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

5.9.1.tar.gz
md5=18742da8b8fe3618e3fa700b7a884fe7
sha512=1c51fdb4d0856c89e2df08a1c0095ef28ebd0f613b07b03d0f66501ca5486515562071291e6d0932e57587ed0c9362c8b92c5c9eddb4d2bb2f5e129986b484a7

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: 15 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 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
  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. 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. DkSDKFFIOCaml_Std
  82. dap
  83. data-encoding < "0.1.1"
  84. devkit >= "1.2"
  85. distributed-lwt
  86. dkim-bin < "0.8.0"
  87. dkim-lwt-unix
  88. dkim-mirage
  89. dlm
  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. dnssd
  103. docker_hub
  104. docteur >= "0.0.2"
  105. docteur-solo5
  106. docteur-unix >= "0.0.5"
  107. doi2bib
  108. dream
  109. dream-httpaf
  110. dream-pure
  111. dream-serve
  112. dropbox
  113. dune >= "3.17.2"
  114. dune-rpc-lwt
  115. earlybird
  116. elasticsearch-cli
  117. emoji >= "2.0.0"
  118. equinoxe
  119. ethernet
  120. ez_api >= "1.2.0"
  121. ezcurl-lwt
  122. ezjs_min < "0.2"
  123. ezjsonm-lwt
  124. ezresto
  125. ezresto-directory >= "0.5"
  126. faraday-lwt
  127. faraday-lwt-unix
  128. fat-filesystem
  129. fiber-lwt
  130. fsevents-lwt
  131. fswatch_lwt
  132. fuseau-lwt
  133. gdbprofiler
  134. git
  135. git-cohttp
  136. git-cohttp-unix
  137. git-kv >= "0.2.0"
  138. git-mirage
  139. git-paf
  140. git-unix >= "3.2.0"
  141. github
  142. github-hooks
  143. github-unix >= "4.4.0"
  144. gitlab-unix
  145. gitlab_pipeline_notifier
  146. gluten-lwt
  147. gluten-lwt-unix < "0.4.0"
  148. gluten-mirage < "0.4.0"
  149. graphql-lwt
  150. gremlin
  151. grpc-lwt
  152. guardian
  153. gufo
  154. h1
  155. h1-lwt-unix
  156. h2-lwt
  157. h2-lwt-unix < "0.10.0"
  158. h2-mirage
  159. happy-eyeballs-lwt
  160. happy-eyeballs-mirage
  161. hidapi-lwt
  162. hiredis >= "0.6"
  163. hl_yaml
  164. hockmd
  165. http-lwt-client
  166. http-mirage-client
  167. http-multipart-formdata >= "2.0.0" & < "3.0.0"
  168. httpaf-lwt-unix
  169. httpun-lwt
  170. httpun-mirage
  171. httpun-ws-lwt
  172. hvsock
  173. i3ipc
  174. influxdb-lwt
  175. inotify >= "2.4"
  176. inquire < "0.3.0"
  177. interface-prime-lwt
  178. ip2location
  179. ip2locationio
  180. ip2whois
  181. ipv6-multicast-lwt
  182. irc-client-lwt
  183. irc-client-lwt-ssl
  184. irc-client-tls
  185. irmin
  186. irmin-bench
  187. irmin-chunk
  188. irmin-cli
  189. irmin-client
  190. irmin-containers
  191. irmin-fs
  192. irmin-git
  193. irmin-graphql
  194. irmin-http
  195. irmin-indexeddb
  196. irmin-layers
  197. irmin-mirage-git
  198. irmin-mirage-graphql
  199. irmin-pack
  200. irmin-server
  201. irmin-test
  202. irmin-unix
  203. irmin-watcher
  204. joolog
  205. jose < "0.9.0"
  206. js_of_ocaml-lwt >= "3.5.0"
  207. jsoo_broadcastchannel
  208. jsoo_storage
  209. jupyter
  210. jupyter-kernel
  211. kafka < "0.5"
  212. kafka_lwt
  213. kappa-library
  214. ke >= "0.5"
  215. kinetic-client
  216. kubecaml
  217. lambda-runtime
  218. lambda-term
  219. lambda_streams_lwt
  220. launchd
  221. ldp
  222. learn-ocaml
  223. learn-ocaml-client
  224. ledgerwallet >= "0.4.0"
  225. letsencrypt
  226. letsencrypt-app
  227. letsencrypt-dns
  228. letters
  229. links >= "0.9.1"
  230. linol-lwt
  231. llama
  232. lru_cache
  233. lwt-canceler
  234. lwt-dllist
  235. lwt-exit
  236. lwt-parallel
  237. lwt-pipe
  238. lwt-pipeline
  239. lwt-watcher
  240. lwt_camlp4
  241. lwt_domain
  242. lwt_eio
  243. lwt_glib
  244. lwt_log
  245. lwt_ppx
  246. lwt_react
  247. lwt_retry
  248. lwt_ssl
  249. mariadb >= "1.2.0"
  250. markup = "0.7.6"
  251. markup-lwt
  252. mdx
  253. mechaml
  254. mehari-lwt-unix
  255. mehari-mirage
  256. memtrace-mirage
  257. metrics-influx
  258. metrics-lwt
  259. metrics-unix
  260. mimic
  261. mindstorm-lwt
  262. mirage < "4.0.0"
  263. mirage-block >= "2.0.1"
  264. mirage-block-ccm
  265. mirage-block-combinators
  266. mirage-block-lwt
  267. mirage-block-partition
  268. mirage-block-ramdisk
  269. mirage-block-solo5
  270. mirage-block-unix >= "2.14.2"
  271. mirage-block-xen
  272. mirage-channel >= "4.0.1"
  273. mirage-channel-lwt
  274. mirage-clock-lwt
  275. mirage-clock-unix < "4.2.0"
  276. mirage-console-lwt
  277. mirage-crypto-rng < "0.11.3"
  278. mirage-crypto-rng-lwt
  279. mirage-crypto-rng-mirage
  280. mirage-device >= "2.0.0"
  281. mirage-flow >= "3.0.0"
  282. mirage-flow-combinators
  283. mirage-flow-lwt
  284. mirage-flow-unix
  285. mirage-fs >= "4.0.0"
  286. mirage-fs-lwt
  287. mirage-kv >= "3.0.1"
  288. mirage-kv-lwt
  289. mirage-kv-unix
  290. mirage-net >= "4.0.0"
  291. mirage-net-lwt
  292. mirage-net-macosx
  293. mirage-net-solo5
  294. mirage-net-unix
  295. mirage-net-xen
  296. mirage-profile
  297. mirage-protocols >= "7.0.0"
  298. mirage-protocols-lwt
  299. mirage-qubes
  300. mirage-qubes-ipv4
  301. mirage-runtime
  302. mirage-sleep
  303. mirage-solo5
  304. mirage-stack = "3.0.0"
  305. mirage-stack-lwt
  306. mirage-time >= "3.0.0"
  307. mirage-time-lwt
  308. mirage-time-unix
  309. mirage-types-lwt
  310. mirage-unix
  311. mirage-vnetif
  312. mirage-xen
  313. monorobot
  314. moonpool-lwt
  315. mqtt
  316. mrmime >= "0.5.0"
  317. multipart-form-data
  318. multipart_form >= "0.2.0" & < "0.4.0"
  319. multipart_form-cohttp-lwt < "0.6.0"
  320. multipart_form-lwt
  321. mwt
  322. naboris
  323. nbd >= "4.0.3"
  324. nbd-tool
  325. nbd-unix
  326. nocrypto
  327. nottui-lwt
  328. nproc
  329. nsq
  330. obuilder
  331. obus >= "1.2.1"
  332. ocluster
  333. ocluster-api
  334. ocluster-worker
  335. ocplib-resto
  336. ocsigenserver
  337. ocsipersist
  338. ocsipersist-dbm
  339. ocsipersist-lib
  340. ocsipersist-pgsql
  341. ocsipersist-sqlite
  342. octez-distributed-lwt-internal
  343. octez-internal-libs
  344. octez-l2-libs
  345. octez-libs
  346. octez-proto-libs
  347. octez-protocol-compiler
  348. octez-proxy-server
  349. octez-rpc-process
  350. octez-shell-libs
  351. octez-smart-rollup-wasm-benchmark-lib
  352. oframl
  353. ojs_base
  354. omigrate
  355. oneffs
  356. opentelemetry-client-cohttp-lwt
  357. opentelemetry-cohttp-lwt >= "0.4"
  358. opentelemetry-lwt
  359. opium
  360. opium-graphql
  361. opium_kernel
  362. opomodoro
  363. order-i3-xfce
  364. ordma
  365. oskel >= "0.3.0"
  366. ounit-lwt < "2.2.0"
  367. ounit2-lwt
  368. owork
  369. ozulip
  370. paf
  371. paf-cohttp
  372. passage
  373. pcap-format < "0.5.2"
  374. petrol
  375. pgx_lwt
  376. pgx_lwt_mirage
  377. pgx_lwt_unix < "2.0"
  378. piaf < "0.2.0"
  379. picos >= "0.3.0" & < "0.5.0"
  380. picos_lwt
  381. picos_meta
  382. plebeia >= "2.0.0"
  383. plist-xml-lwt
  384. plotkicadsch
  385. ppx_defer >= "0.4.0"
  386. ppx_deriving_rpc
  387. ppx_rapper_lwt
  388. proc-smaps
  389. prof_spacetime
  390. prometheus
  391. prometheus-app
  392. promise_jsoo_lwt
  393. protocol-9p
  394. protocol-9p-unix
  395. qcow
  396. qcow-stream
  397. qcow-tool
  398. qcow-types
  399. qfs >= "0.5"
  400. quests
  401. rawlink < "2.1"
  402. rawlink-lwt
  403. rdf_json_ld
  404. rdf_lwt
  405. redis-lwt
  406. reparse-lwt
  407. reparse-lwt-unix
  408. resource-pooling
  409. resp
  410. resp-mirage >= "0.10.0"
  411. resp-unix >= "0.10.0"
  412. resto
  413. resto-cohttp-client >= "0.4"
  414. resto-cohttp-self-serving-client
  415. resto-cohttp-server >= "0.4" & < "0.6" | >= "0.9"
  416. resto-directory >= "0.4"
  417. riak
  418. ringo-lwt
  419. river
  420. rock
  421. rpclib-js
  422. rpclib-lwt
  423. SZXX < "4.0.0"
  424. sanddb
  425. scgi
  426. sendmail-lwt
  427. sendmail-mirage
  428. serial
  429. server-reason-react
  430. session-cohttp-lwt
  431. session-cookie-lwt
  432. session-postgresql-lwt
  433. sessions
  434. shared-block-ring
  435. shared-memory-ring-lwt
  436. sherlodoc
  437. sihl < "0.2.0"
  438. slack
  439. slacko
  440. slipshow
  441. smtml
  442. speed
  443. spin < "0.8.0"
  444. spoke
  445. stk
  446. stog
  447. swapfs
  448. syguslib-utils
  449. syndic >= "1.4" & < "1.6.0"
  450. tar-mirage
  451. tar-unix
  452. tcpip
  453. telegraml
  454. terminus
  455. testo-lwt
  456. tezos-base >= "16.0"
  457. tezos-clic >= "16.0"
  458. tezos-crypto >= "16.0"
  459. tezos-crypto-dal
  460. tezos-error-monad >= "16.0"
  461. tezos-lwt-result-stdlib
  462. tezos-p2p = "12.3"
  463. tezos-protocol-environment
  464. tezos-proxy >= "17.3"
  465. tezos-stdlib
  466. tezos-stdlib-unix >= "16.0"
  467. tezos-test-helpers >= "12.3"
  468. tezos-wasmer
  469. tezos-webassembly-interpreter-extra
  470. tezt
  471. tidy_email
  472. timmy-lwt
  473. tls >= "0.10.6" & < "0.16.0"
  474. tls-lwt
  475. tls-mirage
  476. tube
  477. tuntap
  478. twirp_cohttp_lwt_unix
  479. uring
  480. uspf
  481. uspf-lwt
  482. uspf-mirage
  483. utop
  484. uwt
  485. vchan
  486. vchan-unix
  487. vchan-xen
  488. vercel
  489. vhd-format-lwt
  490. vmnet
  491. vpnkit
  492. vue-jsoo < "0.3"
  493. wayland < "2.0"
  494. webauthn
  495. xen-evtchn
  496. xen-evtchn-unix
  497. xen-gnt
  498. xen-gnt-unix
  499. xenstore
  500. xenstore_transport
  501. xlsx2csv
  502. yocaml_git
  503. yocaml_unix < "2.0.0"
  504. zarr-lwt
  505. zmq-lwt >= "5.2.1"

Conflicts

None

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