61. conduit-lwt-unix
  62. cowabloga
  63. crunch
  64. cstruct-lwt
  65. csv-lwt
  66. ctypes >= "0.15.0" & < "0.21.1"
  67. ctypes-foreign >= "0.21.1"
  68. current
  69. current-albatross-deployer
  70. current_docker
  71. current_examples
  72. current_git
  73. current_github
  74. current_gitlab
  75. current_ocluster
  76. current_rpc
  77. current_slack
  78. current_web
  79. DkSDKFFIOCaml_Std
  80. dap
  81. data-encoding < "0.1.1"
  82. devkit >= "1.2"
  83. distributed-lwt
  84. dkim-bin < "0.8.0"
  85. dkim-lwt-unix
  86. dkim-mirage
  87. dlm
  88. dmarc
  89. dns-certify
  90. dns-cli
  91. dns-client < "7.0.3"
  92. dns-client-lwt
  93. dns-client-mirage
  94. dns-forward
  95. dns-forward-lwt-unix
  96. dns-lwt
  97. dns-mirage
  98. dns-resolver
  99. dns-server
  100. dns-stub
  101. dnssd
  102. docker_hub
  103. docteur >= "0.0.2"
  104. docteur-solo5
  105. docteur-unix >= "0.0.5"
  106. doi2bib
  107. dream
  108. dream-httpaf
  109. dream-pure
  110. dream-serve
  111. dropbox
  112. dune >= "3.17.2"
  113. dune-rpc-lwt
  114. earlybird
  115. elasticsearch-cli
  116. emoji = "2.0.0"
  117. equinoxe
  118. ethernet
  119. ez_api >= "1.2.0"
  120. ezcurl-lwt
  121. ezjs_min < "0.2"
  122. ezjsonm-lwt
  123. ezresto
  124. ezresto-directory >= "0.5"
  125. faraday-lwt
  126. faraday-lwt-unix
  127. fat-filesystem
  128. fiber-lwt
  129. fsevents-lwt
  130. fswatch_lwt
  131. gdbprofiler
  132. git
  133. git-cohttp
  134. git-cohttp-unix
  135. git-kv >= "0.2.0"
  136. git-mirage
  137. git-paf
  138. git-unix >= "3.2.0"
  139. github
  140. github-hooks
  141. github-unix >= "4.4.0"
  142. gitlab-unix
  143. gitlab_pipeline_notifier
  144. gluten-lwt
  145. gluten-lwt-unix < "0.4.0"
  146. gluten-mirage < "0.4.0"
  147. graphql-lwt
  148. gremlin
  149. grpc-lwt
  150. guardian
  151. gufo
  152. h1
  153. h1-lwt-unix
  154. h2-lwt
  155. h2-lwt-unix < "0.10.0"
  156. h2-mirage
  157. happy-eyeballs-lwt
  158. happy-eyeballs-mirage
  159. hidapi-lwt
  160. hiredis >= "0.6"
  161. hl_yaml
  162. hockmd
  163. http-lwt-client
  164. http-mirage-client
  165. http-multipart-formdata >= "2.0.0" & < "3.0.0"
  166. httpaf-lwt-unix
  167. httpun-lwt
  168. httpun-mirage
  169. httpun-ws-lwt
  170. hvsock
  171. i3ipc
  172. influxdb-lwt
  173. inotify >= "2.4"
  174. inquire < "0.3.0"
  175. interface-prime-lwt
  176. ip2location
  177. ip2locationio
  178. ip2whois
  179. ipv6-multicast-lwt
  180. irc-client-lwt
  181. irc-client-lwt-ssl
  182. irc-client-tls
  183. irmin
  184. irmin-bench
  185. irmin-chunk
  186. irmin-cli
  187. irmin-client
  188. irmin-containers
  189. irmin-fs
  190. irmin-git
  191. irmin-graphql
  192. irmin-http
  193. irmin-indexeddb
  194. irmin-layers
  195. irmin-mirage-git
  196. irmin-mirage-graphql
  197. irmin-pack
  198. irmin-server
  199. irmin-test
  200. irmin-unix
  201. irmin-watcher
  202. joolog
  203. jose < "0.9.0"
  204. js_of_ocaml-lwt >= "3.5.0"
  205. jsoo_broadcastchannel
  206. jsoo_storage
  207. jupyter
  208. jupyter-kernel
  209. kafka < "0.5"
  210. kafka_lwt
  211. kappa-library
  212. ke >= "0.5"
  213. kinetic-client
  214. kubecaml
  215. lambda-runtime
  216. lambda-term >= "3.3.3"
  217. lambda_streams_lwt
  218. launchd
  219. ldp
  220. learn-ocaml
  221. learn-ocaml-client
  222. ledgerwallet >= "0.4.0"
  223. letsencrypt
  224. letsencrypt-app
  225. letsencrypt-dns
  226. letters
  227. lichess_api
  228. links >= "0.9.1"
  229. llama
  230. lru_cache
  231. lwt-canceler
  232. lwt-dllist
  233. lwt-exit
  234. lwt-parallel
  235. lwt-pipe
  236. lwt-pipeline
  237. lwt-watcher
  238. lwt_camlp4
  239. lwt_direct
  240. lwt_ppx = "6.0.0"
  241. lwt_react
  242. lwt_retry
  243. lwt_ssl
  244. mariadb >= "1.2.0"
  245. markup = "0.7.6"
  246. markup-lwt
  247. mdx
  248. mechaml
  249. mehari-lwt-unix
  250. mehari-mirage
  251. memtrace-mirage
  252. metrics-influx
  253. metrics-lwt
  254. metrics-unix
  255. mimic
  256. mindstorm-lwt
  257. mirage < "4.0.0"
  258. mirage-block >= "2.0.1"
  259. mirage-block-ccm
  260. mirage-block-combinators
  261. mirage-block-lwt
  262. mirage-block-partition
  263. mirage-block-ramdisk
  264. mirage-block-solo5
  265. mirage-block-unikraft
  266. mirage-block-unix >= "2.14.2"
  267. mirage-block-xen
  268. mirage-channel >= "4.0.1"
  269. mirage-channel-lwt
  270. mirage-clock-lwt
  271. mirage-clock-unix < "4.2.0"
  272. mirage-console-lwt
  273. mirage-crypto-rng < "0.11.3"
  274. mirage-crypto-rng-lwt
  275. mirage-crypto-rng-mirage
  276. mirage-device >= "2.0.0"
  277. mirage-flow >= "3.0.0"
  278. mirage-flow-combinators
  279. mirage-flow-lwt
  280. mirage-flow-unix
  281. mirage-fs >= "4.0.0"
  282. mirage-fs-lwt
  283. mirage-kv >= "3.0.1"
  284. mirage-kv-lwt
  285. mirage-kv-unix
  286. mirage-net >= "4.0.0"
  287. mirage-net-lwt
  288. mirage-net-macosx
  289. mirage-net-solo5
  290. mirage-net-unikraft
  291. mirage-net-unix
  292. mirage-net-xen
  293. mirage-profile
  294. mirage-protocols >= "7.0.0"
  295. mirage-protocols-lwt
  296. mirage-qubes
  297. mirage-qubes-ipv4
  298. mirage-runtime
  299. mirage-sleep
  300. mirage-solo5
  301. mirage-stack = "3.0.0"
  302. mirage-stack-lwt
  303. mirage-time >= "3.0.0"
  304. mirage-time-lwt
  305. mirage-time-unix
  306. mirage-types-lwt
  307. mirage-unikraft
  308. mirage-unix
  309. mirage-vnetif
  310. mirage-xen
  311. monorobot
  312. mqtt
  313. mrmime >= "0.5.0"
  314. multipart-form-data
  315. multipart_form >= "0.2.0" & < "0.4.0"
  316. multipart_form-cohttp-lwt < "0.6.0"
  317. multipart_form-lwt
  318. naboris
  319. nbd >= "4.0.3"
  320. nbd-tool
  321. nbd-unix
  322. neo4j_bolt
  323. nottui-lwt
  324. notty-community
  325. nproc
  326. nsq
  327. obuilder
  328. obus >= "1.2.1"
  329. ocluster-api
  330. ocluster-worker
  331. ocplib-resto
  332. ocsigenserver
  333. ocsipersist
  334. ocsipersist-dbm
  335. ocsipersist-lib
  336. ocsipersist-pgsql
  337. ocsipersist-sqlite
  338. octez-distributed-lwt-internal
  339. octez-internal-libs
  340. octez-l2-libs
  341. octez-libs
  342. octez-proto-libs
  343. octez-protocol-compiler
  344. octez-proxy-server
  345. octez-rpc-process
  346. octez-shell-libs
  347. octez-smart-rollup-wasm-benchmark-lib
  348. oframl
  349. ojs_base
  350. omigrate
  351. oneffs
  352. opam-check-npm-deps >= "4.1.0"
  353. opencage
  354. opentelemetry-client-cohttp-lwt
  355. opentelemetry-cohttp-lwt >= "0.4"
  356. opentelemetry-lwt
  357. opium
  358. opium-graphql
  359. opium_kernel
  360. opomodoro
  361. order-i3-xfce
  362. ordma
  363. oskel >= "0.3.0"
  364. ounit-lwt < "2.2.0"
  365. ounit2-lwt
  366. owork
  367. ozulip
  368. paf
  369. paf-cohttp
  370. passage < "0.1.8"
  371. pcap-format < "0.5.2"
  372. petrol
  373. pgn_parser
  374. pgx_lwt
  375. pgx_lwt_mirage
  376. pgx_lwt_unix < "2.0"
  377. piaf < "0.2.0"
  378. picos_meta
  379. plebeia >= "2.0.0"
  380. plotkicadsch
  381. ppx_defer >= "0.4.0"
  382. ppx_deriving_rpc
  383. ppx_rapper_lwt
  384. proc-smaps
  385. prof_spacetime
  386. prometheus
  387. prometheus-app
  388. promise_jsoo_lwt
  389. protocol-9p
  390. protocol-9p-unix
  391. proton
  392. qcow
  393. qcow-stream
  394. qcow-tool
  395. qcow-types
  396. qdrant
  397. qfs >= "0.5"
  398. quests
  399. rawlink < "2.1"
  400. rawlink-lwt
  401. rdf_json_ld
  402. rdf_lwt
  403. redis-lwt
  404. reparse-lwt
  405. reparse-lwt-unix
  406. resource-pooling
  407. resp
  408. resp-mirage >= "0.10.0"
  409. resp-unix >= "0.10.0"
  410. resto
  411. resto-cohttp-client >= "0.4"
  412. resto-cohttp-self-serving-client
  413. resto-cohttp-server >= "0.4" & < "0.6" | >= "0.9"
  414. resto-directory >= "0.4"
  415. riak
  416. ringo-lwt
  417. river
  418. rock
  419. rpclib-js
  420. rpclib-lwt
  421. SZXX < "4.0.0"
  422. sanddb
  423. scgi
  424. sendmail-lwt
  425. sendmail-mirage
  426. serial
  427. server-reason-react
  428. session-cohttp-lwt
  429. session-cookie-lwt
  430. session-postgresql-lwt
  431. sessions
  432. shared-block-ring
  433. shared-memory-ring-lwt
  434. sherlodoc
  435. sihl < "0.2.0"
  436. slack
  437. slacko
  438. slipshow
  439. smtml >= "0.7.0"
  440. speed
  441. spin < "0.8.0"
  442. spoke
  443. statocaml
  444. stk
  445. stog
  446. swapfs
  447. syguslib-utils
  448. syndic >= "1.4" & < "1.6.0"
  449. tar-mirage
  450. tar-unix
  451. tcpip
  452. telegraml
  453. terminus
  454. testo-lwt
  455. tezos-base >= "16.0"
  456. tezos-clic >= "16.0"
  457. tezos-crypto >= "16.0"
  458. tezos-crypto-dal
  459. tezos-error-monad >= "16.0"
  460. tezos-lwt-result-stdlib
  461. tezos-p2p = "12.3"
  462. tezos-protocol-environment
  463. tezos-proxy >= "17.3"
  464. tezos-stdlib
  465. tezos-stdlib-unix >= "16.0"
  466. tezos-test-helpers >= "12.3"
  467. tezos-webassembly-interpreter-extra
  468. tidy_email
  469. timmy-lwt
  470. tls >= "0.10.6" & < "0.16.0"
  471. tls-lwt
  472. tls-mirage
  473. tube
  474. tuntap
  475. twirp_cohttp_lwt_unix
  476. uring
  477. uspf
  478. uspf-lwt
  479. uspf-mirage
  480. utop
  481. uwt
  482. vchan
  483. vchan-unix
  484. vchan-xen
  485. vercel
  486. vhd-format-lwt
  487. vmnet
  488. vpnkit
  489. vue-jsoo < "0.3"
  490. wayland < "2.0"
  491. webauthn
  492. xen-evtchn
  493. xen-evtchn-unix
  494. xen-gnt
  495. xen-gnt-unix
  496. xenstore
  497. xenstore-tool
  498. xenstore_transport
  499. xlsx2csv
  500. yocaml_git
  501. yocaml_unix < "2.0.0"
  502. zarr-lwt
  503. zmq-lwt >= "5.2.1"

Conflicts

None