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

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 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. 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_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
  239. lwt_ppx != "5.8.1"
  240. lwt_react
  241. lwt_retry
  242. lwt_ssl
  243. mariadb >= "1.2.0"
  244. markup = "0.7.6"
  245. markup-lwt
  246. mdx
  247. mechaml
  248. mehari-lwt-unix
  249. mehari-mirage
  250. memtrace-mirage
  251. metrics-influx
  252. metrics-lwt
  253. metrics-unix
  254. mimic
  255. mindstorm-lwt
  256. mirage < "4.0.0"
  257. mirage-block >= "2.0.1"
  258. mirage-block-ccm
  259. mirage-block-combinators
  260. mirage-block-lwt
  261. mirage-block-partition
  262. mirage-block-ramdisk
  263. mirage-block-solo5
  264. mirage-block-unikraft
  265. mirage-block-unix >= "2.14.2"
  266. mirage-block-xen
  267. mirage-channel >= "4.0.1"
  268. mirage-channel-lwt
  269. mirage-clock-lwt
  270. mirage-clock-unix < "4.2.0"
  271. mirage-console-lwt
  272. mirage-crypto-rng < "0.11.3"
  273. mirage-crypto-rng-lwt
  274. mirage-crypto-rng-mirage
  275. mirage-device >= "2.0.0"
  276. mirage-flow >= "3.0.0"
  277. mirage-flow-combinators
  278. mirage-flow-lwt
  279. mirage-flow-unix
  280. mirage-fs >= "4.0.0"
  281. mirage-fs-lwt
  282. mirage-kv >= "3.0.1"
  283. mirage-kv-lwt
  284. mirage-kv-unix
  285. mirage-net >= "4.0.0"
  286. mirage-net-lwt
  287. mirage-net-macosx
  288. mirage-net-solo5
  289. mirage-net-unikraft
  290. mirage-net-unix
  291. mirage-net-xen
  292. mirage-profile
  293. mirage-protocols >= "7.0.0"
  294. mirage-protocols-lwt
  295. mirage-qubes
  296. mirage-qubes-ipv4
  297. mirage-runtime
  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. moonpool-lwt >= "0.9"
  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. 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. opencage
  352. opentelemetry-client-cohttp-lwt
  353. opentelemetry-cohttp-lwt >= "0.4"
  354. opentelemetry-lwt
  355. opium
  356. opium-graphql
  357. opium_kernel
  358. opomodoro
  359. order-i3-xfce
  360. ordma
  361. oskel >= "0.3.0"
  362. ounit-lwt < "2.2.0"
  363. ounit2-lwt
  364. owork
  365. ozulip
  366. paf
  367. paf-cohttp
  368. passage < "0.1.8"
  369. pcap-format < "0.5.2"
  370. petrol
  371. pgn_parser
  372. pgx_lwt
  373. pgx_lwt_mirage
  374. pgx_lwt_unix < "2.0"
  375. piaf < "0.2.0"
  376. picos_meta
  377. plebeia >= "2.0.0"
  378. plotkicadsch
  379. ppx_defer >= "0.4.0"
  380. ppx_deriving_rpc
  381. ppx_rapper_lwt
  382. proc-smaps
  383. prof_spacetime
  384. prometheus
  385. prometheus-app
  386. promise_jsoo_lwt
  387. protocol-9p
  388. protocol-9p-unix
  389. proton
  390. qcow
  391. qcow-stream
  392. qcow-tool
  393. qcow-types
  394. qfs >= "0.5"
  395. quests
  396. rawlink < "2.1"
  397. rawlink-lwt
  398. rdf_json_ld
  399. rdf_lwt
  400. redis-lwt
  401. reparse-lwt
  402. reparse-lwt-unix
  403. resource-pooling
  404. resp
  405. resp-mirage >= "0.10.0"
  406. resp-unix >= "0.10.0"
  407. resto
  408. resto-cohttp-client >= "0.4"
  409. resto-cohttp-self-serving-client
  410. resto-cohttp-server >= "0.4" & < "0.6" | >= "0.9"
  411. resto-directory >= "0.4"
  412. riak
  413. ringo-lwt
  414. river
  415. rock
  416. rpclib-js
  417. rpclib-lwt
  418. SZXX < "4.0.0"
  419. sanddb
  420. scgi
  421. sendmail-lwt
  422. sendmail-mirage
  423. serial
  424. server-reason-react
  425. session-cohttp-lwt
  426. session-cookie-lwt
  427. session-postgresql-lwt
  428. sessions
  429. shared-block-ring
  430. shared-memory-ring-lwt
  431. sherlodoc
  432. sihl < "0.2.0"
  433. slack
  434. slacko
  435. slipshow
  436. smtml >= "0.7.0"
  437. speed
  438. spin < "0.8.0"
  439. spoke
  440. statocaml
  441. stk
  442. stog
  443. swapfs
  444. syguslib-utils
  445. syndic >= "1.4" & < "1.6.0"
  446. tar-mirage
  447. tar-unix
  448. tcpip
  449. telegraml
  450. terminus
  451. testo-lwt
  452. tezos-base >= "16.0"
  453. tezos-clic >= "16.0"
  454. tezos-crypto >= "16.0"
  455. tezos-crypto-dal
  456. tezos-error-monad >= "16.0"
  457. tezos-lwt-result-stdlib
  458. tezos-p2p = "12.3"
  459. tezos-protocol-environment
  460. tezos-proxy >= "17.3"
  461. tezos-stdlib
  462. tezos-stdlib-unix >= "16.0"
  463. tezos-test-helpers >= "12.3"
  464. tezos-webassembly-interpreter-extra
  465. tezt < "3.1.0" | >= "4.3.0"
  466. tidy_email
  467. timmy-lwt
  468. tls >= "0.10.6" & < "0.16.0"
  469. tls-lwt
  470. tls-mirage
  471. tube
  472. tuntap
  473. twirp_cohttp_lwt_unix
  474. uring
  475. uspf
  476. uspf-lwt
  477. uspf-mirage
  478. utop
  479. uwt
  480. vchan
  481. vchan-unix
  482. vchan-xen
  483. vercel
  484. vhd-format-lwt
  485. vmnet
  486. vpnkit
  487. vue-jsoo < "0.3"
  488. wayland < "2.0"
  489. webauthn
  490. xen-evtchn
  491. xen-evtchn-unix
  492. xen-gnt
  493. xen-gnt-unix
  494. xenstore
  495. xenstore-tool
  496. xenstore_transport
  497. xlsx2csv
  498. yocaml_git
  499. yocaml_unix < "2.0.0"
  500. zarr-lwt
  501. zmq-lwt >= "5.2.1"

Conflicts

None