package lwt

  1. Overview
  2. Docs
Promises and event-driven I/O

Install

Dune Dependency

github.com Readme Changelog MIT License Edit opam file Versions (25)

Authors

  1. J
    Jérôme Vouillon
  2. J
    Jérémie Dimino

Maintainers

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

Sources

5.8.1.tar.gz
md5=d0f824f75ce5297975aec75366fed36c
sha512=c43fff2e9e808dc49ddf09caf7040e512ba36aebe2c600d524095ced11c196c3ac619953a11cb3e3a9f7c6c99e798d43bf179d5f790ab258fb9f746fae4c1125

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: 09 Mar 2025

README

Lwt

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 in /r/ocaml, and 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 (2)

  1. odoc with-doc & >= "2.3.0"
  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 >= "2.3.1"
  15. arp-mirage >= "2.2.1"
  16. awa-lwt
  17. awa-mirage
  18. aws-lwt
  19. aws-s3-lwt < "4.4.0" | >= "4.8.1"
  20. awsm-lwt
  21. azure-cosmos-db
  22. balancer
  23. bastet_lwt
  24. bimage-lwt
  25. bistro
  26. brisk-reconciler
  27. brozip
  28. builder
  29. builder-web
  30. bun >= "0.3.3"
  31. cachet-lwt
  32. calculon
  33. caldav
  34. camltc
  35. canary
  36. capnp-rpc-lwt < "2.0"
  37. capnp-rpc-unix >= "0.9.0" & < "2.0"
  38. caqti-lwt
  39. caqti-mirage
  40. carton < "1.0.0"
  41. carton-git
  42. carton-lwt
  43. catala-format >= "0.2.0"
  44. cf-lwt
  45. chamelon
  46. chamelon-unix
  47. chamo
  48. charrua-client >= "1.3.0"
  49. charrua-client-lwt
  50. charrua-client-mirage
  51. charrua-unix
  52. clz
  53. cmdtui-lambda-term
  54. coap
  55. coap-server-lwt
  56. cohttp-curl-lwt
  57. cohttp-lwt
  58. cohttp-lwt-jsoo
  59. cohttp-lwt-unix
  60. cohttp-mirage
  61. cohttp-server-lwt-unix
  62. comby
  63. comby-semantic
  64. conan-lwt
  65. conduit-lwt
  66. conduit-lwt-unix
  67. cowabloga
  68. crunch >= "2.0.0"
  69. cstruct-lwt
  70. csv-lwt
  71. ctypes >= "0.15.0" & < "0.21.1"
  72. ctypes-foreign >= "0.21.1"
  73. current
  74. current-albatross-deployer
  75. current_docker
  76. current_examples
  77. current_git
  78. current_github
  79. current_gitlab
  80. current_ocluster
  81. current_rpc >= "0.4"
  82. current_slack
  83. current_web
  84. DkSDKFFIOCaml_Std
  85. dap
  86. data-encoding < "0.1.1"
  87. devkit >= "1.2"
  88. distributed-lwt
  89. dkim-bin >= "0.6.0"
  90. dkim-mirage
  91. dlm
  92. dns-certify
  93. dns-cli >= "4.6.3"
  94. dns-client < "7.0.0"
  95. dns-client-lwt
  96. dns-client-mirage
  97. dns-forward
  98. dns-forward-lwt-unix
  99. dns-lwt
  100. dns-mirage
  101. dns-resolver
  102. dns-server
  103. dns-stub
  104. dnssd
  105. docker_hub
  106. docteur >= "0.0.2"
  107. docteur-solo5
  108. docteur-unix >= "0.0.5"
  109. doi2bib
  110. dream
  111. dream-httpaf
  112. dream-pure
  113. dream-serve
  114. dropbox
  115. dune >= "3.17.0"
  116. dune-rpc-lwt >= "3.7.0" & != "3.10.0"
  117. earlybird
  118. elasticsearch-cli
  119. emoji >= "2.0.0"
  120. equinoxe
  121. ethernet
  122. ez_api >= "1.2.0"
  123. ezcurl-lwt
  124. ezjs_min < "0.2"
  125. ezjsonm-lwt
  126. ezresto
  127. ezresto-directory >= "0.5"
  128. faraday-lwt
  129. faraday-lwt-unix
  130. fat-filesystem
  131. fiber-lwt
  132. fsevents-lwt
  133. fswatch_lwt
  134. fuseau-lwt
  135. gdbprofiler
  136. git
  137. git-cohttp
  138. git-cohttp-mirage
  139. git-cohttp-unix
  140. git-mirage >= "3.0.0"
  141. git-paf
  142. git-unix >= "3.0.0"
  143. github
  144. github-hooks
  145. github-unix >= "4.4.0"
  146. gitlab-unix
  147. gitlab_pipeline_notifier
  148. gluten-lwt
  149. gluten-lwt-unix < "0.4.0"
  150. gluten-mirage < "0.4.0"
  151. graphql-lwt
  152. gremlin
  153. grpc-lwt
  154. guardian
  155. gufo
  156. h1
  157. h1-lwt-unix
  158. h2-lwt
  159. h2-lwt-unix < "0.10.0"
  160. h2-mirage
  161. happy-eyeballs-lwt
  162. happy-eyeballs-mirage
  163. hidapi-lwt
  164. hiredis >= "0.6"
  165. hl_yaml
  166. hockmd
  167. http-lwt-client
  168. http-mirage-client
  169. http-multipart-formdata >= "2.0.0" & < "3.0.0"
  170. httpaf-lwt-unix
  171. httpun-lwt
  172. httpun-mirage
  173. httpun-ws-lwt
  174. hvsock
  175. i3ipc
  176. influxdb-lwt
  177. inotify >= "2.4"
  178. inquire < "0.3.0"
  179. interface-prime-lwt
  180. ip2location
  181. ip2locationio
  182. ip2whois
  183. ipv6-multicast-lwt
  184. irc-client-lwt
  185. irc-client-lwt-ssl
  186. irc-client-tls
  187. irmin
  188. irmin-bench
  189. irmin-chunk
  190. irmin-cli
  191. irmin-client
  192. irmin-containers
  193. irmin-fs >= "2.3.0"
  194. irmin-git >= "2.3.0"
  195. irmin-graphql >= "2.3.0"
  196. irmin-http >= "2.3.0"
  197. irmin-indexeddb
  198. irmin-layers
  199. irmin-mem >= "2.3.0"
  200. irmin-mirage-git >= "2.3.0"
  201. irmin-mirage-graphql >= "2.3.0"
  202. irmin-pack
  203. irmin-server
  204. irmin-test >= "2.3.0"
  205. irmin-unix >= "2.3.0"
  206. irmin-watcher
  207. joolog
  208. jose < "0.9.0"
  209. js_of_ocaml-lwt >= "3.5.0"
  210. jsoo_broadcastchannel
  211. jsoo_storage
  212. jupyter
  213. jupyter-kernel
  214. kafka < "0.5"
  215. kafka_lwt
  216. kappa-library
  217. ke >= "0.5"
  218. kinetic-client
  219. kubecaml
  220. lablqml < "0.6"
  221. lambda-runtime
  222. lambda-term
  223. lambda_streams_lwt
  224. launchd
  225. ldp
  226. learn-ocaml
  227. learn-ocaml-client
  228. ledgerwallet >= "0.4.0"
  229. letsencrypt
  230. letsencrypt-app
  231. letsencrypt-dns
  232. letters
  233. links >= "0.9.1"
  234. linol-lwt
  235. llama
  236. lru_cache
  237. lwt-canceler
  238. lwt-dllist
  239. lwt-exit
  240. lwt-parallel
  241. lwt-pipe
  242. lwt-pipeline
  243. lwt-watcher
  244. lwt_camlp4
  245. lwt_domain
  246. lwt_eio
  247. lwt_glib
  248. lwt_log
  249. lwt_ppx
  250. lwt_react
  251. lwt_retry
  252. lwt_ssl
  253. mariadb >= "1.2.0"
  254. markup = "0.7.6"
  255. markup-lwt
  256. mdx
  257. mechaml
  258. mehari-lwt-unix
  259. mehari-mirage
  260. memtrace-mirage
  261. metrics-influx
  262. metrics-lwt
  263. metrics-mirage
  264. metrics-unix
  265. mimic
  266. mindstorm-lwt
  267. mirage < "3.1.1"
  268. mirage-block >= "2.0.1"
  269. mirage-block-ccm
  270. mirage-block-combinators
  271. mirage-block-lwt
  272. mirage-block-partition
  273. mirage-block-ramdisk
  274. mirage-block-solo5
  275. mirage-block-unix < "2.8.3" | >= "2.13.0"
  276. mirage-block-xen
  277. mirage-bootvar-solo5
  278. mirage-bootvar-unix
  279. mirage-bootvar-xen >= "0.4.0"
  280. mirage-channel >= "4.0.1"
  281. mirage-channel-lwt
  282. mirage-clock-freestanding < "3.0.1"
  283. mirage-clock-lwt
  284. mirage-clock-unix >= "1.3.0" & < "3.0.1"
  285. mirage-console >= "3.0.2"
  286. mirage-console-lwt
  287. mirage-console-solo5
  288. mirage-console-unix
  289. mirage-console-xen >= "5.0.0"
  290. mirage-console-xen-backend
  291. mirage-crypto-entropy
  292. mirage-crypto-rng >= "0.8.0" & < "0.11.0"
  293. mirage-crypto-rng-lwt
  294. mirage-crypto-rng-mirage >= "0.8.8"
  295. mirage-device >= "2.0.0"
  296. mirage-entropy
  297. mirage-flow >= "2.0.1"
  298. mirage-flow-combinators
  299. mirage-flow-lwt < "1.3.0" | >= "1.5.0"
  300. mirage-flow-rawlink
  301. mirage-flow-unix >= "1.3.0"
  302. mirage-fs >= "3.0.1"
  303. mirage-fs-lwt
  304. mirage-fs-unix
  305. mirage-kv >= "3.0.1"
  306. mirage-kv-lwt
  307. mirage-kv-unix
  308. mirage-logs != "0.3.0" & < "3.0.0"
  309. mirage-nat < "3.0.0"
  310. mirage-net >= "3.0.1"
  311. mirage-net-fd
  312. mirage-net-lwt
  313. mirage-net-macosx
  314. mirage-net-solo5
  315. mirage-net-unix
  316. mirage-net-xen
  317. mirage-os-shim
  318. mirage-profile
  319. mirage-protocols >= "4.0.1" & < "8.0.0"
  320. mirage-protocols-lwt
  321. mirage-qubes
  322. mirage-qubes-ipv4
  323. mirage-random-stdlib >= "0.1.0"
  324. mirage-runtime >= "3.7.0"
  325. mirage-sleep
  326. mirage-solo5
  327. mirage-stack >= "2.0.1" & < "4.0.0"
  328. mirage-stack-lwt
  329. mirage-time >= "2.0.1"
  330. mirage-time-lwt
  331. mirage-time-unix
  332. mirage-types-lwt < "3.7.1"
  333. mirage-unix
  334. mirage-vnetif
  335. mirage-vnetif-stack
  336. mirage-xen
  337. monorobot
  338. moonpool-lwt
  339. mqtt
  340. mrmime >= "0.5.0"
  341. multipart-form-data
  342. multipart_form >= "0.2.0" & < "0.4.0"
  343. multipart_form-cohttp-lwt < "0.6.0"
  344. multipart_form-lwt
  345. mwt
  346. naboris
  347. nbd >= "4.0.3"
  348. nbd-tool
  349. nbd-unix
  350. netchannel
  351. nocrypto
  352. nottui-lwt
  353. nproc
  354. nsq
  355. obuilder
  356. obus >= "1.2.1"
  357. ocaml-variants >= "4.00.1+mirage-unix" & < "4.00.1+open-types"
  358. ocluster
  359. ocluster-api
  360. ocluster-worker
  361. ocplib-resto
  362. ocsigenserver
  363. ocsipersist
  364. ocsipersist-dbm
  365. ocsipersist-lib
  366. ocsipersist-pgsql
  367. ocsipersist-sqlite
  368. octez-distributed-lwt-internal
  369. octez-internal-libs
  370. octez-l2-libs
  371. octez-libs
  372. octez-proto-libs
  373. octez-protocol-compiler
  374. octez-proxy-server
  375. octez-rpc-process
  376. octez-shell-libs
  377. octez-smart-rollup-wasm-benchmark-lib
  378. odoc >= "2.0.0" & < "2.1.0"
  379. oframl
  380. ojs_base
  381. omigrate
  382. oneffs
  383. opentelemetry-client-cohttp-lwt
  384. opentelemetry-cohttp-lwt >= "0.4"
  385. opentelemetry-lwt
  386. opium
  387. opium-graphql
  388. opium_kernel
  389. opomodoro
  390. order-i3-xfce
  391. ordma
  392. oskel >= "0.3.0"
  393. ounit-lwt < "2.2.0"
  394. ounit2-lwt
  395. owork
  396. ozulip
  397. paf
  398. paf-cohttp
  399. passage
  400. pcap-format < "0.5.2"
  401. petrol
  402. pgx_lwt
  403. pgx_lwt_mirage
  404. pgx_lwt_unix < "2.0"
  405. piaf < "0.2.0"
  406. picos >= "0.3.0" & < "0.5.0"
  407. picos_lwt
  408. picos_meta
  409. plebeia >= "2.0.0"
  410. plist-xml-lwt
  411. plotkicadsch
  412. ppx_defer >= "0.4.0"
  413. ppx_deriving_rpc
  414. ppx_rapper_lwt
  415. proc-smaps
  416. prof_spacetime
  417. prometheus
  418. prometheus-app
  419. promise_jsoo_lwt
  420. protocol-9p
  421. protocol-9p-unix
  422. qcow
  423. qcow-tool
  424. qfs >= "0.5"
  425. quests
  426. rawlink < "2.1"
  427. rawlink-lwt
  428. rdf_json_ld
  429. rdf_lwt
  430. redis-lwt
  431. reparse-lwt
  432. reparse-lwt-unix
  433. resource-pooling
  434. resp
  435. resp-mirage >= "0.10.0"
  436. resp-unix >= "0.10.0"
  437. resto
  438. resto-cohttp-client >= "0.4"
  439. resto-cohttp-self-serving-client
  440. resto-cohttp-server >= "0.4" & < "0.6" | >= "0.9"
  441. resto-directory >= "0.4"
  442. riak
  443. ringo-lwt
  444. river
  445. rock
  446. rpc < "7.1.0"
  447. rpclib-js
  448. rpclib-lwt
  449. SZXX < "4.0.0"
  450. sanddb
  451. scgi
  452. sendmail-lwt
  453. sendmail-mirage
  454. serial
  455. server-reason-react
  456. session-cohttp-lwt
  457. session-cookie-lwt
  458. session-postgresql-lwt
  459. sessions
  460. shared-block-ring
  461. shared-memory-ring-lwt
  462. sherlodoc
  463. sihl < "0.2.0"
  464. slack
  465. slacko
  466. slipshow
  467. smtml >= "0.3.1"
  468. speed
  469. spin < "0.8.0"
  470. spoke
  471. stk
  472. stog
  473. swapfs
  474. syguslib-utils
  475. syndic >= "1.4" & < "1.6.0"
  476. tar-mirage
  477. tar-unix
  478. tcpip >= "4.1.0"
  479. telegraml
  480. terminus
  481. testo-lwt
  482. tezos-base >= "16.0"
  483. tezos-clic >= "16.0"
  484. tezos-crypto >= "16.0"
  485. tezos-crypto-dal
  486. tezos-error-monad >= "16.0"
  487. tezos-lwt-result-stdlib
  488. tezos-p2p = "12.3"
  489. tezos-protocol-environment
  490. tezos-proxy >= "17.3"
  491. tezos-stdlib
  492. tezos-stdlib-unix >= "16.0"
  493. tezos-test-helpers >= "12.3"
  494. tezos-wasmer
  495. tezos-webassembly-interpreter-extra
  496. tezt
  497. tidy_email
  498. timmy-lwt
  499. tls >= "0.10.6" & < "0.16.0"
  500. tls-lwt
  501. tls-mirage
  502. tube
  503. tuntap < "1.7.0" | >= "2.0.0"
  504. twirp_cohttp_lwt_unix
  505. u2f
  506. uring
  507. uspf
  508. uspf-lwt
  509. uspf-mirage
  510. utop
  511. uwt
  512. vchan
  513. vchan-unix
  514. vchan-xen
  515. vercel
  516. vhd-format-lwt
  517. vmnet
  518. vpnkit
  519. vue-jsoo < "0.3"
  520. wayland < "2.0"
  521. webauthn
  522. xen-evtchn >= "2.0.0"
  523. xen-evtchn-unix
  524. xen-gnt
  525. xen-gnt-unix >= "4.0.2"
  526. xenstore
  527. xenstore_transport
  528. xlsx2csv
  529. yocaml_git
  530. yocaml_unix < "2.0.0"
  531. zarr-lwt
  532. zmq-lwt >= "5.2.1"

Conflicts

None

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