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 (31)

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. curl_lwt
  69. current
  70. current-albatross-deployer
  71. current_docker
  72. current_examples
  73. current_git
  74. current_github
  75. current_gitlab
  76. current_ocluster
  77. current_rpc
  78. current_slack
  79. current_web
  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 != "6.0.0~beta01"
  240. lwt_ppx >= "6.0.0"
  241. lwt_react
  242. lwt_retry
  243. lwt_ssl
  244. mariadb >= "1.2.0"
  245. markdown_monolith
  246. markup = "0.7.6"
  247. markup-lwt
  248. mdx
  249. mechaml
  250. mehari-lwt-unix
  251. mehari-mirage
  252. memtrace-mirage
  253. metrics-influx
  254. metrics-lwt
  255. metrics-unix
  256. mimic
  257. mindstorm-lwt
  258. mirage < "4.0.0"
  259. mirage-block >= "2.0.1"
  260. mirage-block-ccm
  261. mirage-block-combinators
  262. mirage-block-lwt
  263. mirage-block-partition
  264. mirage-block-ramdisk
  265. mirage-block-solo5
  266. mirage-block-unikraft
  267. mirage-block-unix >= "2.14.2"
  268. mirage-block-xen
  269. mirage-channel >= "4.0.1"
  270. mirage-channel-lwt
  271. mirage-clock-lwt
  272. mirage-clock-unix < "4.2.0"
  273. mirage-console-lwt
  274. mirage-crypto-rng < "0.11.3"
  275. mirage-crypto-rng-lwt
  276. mirage-crypto-rng-mirage
  277. mirage-device >= "2.0.0"
  278. mirage-flow >= "3.0.0"
  279. mirage-flow-combinators
  280. mirage-flow-lwt
  281. mirage-flow-unix
  282. mirage-fs >= "4.0.0"
  283. mirage-fs-lwt
  284. mirage-kv >= "3.0.1"
  285. mirage-kv-lwt
  286. mirage-kv-unix
  287. mirage-net >= "4.0.0"
  288. mirage-net-lwt
  289. mirage-net-macosx
  290. mirage-net-solo5
  291. mirage-net-unikraft
  292. mirage-net-unix
  293. mirage-net-xen
  294. mirage-profile
  295. mirage-protocols >= "7.0.0"
  296. mirage-protocols-lwt
  297. mirage-qubes
  298. mirage-qubes-ipv4
  299. mirage-runtime
  300. mirage-sleep
  301. mirage-solo5
  302. mirage-stack = "3.0.0"
  303. mirage-stack-lwt
  304. mirage-time >= "3.0.0"
  305. mirage-time-lwt
  306. mirage-time-unix
  307. mirage-types-lwt
  308. mirage-unikraft
  309. mirage-unix
  310. mirage-vnetif
  311. mirage-xen
  312. monorobot
  313. mqtt
  314. mrmime >= "0.5.0"
  315. multipart-form-data
  316. multipart_form >= "0.2.0" & < "0.4.0"
  317. multipart_form-cohttp-lwt < "0.6.0"
  318. multipart_form-lwt
  319. naboris
  320. nbd >= "4.0.3"
  321. nbd-tool
  322. nbd-unix
  323. neo4j_bolt
  324. nottui-lwt
  325. notty-community
  326. nproc
  327. nsq
  328. obuilder
  329. obus >= "1.2.1"
  330. ocluster-api
  331. ocluster-worker
  332. ocplib-resto
  333. ocsigenserver
  334. ocsipersist
  335. ocsipersist-dbm
  336. ocsipersist-lib
  337. ocsipersist-pgsql
  338. ocsipersist-sqlite
  339. oframl
  340. ojs_base
  341. omigrate
  342. oneffs
  343. opam-check-npm-deps >= "4.1.0"
  344. opam-publish >= "3.0.0"
  345. opencage
  346. opentelemetry-client-cohttp-lwt
  347. opentelemetry-cohttp-lwt >= "0.4"
  348. opentelemetry-lwt
  349. opium
  350. opium-graphql
  351. opium_kernel
  352. opomodoro
  353. order-i3-xfce
  354. ordma
  355. oskel >= "0.3.0"
  356. ounit-lwt < "2.2.0"
  357. ounit2-lwt
  358. owork
  359. ozulip
  360. paf
  361. paf-cohttp
  362. passage < "0.1.8"
  363. pcap-format < "0.5.2"
  364. petrol
  365. pgn_parser
  366. pgx_lwt
  367. pgx_lwt_mirage
  368. pgx_lwt_unix < "2.0"
  369. piaf < "0.2.0"
  370. picos_meta
  371. plebeia >= "2.0.0"
  372. plotkicadsch
  373. ppx_defer >= "0.4.0"
  374. ppx_deriving_rpc
  375. ppx_rapper_lwt
  376. proc-smaps
  377. prof_spacetime
  378. prometheus
  379. prometheus-app
  380. promise_jsoo_lwt
  381. protocol-9p
  382. protocol-9p-unix
  383. proton
  384. pxshot
  385. qcow
  386. qcow-stream
  387. qcow-tool
  388. qcow-types
  389. qdrant < "0.2.0"
  390. qfs >= "0.5"
  391. quests
  392. quickterface
  393. rawlink < "2.1"
  394. rawlink-lwt
  395. rdf_json_ld
  396. rdf_lwt
  397. redis-lwt
  398. reparse-lwt
  399. reparse-lwt-unix
  400. resource-pooling
  401. resp
  402. resp-mirage >= "0.10.0"
  403. resp-unix >= "0.10.0"
  404. resto
  405. resto-cohttp-client >= "0.4"
  406. resto-cohttp-self-serving-client
  407. resto-cohttp-server >= "0.4" & < "0.6" | >= "0.9"
  408. resto-directory >= "0.4"
  409. riak
  410. ringo-lwt
  411. river
  412. rock
  413. rpclib-js
  414. rpclib-lwt
  415. SZXX < "4.0.0"
  416. sanddb
  417. scgi
  418. sendmail-lwt
  419. sendmail-mirage
  420. serial
  421. server-reason-react
  422. session-cohttp-lwt
  423. session-cookie-lwt
  424. session-postgresql-lwt
  425. sessions
  426. shared-block-ring
  427. shared-memory-ring-lwt
  428. sherlodoc
  429. sihl < "0.2.0"
  430. slack
  431. slacko
  432. slipshow
  433. smtml >= "0.7.0"
  434. speed
  435. spin < "0.8.0"
  436. spoke
  437. statocaml
  438. stk
  439. stog
  440. swapfs
  441. syguslib-utils
  442. syndic >= "1.4" & < "1.6.0"
  443. tar-mirage
  444. tar-unix
  445. tcpip
  446. telegraml
  447. terminus
  448. testcontainers
  449. testo-lwt
  450. tidy_email
  451. timmy-lwt
  452. tls >= "0.10.6" & < "0.16.0"
  453. tls-lwt
  454. tls-mirage
  455. tube
  456. tuntap
  457. twirp_cohttp_lwt_unix
  458. uring
  459. uspf
  460. uspf-lwt
  461. uspf-mirage
  462. utcp
  463. utop
  464. uwt
  465. vchan
  466. vchan-unix
  467. vchan-xen
  468. vercel
  469. vhd-format-lwt
  470. vmnet
  471. vue-jsoo < "0.3"
  472. wayland < "2.0"
  473. webauthn
  474. xen-evtchn
  475. xen-evtchn-unix
  476. xen-gnt
  477. xen-gnt-unix
  478. xenstore
  479. xenstore-tool
  480. xenstore_transport
  481. xlsx2csv
  482. yocaml_git
  483. yocaml_unix < "2.0.0"
  484. zarr-lwt
  485. zmq-lwt >= "5.2.1"

Conflicts

None