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

6.1.1.tar.gz
md5=9bccdb35c84cb8097233b3a875f08826
sha512=3c92e3359ee742b9647dc8392b71c2d725b23549327aaab772d48d016d7abb8f0d710d0e9bdc0fd2406e0cbc4ea182cd134446fe4356538795d173cc6a9e0a27

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 Feb 2026

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 resolved 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:

let () =
  let request =
    let%lwt 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) ->
      write outgoing "GET / HTTP/1.1\r\n";%lwt
      write outgoing "Connection: close\r\n\r\n";%lwt
      let%lwt response = read incoming in
      Lwt.return (Some response)))
  in

  let timeout =
    Lwt_unix.sleep 5.;%lwt
    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,lwt_ppx -linkpkg example.ml && ./a.out *)

If you are not using the lwt_ppx syntax extension, you can use the let* binding opoerators from the Lwt.Syntax module instead.

let () =
  let open Lwt.Syntax in
  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,lwt_ppx -linkpkg example.ml && ./a.out *)

In the program above, functions such as Lwt_io.write create promises. The let%lwt ... in construct (provided by lwt_ppx) or the let* ... in construct (provided by Lwt.Syntax) are used to wait for a promise to resolve. The code after in is scheduled to run after the code inside the let...in has resolved.

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 >= "3.18"

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. 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. ciao_lwt
  48. clz
  49. cmdtui-lambda-term
  50. coap
  51. coap-server-lwt
  52. cohttp-curl-lwt
  53. cohttp-lwt
  54. cohttp-lwt-jsoo
  55. cohttp-lwt-unix
  56. cohttp-mirage
  57. cohttp-server-lwt-unix
  58. comby
  59. comby-semantic
  60. conan-lwt
  61. conduit-lwt
  62. conduit-lwt-unix
  63. cowabloga
  64. crunch
  65. cstruct-lwt
  66. csv-lwt
  67. ctypes >= "0.15.0" & < "0.21.1"
  68. ctypes-foreign >= "0.21.1"
  69. curl_lwt
  70. current
  71. current-albatross-deployer
  72. current_docker
  73. current_examples
  74. current_git
  75. current_github
  76. current_gitlab
  77. current_ocluster
  78. current_rpc
  79. current_slack
  80. current_web
  81. dap
  82. data-encoding < "0.1.1"
  83. devkit >= "1.2"
  84. distributed-lwt
  85. dkim-bin < "0.8.0"
  86. dkim-lwt-unix
  87. dkim-mirage
  88. dlm
  89. dmarc
  90. dns-certify
  91. dns-cli
  92. dns-client < "7.0.3"
  93. dns-client-lwt
  94. dns-client-mirage
  95. dns-forward
  96. dns-forward-lwt-unix
  97. dns-lwt
  98. dns-mirage
  99. dns-resolver
  100. dns-server
  101. dns-stub
  102. dnsrobot
  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. faraday-lwt
  127. faraday-lwt-unix
  128. fat-filesystem
  129. fiber-lwt
  130. fsevents-lwt
  131. fswatch_lwt
  132. gdbprofiler
  133. git
  134. git-cohttp
  135. git-cohttp-unix
  136. git-kv >= "0.2.0"
  137. git-mirage
  138. git-paf
  139. git-unix >= "3.2.0"
  140. github
  141. github-hooks
  142. github-unix >= "4.4.0"
  143. gitlab-unix
  144. gitlab_pipeline_notifier
  145. gluten-lwt
  146. gluten-lwt-unix < "0.4.0"
  147. gluten-mirage < "0.4.0"
  148. graphql-lwt
  149. gremlin
  150. grpc-lwt
  151. guardian
  152. gufo
  153. h1
  154. h1-lwt-unix
  155. h2-lwt
  156. h2-lwt-unix < "0.10.0"
  157. h2-mirage
  158. happy-eyeballs-lwt
  159. happy-eyeballs-mirage
  160. hidapi-lwt
  161. hiredis >= "0.6"
  162. hl_yaml
  163. hockmd
  164. http-lwt-client
  165. http-mirage-client
  166. http-multipart-formdata >= "2.0.0" & < "3.0.0"
  167. httpaf-lwt-unix
  168. httpun-lwt
  169. httpun-mirage
  170. httpun-ws-lwt
  171. hvsock
  172. i3ipc
  173. influxdb-lwt
  174. inotify >= "2.4"
  175. inquire < "0.3.0"
  176. interface-prime-lwt
  177. ip2location
  178. ip2locationio
  179. ip2whois
  180. ipv6-multicast-lwt
  181. irc-client-lwt
  182. irc-client-lwt-ssl
  183. irc-client-tls
  184. irmin
  185. irmin-bench
  186. irmin-chunk
  187. irmin-cli
  188. irmin-client
  189. irmin-containers
  190. irmin-fs
  191. irmin-git
  192. irmin-graphql
  193. irmin-http
  194. irmin-indexeddb
  195. irmin-layers
  196. irmin-mirage-git
  197. irmin-mirage-graphql
  198. irmin-pack
  199. irmin-server
  200. irmin-test
  201. irmin-unix
  202. irmin-watcher
  203. joolog
  204. jose < "0.9.0"
  205. js_of_ocaml-lwt >= "3.5.0"
  206. jsoo_broadcastchannel
  207. jsoo_storage
  208. jupyter
  209. jupyter-kernel
  210. kafka < "0.5"
  211. kafka_lwt
  212. kappa-library
  213. ke >= "0.5"
  214. kinetic-client
  215. kubecaml
  216. lambda-runtime
  217. lambda-term >= "3.3.3"
  218. lambda_streams_lwt
  219. launchd
  220. ldp
  221. learn-ocaml
  222. learn-ocaml-client
  223. ledgerwallet >= "0.4.0"
  224. letsencrypt
  225. letsencrypt-app
  226. letsencrypt-dns
  227. letters
  228. lichess_api
  229. links >= "0.9.1"
  230. llama
  231. lru_cache
  232. lwt-canceler
  233. lwt-dllist
  234. lwt-exit
  235. lwt-parallel
  236. lwt-pipe
  237. lwt-pipeline
  238. lwt-watcher
  239. lwt_camlp4
  240. lwt_direct >= "6.0.0"
  241. lwt_ppx >= "6.0.0"
  242. lwt_react
  243. lwt_retry
  244. lwt_ssl
  245. mariadb >= "1.2.0"
  246. markdown_monolith
  247. markup = "0.7.6"
  248. markup-lwt
  249. mdx
  250. mechaml
  251. mehari-lwt-unix
  252. mehari-mirage
  253. memtrace-mirage
  254. metrics-influx
  255. metrics-lwt
  256. metrics-unix
  257. mimic
  258. mindstorm-lwt
  259. mirage < "4.0.0"
  260. mirage-block >= "2.0.1"
  261. mirage-block-ccm
  262. mirage-block-combinators
  263. mirage-block-lwt
  264. mirage-block-partition
  265. mirage-block-ramdisk
  266. mirage-block-solo5
  267. mirage-block-unikraft
  268. mirage-block-unix >= "2.14.2"
  269. mirage-block-xen
  270. mirage-channel >= "4.0.1"
  271. mirage-channel-lwt
  272. mirage-clock-lwt
  273. mirage-clock-unix < "4.2.0"
  274. mirage-console-lwt
  275. mirage-crypto-rng < "0.11.3"
  276. mirage-crypto-rng-lwt
  277. mirage-crypto-rng-mirage
  278. mirage-device >= "2.0.0"
  279. mirage-flow >= "3.0.0"
  280. mirage-flow-combinators
  281. mirage-flow-lwt
  282. mirage-flow-unix
  283. mirage-fs >= "4.0.0"
  284. mirage-fs-lwt
  285. mirage-kv >= "3.0.1"
  286. mirage-kv-lwt
  287. mirage-kv-unix
  288. mirage-net >= "4.0.0"
  289. mirage-net-lwt
  290. mirage-net-macosx
  291. mirage-net-solo5
  292. mirage-net-unikraft
  293. mirage-net-unix
  294. mirage-net-xen
  295. mirage-profile
  296. mirage-protocols >= "7.0.0"
  297. mirage-protocols-lwt
  298. mirage-qubes
  299. mirage-qubes-ipv4
  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. posix-getopt >= "4.0.2"
  374. ppx_defer >= "0.4.0"
  375. ppx_deriving_rpc
  376. ppx_rapper_lwt
  377. proc-smaps
  378. prof_spacetime
  379. prometheus
  380. prometheus-app
  381. promise_jsoo_lwt
  382. protocol-9p
  383. protocol-9p-unix
  384. proton
  385. pxshot
  386. qcow
  387. qcow-stream
  388. qcow-tool
  389. qcow-types
  390. qdrant < "0.2.0"
  391. qfs >= "0.5"
  392. quests
  393. quickterface
  394. rawlink < "2.1"
  395. rawlink-lwt
  396. rdf_json_ld
  397. rdf_lwt
  398. redis-lwt
  399. reparse-lwt
  400. reparse-lwt-unix
  401. resource-pooling
  402. resp
  403. resp-mirage >= "0.10.0"
  404. resp-unix >= "0.10.0"
  405. resto
  406. resto-cohttp-client = "0.4"
  407. resto-cohttp-server = "0.4"
  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. tls >= "0.10.6" & < "0.16.0"
  452. tls-lwt
  453. tls-mirage
  454. tube
  455. tuntap
  456. twirp_cohttp_lwt_unix
  457. uring
  458. uspf
  459. uspf-lwt
  460. uspf-mirage
  461. utcp
  462. utop
  463. uwt
  464. vchan
  465. vchan-unix
  466. vchan-xen
  467. vercel
  468. vhd-format-lwt
  469. vmnet
  470. vue-jsoo < "0.3"
  471. wayland < "2.0"
  472. webauthn
  473. xen-evtchn
  474. xen-evtchn-unix
  475. xen-gnt
  476. xen-gnt-unix
  477. xenstore
  478. xenstore-tool
  479. xenstore_transport
  480. xlsx2csv
  481. yocaml_git
  482. yocaml_unix < "2.0.0"
  483. zarr-lwt
  484. zmq-lwt >= "5.2.1"

Conflicts

None