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 (27)

Authors

  1. J
    Jérôme Vouillon
  2. J
    Jérémie Dimino

Maintainers

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

Sources

5.9.2.tar.gz
md5=9687c14532a90af1098b646bde219a70
sha512=44ad793741a3ba52dfc07a190790d6e3207f146c42a4e1e11adc76f9d7fc9bee93d7fe18376882bb27e7e7e286be5807498884367b172a4ebc067028fa0c824c

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: 27 Aug 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 >= "4.08"
  5. dune >= "3.15"

Dev Dependencies (2)

  1. odoc with-doc & >= "2.3"
  2. ocamlfind dev & >= "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. bimage-lwt
  23. bistro
  24. brisk-reconciler
  25. brozip
  26. builder
  27. builder-web
  28. bun >= "0.3.3"
  29. cachet-lwt
  30. calculon
  31. caldav
  32. camltc
  33. canary
  34. capnp-rpc-lwt < "2.0"
  35. capnp-rpc-unix < "2.1"
  36. caqti-lwt
  37. caqti-mirage
  38. carton < "1.0.0"
  39. carton-git
  40. carton-lwt
  41. catala-format >= "0.2.0"
  42. cf-lwt
  43. chamelon
  44. chamelon-unix
  45. chamo
  46. charrua-client
  47. charrua-unix
  48. chess_com_api
  49. clz
  50. cmdtui-lambda-term
  51. coap
  52. coap-server-lwt
  53. cohttp-curl-lwt
  54. cohttp-lwt
  55. cohttp-lwt-jsoo
  56. cohttp-lwt-unix
  57. cohttp-mirage
  58. cohttp-server-lwt-unix
  59. comby
  60. comby-semantic
  61. conan-lwt
  62. conduit-lwt
  63. conduit-lwt-unix
  64. cowabloga
  65. crunch
  66. cstruct-lwt
  67. csv-lwt
  68. ctypes >= "0.15.0" & < "0.21.1"
  69. ctypes-foreign >= "0.21.1"
  70. curl_lwt
  71. current
  72. current-albatross-deployer
  73. current_docker
  74. current_examples
  75. current_git
  76. current_github
  77. current_gitlab
  78. current_ocluster
  79. current_rpc
  80. current_slack
  81. current_web
  82. DkSDKFFIOCaml_Std
  83. dap
  84. data-encoding < "0.1.1"
  85. devkit >= "1.2"
  86. distributed-lwt
  87. dkim-bin < "0.8.0"
  88. dkim-lwt-unix
  89. dkim-mirage
  90. dlm
  91. dns-certify
  92. dns-cli
  93. dns-client < "7.0.3"
  94. dns-client-lwt
  95. dns-client-mirage
  96. dns-forward
  97. dns-forward-lwt-unix
  98. dns-lwt
  99. dns-mirage
  100. dns-resolver
  101. dns-server
  102. dns-stub
  103. dnssd
  104. docker_hub
  105. docteur >= "0.0.2"
  106. docteur-solo5
  107. docteur-unix >= "0.0.5"
  108. doi2bib
  109. dream
  110. dream-httpaf
  111. dream-pure
  112. dream-serve
  113. dropbox
  114. dune >= "3.17.2"
  115. dune-rpc-lwt
  116. earlybird
  117. elasticsearch-cli
  118. emoji = "2.0.0"
  119. equinoxe
  120. ethernet
  121. ez_api >= "1.2.0"
  122. ezcurl-lwt
  123. ezjs_min < "0.2"
  124. ezjsonm-lwt
  125. ezresto
  126. ezresto-directory >= "0.5"
  127. faraday-lwt
  128. faraday-lwt-unix
  129. fat-filesystem
  130. fiber-lwt
  131. fsevents-lwt
  132. fswatch_lwt
  133. fuseau-lwt
  134. gdbprofiler
  135. git
  136. git-cohttp
  137. git-cohttp-unix
  138. git-kv >= "0.2.0"
  139. git-mirage
  140. git-paf
  141. git-unix >= "3.2.0"
  142. github
  143. github-hooks
  144. github-unix >= "4.4.0"
  145. gitlab-unix
  146. gitlab_pipeline_notifier
  147. gluten-lwt
  148. gluten-lwt-unix < "0.4.0"
  149. gluten-mirage < "0.4.0"
  150. graphql-lwt
  151. gremlin
  152. grpc-lwt
  153. guardian
  154. gufo
  155. h1
  156. h1-lwt-unix
  157. h2-lwt
  158. h2-lwt-unix < "0.10.0"
  159. h2-mirage
  160. happy-eyeballs-lwt
  161. happy-eyeballs-mirage
  162. hidapi-lwt
  163. hiredis >= "0.6"
  164. hl_yaml
  165. hockmd
  166. http-lwt-client
  167. http-mirage-client
  168. http-multipart-formdata >= "2.0.0" & < "3.0.0"
  169. httpaf-lwt-unix
  170. httpun-lwt
  171. httpun-mirage
  172. httpun-ws-lwt
  173. hvsock
  174. i3ipc
  175. influxdb-lwt
  176. inotify >= "2.4"
  177. inquire < "0.3.0"
  178. interface-prime-lwt
  179. ip2location
  180. ip2locationio
  181. ip2whois
  182. ipv6-multicast-lwt
  183. irc-client-lwt
  184. irc-client-lwt-ssl
  185. irc-client-tls
  186. irmin
  187. irmin-bench
  188. irmin-chunk
  189. irmin-cli
  190. irmin-client
  191. irmin-containers
  192. irmin-fs
  193. irmin-git
  194. irmin-graphql
  195. irmin-http
  196. irmin-indexeddb
  197. irmin-layers
  198. irmin-mirage-git
  199. irmin-mirage-graphql
  200. irmin-pack
  201. irmin-server
  202. irmin-test
  203. irmin-unix
  204. irmin-watcher
  205. joolog
  206. jose < "0.9.0"
  207. js_of_ocaml-lwt >= "3.5.0"
  208. jsoo_broadcastchannel
  209. jsoo_storage
  210. jupyter
  211. jupyter-kernel
  212. kafka < "0.5"
  213. kafka_lwt
  214. kappa-library
  215. ke >= "0.5"
  216. kinetic-client
  217. kubecaml
  218. lambda-runtime
  219. lambda-term
  220. lambda_streams_lwt
  221. launchd
  222. ldp
  223. learn-ocaml
  224. learn-ocaml-client
  225. ledgerwallet >= "0.4.0"
  226. letsencrypt
  227. letsencrypt-app
  228. letsencrypt-dns
  229. letters
  230. lichess_api
  231. links >= "0.9.1"
  232. linol-lwt
  233. llama
  234. lru_cache
  235. lwt-canceler
  236. lwt-dllist
  237. lwt-exit
  238. lwt-parallel
  239. lwt-pipe
  240. lwt-pipeline
  241. lwt-watcher
  242. lwt_camlp4
  243. lwt_domain
  244. lwt_eio
  245. lwt_glib
  246. lwt_log
  247. lwt_ppx
  248. lwt_react
  249. lwt_retry
  250. lwt_ssl
  251. mariadb >= "1.2.0"
  252. markup = "0.7.6"
  253. markup-lwt
  254. mdx
  255. mechaml
  256. mehari-lwt-unix
  257. mehari-mirage
  258. memtrace-mirage
  259. metrics-influx
  260. metrics-lwt
  261. metrics-unix
  262. mimic
  263. mindstorm-lwt
  264. mirage < "4.0.0"
  265. mirage-block >= "2.0.1"
  266. mirage-block-ccm
  267. mirage-block-combinators
  268. mirage-block-lwt
  269. mirage-block-partition
  270. mirage-block-ramdisk
  271. mirage-block-solo5
  272. mirage-block-unikraft
  273. mirage-block-unix >= "2.14.2"
  274. mirage-block-xen
  275. mirage-channel >= "4.0.1"
  276. mirage-channel-lwt
  277. mirage-clock-lwt
  278. mirage-clock-unix < "4.2.0"
  279. mirage-console-lwt
  280. mirage-crypto-rng < "0.11.3"
  281. mirage-crypto-rng-lwt
  282. mirage-crypto-rng-mirage
  283. mirage-device >= "2.0.0"
  284. mirage-flow >= "3.0.0"
  285. mirage-flow-combinators
  286. mirage-flow-lwt
  287. mirage-flow-unix
  288. mirage-fs >= "4.0.0"
  289. mirage-fs-lwt
  290. mirage-kv >= "3.0.1"
  291. mirage-kv-lwt
  292. mirage-kv-unix
  293. mirage-net >= "4.0.0"
  294. mirage-net-lwt
  295. mirage-net-macosx
  296. mirage-net-solo5
  297. mirage-net-unikraft
  298. mirage-net-unix
  299. mirage-net-xen
  300. mirage-profile
  301. mirage-protocols >= "7.0.0"
  302. mirage-protocols-lwt
  303. mirage-qubes
  304. mirage-qubes-ipv4
  305. mirage-runtime
  306. mirage-sleep
  307. mirage-solo5
  308. mirage-stack = "3.0.0"
  309. mirage-stack-lwt
  310. mirage-time >= "3.0.0"
  311. mirage-time-lwt
  312. mirage-time-unix
  313. mirage-types-lwt
  314. mirage-unikraft
  315. mirage-unix
  316. mirage-vnetif
  317. mirage-xen
  318. monorobot
  319. moonpool-lwt
  320. mqtt
  321. mrmime >= "0.5.0"
  322. multipart-form-data
  323. multipart_form >= "0.2.0" & < "0.4.0"
  324. multipart_form-cohttp-lwt < "0.6.0"
  325. multipart_form-lwt
  326. mwt
  327. naboris
  328. nbd >= "4.0.3"
  329. nbd-tool
  330. nbd-unix
  331. nocrypto
  332. nottui-lwt
  333. notty-community
  334. nproc
  335. nsq
  336. obuilder
  337. obus >= "1.2.1"
  338. ocluster
  339. ocluster-api
  340. ocluster-worker
  341. ocplib-resto
  342. ocsigenserver
  343. ocsipersist
  344. ocsipersist-dbm
  345. ocsipersist-lib
  346. ocsipersist-pgsql
  347. ocsipersist-sqlite
  348. octez-distributed-lwt-internal
  349. octez-internal-libs
  350. octez-l2-libs
  351. octez-libs
  352. octez-proto-libs
  353. octez-protocol-compiler
  354. octez-proxy-server
  355. octez-rpc-process
  356. octez-shell-libs
  357. octez-smart-rollup-wasm-benchmark-lib
  358. oframl
  359. ojs_base
  360. omigrate
  361. oneffs
  362. opencage
  363. opentelemetry-client-cohttp-lwt
  364. opentelemetry-cohttp-lwt >= "0.4"
  365. opentelemetry-lwt
  366. opium
  367. opium-graphql
  368. opium_kernel
  369. opomodoro
  370. order-i3-xfce
  371. ordma
  372. oskel >= "0.3.0"
  373. ounit-lwt < "2.2.0"
  374. ounit2-lwt
  375. owork
  376. ozulip
  377. paf
  378. paf-cohttp
  379. passage
  380. pcap-format < "0.5.2"
  381. petrol
  382. pgn_parser
  383. pgx_lwt
  384. pgx_lwt_mirage
  385. pgx_lwt_unix < "2.0"
  386. piaf < "0.2.0"
  387. picos >= "0.3.0" & < "0.5.0"
  388. picos_lwt
  389. picos_meta
  390. plebeia >= "2.0.0"
  391. plist-xml-lwt
  392. plotkicadsch
  393. ppx_defer >= "0.4.0"
  394. ppx_deriving_rpc
  395. ppx_rapper_lwt
  396. proc-smaps
  397. prof_spacetime
  398. prometheus
  399. prometheus-app
  400. promise_jsoo_lwt
  401. protocol-9p
  402. protocol-9p-unix
  403. proton
  404. qcow
  405. qcow-stream
  406. qcow-tool
  407. qcow-types
  408. qfs >= "0.5"
  409. quests
  410. rawlink < "2.1"
  411. rawlink-lwt
  412. rdf_json_ld
  413. rdf_lwt
  414. redis-lwt
  415. reparse-lwt
  416. reparse-lwt-unix
  417. resource-pooling
  418. resp
  419. resp-mirage >= "0.10.0"
  420. resp-unix >= "0.10.0"
  421. resto
  422. resto-cohttp-client >= "0.4"
  423. resto-cohttp-self-serving-client
  424. resto-cohttp-server >= "0.4" & < "0.6" | >= "0.9"
  425. resto-directory >= "0.4"
  426. riak
  427. ringo-lwt
  428. river
  429. rock
  430. rpclib-js
  431. rpclib-lwt
  432. SZXX < "4.0.0"
  433. sanddb
  434. scgi
  435. sendmail-lwt
  436. sendmail-mirage
  437. serial
  438. server-reason-react
  439. session-cohttp-lwt
  440. session-cookie-lwt
  441. session-postgresql-lwt
  442. sessions
  443. shared-block-ring
  444. shared-memory-ring-lwt
  445. sherlodoc
  446. sihl < "0.2.0"
  447. slack
  448. slacko
  449. slipshow
  450. smtml >= "0.7.0"
  451. speed
  452. spin < "0.8.0"
  453. spoke
  454. statocaml
  455. stk
  456. stog
  457. swapfs
  458. syguslib-utils
  459. syndic >= "1.4" & < "1.6.0"
  460. tar-mirage
  461. tar-unix
  462. tcpip
  463. telegraml
  464. terminus
  465. testo-lwt
  466. tezos-base >= "16.0"
  467. tezos-clic >= "16.0"
  468. tezos-crypto >= "16.0"
  469. tezos-crypto-dal
  470. tezos-error-monad >= "16.0"
  471. tezos-lwt-result-stdlib
  472. tezos-p2p = "12.3"
  473. tezos-protocol-environment
  474. tezos-proxy >= "17.3"
  475. tezos-stdlib
  476. tezos-stdlib-unix >= "16.0"
  477. tezos-test-helpers >= "12.3"
  478. tezos-wasmer
  479. tezos-webassembly-interpreter-extra
  480. tezt
  481. tidy_email
  482. timmy-lwt
  483. tls >= "0.10.6" & < "0.16.0"
  484. tls-lwt
  485. tls-mirage
  486. tube
  487. tuntap
  488. twirp_cohttp_lwt_unix
  489. uring
  490. uspf
  491. uspf-lwt
  492. uspf-mirage
  493. utop
  494. uwt
  495. vchan
  496. vchan-unix
  497. vchan-xen
  498. vercel
  499. vhd-format-lwt
  500. vmnet
  501. vpnkit
  502. vue-jsoo < "0.3"
  503. wayland < "2.0"
  504. webauthn
  505. xen-evtchn
  506. xen-evtchn-unix
  507. xen-gnt
  508. xen-gnt-unix
  509. xenstore
  510. xenstore-tool
  511. xenstore_transport
  512. xlsx2csv
  513. yocaml_git
  514. yocaml_unix < "2.0.0"
  515. zarr-lwt
  516. zmq-lwt >= "5.2.1"

Conflicts

None