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 (30)

Authors

  1. J
    Jérôme Vouillon
  2. J
    Jérémie Dimino

Maintainers

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

Sources

6.0.0-beta01.tar.gz
md5=6bb978c590433a218a53722b985a01d4
sha512=529c8c920716cf204efd60a2bad3f817eed6eed120a8e8a8dae763ab7f4c45799a74004d0d0a2b8ec08f37c81b77f1a4813426a25f256b254dfa846c86debe44

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: 15 Dec 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"
  4. ocaml >= "4.14"
  5. dune >= "3.15"

Dev Dependencies (2)

  1. odoc with-doc & >= "2.3"
  2. 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. faraday-lwt
  125. faraday-lwt-unix
  126. fat-filesystem
  127. fiber-lwt
  128. fsevents-lwt
  129. fswatch_lwt
  130. gdbprofiler
  131. git
  132. git-cohttp
  133. git-cohttp-unix
  134. git-kv >= "0.2.0"
  135. git-mirage
  136. git-paf
  137. git-unix >= "3.2.0"
  138. github
  139. github-hooks
  140. github-unix >= "4.4.0"
  141. gitlab-unix
  142. gitlab_pipeline_notifier
  143. gluten-lwt
  144. gluten-lwt-unix < "0.4.0"
  145. gluten-mirage < "0.4.0"
  146. graphql-lwt
  147. gremlin
  148. grpc-lwt
  149. guardian
  150. gufo
  151. h1
  152. h1-lwt-unix
  153. h2-lwt
  154. h2-lwt-unix < "0.10.0"
  155. h2-mirage
  156. happy-eyeballs-lwt
  157. happy-eyeballs-mirage
  158. hidapi-lwt
  159. hiredis >= "0.6"
  160. hl_yaml
  161. hockmd
  162. http-lwt-client
  163. http-mirage-client
  164. http-multipart-formdata >= "2.0.0" & < "3.0.0"
  165. httpaf-lwt-unix
  166. httpun-lwt
  167. httpun-mirage
  168. httpun-ws-lwt
  169. hvsock
  170. i3ipc
  171. influxdb-lwt
  172. inotify >= "2.4"
  173. inquire < "0.3.0"
  174. interface-prime-lwt
  175. ip2location
  176. ip2locationio
  177. ip2whois
  178. ipv6-multicast-lwt
  179. irc-client-lwt
  180. irc-client-lwt-ssl
  181. irc-client-tls
  182. irmin
  183. irmin-bench
  184. irmin-chunk
  185. irmin-cli
  186. irmin-client
  187. irmin-containers
  188. irmin-fs
  189. irmin-git
  190. irmin-graphql
  191. irmin-http
  192. irmin-indexeddb
  193. irmin-layers
  194. irmin-mirage-git
  195. irmin-mirage-graphql
  196. irmin-pack
  197. irmin-server
  198. irmin-test
  199. irmin-unix
  200. irmin-watcher
  201. joolog
  202. jose < "0.9.0"
  203. js_of_ocaml-lwt >= "3.5.0"
  204. jsoo_broadcastchannel
  205. jsoo_storage
  206. jupyter
  207. jupyter-kernel
  208. kafka < "0.5"
  209. kafka_lwt
  210. kappa-library
  211. ke >= "0.5"
  212. kinetic-client
  213. kubecaml
  214. lambda-runtime
  215. lambda-term >= "3.3.3"
  216. lambda_streams_lwt
  217. launchd
  218. ldp
  219. learn-ocaml
  220. learn-ocaml-client
  221. ledgerwallet >= "0.4.0"
  222. letsencrypt
  223. letsencrypt-app
  224. letsencrypt-dns
  225. letters
  226. lichess_api
  227. links >= "0.9.1"
  228. llama
  229. lru_cache
  230. lwt-canceler
  231. lwt-dllist
  232. lwt-exit
  233. lwt-parallel
  234. lwt-pipe
  235. lwt-pipeline
  236. lwt-watcher
  237. lwt_camlp4
  238. lwt_direct >= "6.0.0"
  239. lwt_ppx >= "6.0.0"
  240. lwt_react
  241. lwt_retry
  242. lwt_ssl
  243. mariadb >= "1.2.0"
  244. markdown_monolith
  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-sleep
  299. mirage-solo5
  300. mirage-stack = "3.0.0"
  301. mirage-stack-lwt
  302. mirage-time >= "3.0.0"
  303. mirage-time-lwt
  304. mirage-time-unix
  305. mirage-types-lwt
  306. mirage-unikraft
  307. mirage-unix
  308. mirage-vnetif
  309. mirage-xen
  310. monorobot
  311. mqtt
  312. mrmime >= "0.5.0"
  313. multipart-form-data
  314. multipart_form >= "0.2.0" & < "0.4.0"
  315. multipart_form-cohttp-lwt < "0.6.0"
  316. multipart_form-lwt
  317. naboris
  318. nbd >= "4.0.3"
  319. nbd-tool
  320. nbd-unix
  321. neo4j_bolt
  322. nottui-lwt
  323. notty-community
  324. nproc
  325. nsq
  326. obuilder
  327. obus >= "1.2.1"
  328. ocluster-api
  329. ocluster-worker
  330. ocplib-resto
  331. ocsigenserver
  332. ocsipersist
  333. ocsipersist-dbm
  334. ocsipersist-lib
  335. ocsipersist-pgsql
  336. ocsipersist-sqlite
  337. octez-distributed-lwt-internal
  338. octez-internal-libs
  339. octez-l2-libs
  340. octez-libs
  341. octez-proto-libs
  342. octez-protocol-compiler
  343. octez-proxy-server
  344. octez-rpc-process
  345. octez-shell-libs
  346. octez-smart-rollup-wasm-benchmark-lib
  347. oframl
  348. ojs_base
  349. omigrate
  350. oneffs
  351. opam-check-npm-deps >= "4.1.0"
  352. opencage
  353. opentelemetry-client-cohttp-lwt
  354. opentelemetry-cohttp-lwt >= "0.4"
  355. opentelemetry-lwt
  356. opium
  357. opium-graphql
  358. opium_kernel
  359. opomodoro
  360. order-i3-xfce
  361. ordma
  362. oskel >= "0.3.0"
  363. ounit-lwt < "2.2.0"
  364. ounit2-lwt
  365. owork
  366. ozulip
  367. paf
  368. paf-cohttp
  369. passage < "0.1.8"
  370. pcap-format < "0.5.2"
  371. petrol
  372. pgn_parser
  373. pgx_lwt
  374. pgx_lwt_mirage
  375. pgx_lwt_unix < "2.0"
  376. piaf < "0.2.0"
  377. picos_meta
  378. plebeia >= "2.0.0"
  379. plotkicadsch
  380. ppx_defer >= "0.4.0"
  381. ppx_deriving_rpc
  382. ppx_rapper_lwt
  383. proc-smaps
  384. prof_spacetime
  385. prometheus
  386. prometheus-app
  387. promise_jsoo_lwt
  388. protocol-9p
  389. protocol-9p-unix
  390. proton
  391. pxshot
  392. qcow
  393. qcow-stream
  394. qcow-tool
  395. qcow-types
  396. qdrant < "0.2.0"
  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-server = "0.4"
  413. resto-directory = "0.4"
  414. riak
  415. ringo-lwt
  416. river
  417. rock
  418. rpclib-js
  419. rpclib-lwt
  420. SZXX < "4.0.0"
  421. sanddb
  422. scgi
  423. sendmail-lwt
  424. sendmail-mirage
  425. serial
  426. server-reason-react
  427. session-cohttp-lwt
  428. session-cookie-lwt
  429. session-postgresql-lwt
  430. sessions
  431. shared-block-ring
  432. shared-memory-ring-lwt
  433. sherlodoc
  434. sihl < "0.2.0"
  435. slack
  436. slacko
  437. slipshow
  438. smtml >= "0.7.0"
  439. speed
  440. spin < "0.8.0"
  441. spoke
  442. statocaml
  443. stk
  444. stog
  445. swapfs
  446. syguslib-utils
  447. syndic >= "1.4" & < "1.6.0"
  448. tar-mirage
  449. tar-unix
  450. tcpip
  451. telegraml
  452. terminus
  453. testo-lwt
  454. tezos-base >= "16.0"
  455. tezos-clic >= "16.0"
  456. tezos-crypto >= "16.0"
  457. tezos-crypto-dal
  458. tezos-error-monad >= "16.0"
  459. tezos-lwt-result-stdlib
  460. tezos-p2p = "12.3"
  461. tezos-protocol-environment
  462. tezos-proxy >= "17.3"
  463. tezos-stdlib
  464. tezos-stdlib-unix >= "16.0"
  465. tezos-test-helpers >= "12.3"
  466. tezos-webassembly-interpreter-extra
  467. tidy_email
  468. timmy-lwt
  469. tls >= "0.10.6" & < "0.16.0"
  470. tls-lwt
  471. tls-mirage
  472. tube
  473. tuntap
  474. twirp_cohttp_lwt_unix
  475. uring
  476. uspf
  477. uspf-lwt
  478. uspf-mirage
  479. utop
  480. uwt
  481. vchan
  482. vchan-unix
  483. vchan-xen
  484. vercel
  485. vhd-format-lwt
  486. vmnet
  487. vpnkit
  488. vue-jsoo < "0.3"
  489. wayland < "2.0"
  490. webauthn
  491. xen-evtchn
  492. xen-evtchn-unix
  493. xen-gnt
  494. xen-gnt-unix
  495. xenstore
  496. xenstore-tool
  497. xenstore_transport
  498. xlsx2csv
  499. yocaml_git
  500. yocaml_unix < "2.0.0"
  501. zarr-lwt
  502. zmq-lwt >= "5.2.1"

Conflicts

None