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 (30)

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

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 (2)

  1. odoc with-doc & >= "2.3.0"
  2. 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. 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. chess_com_api
  49. clz
  50. cmdtui-lambda-term
  51. coap
  52. coap-server-lwt
  53. cohttp-curl-lwt
  54. cohttp-lwt
  55. cohttp-lwt-jsoo
  56. cohttp-lwt-unix
  57. cohttp-mirage
  58. cohttp-server-lwt-unix
  59. comby
  60. comby-semantic
  61. conan-lwt
  62. conduit-lwt
  63. conduit-lwt-unix
  64. cowabloga
  65. crunch
  66. cstruct-lwt
  67. csv-lwt
  68. ctypes >= "0.15.0" & < "0.21.1"
  69. ctypes-foreign >= "0.21.1"
  70. curl_lwt
  71. current
  72. current-albatross-deployer
  73. current_docker
  74. current_examples
  75. current_git
  76. current_github
  77. current_gitlab
  78. current_ocluster
  79. current_rpc
  80. current_slack
  81. current_web
  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. dmarc
  91. dns-certify
  92. dns-cli
  93. dns-client < "7.0.3"
  94. dns-client-lwt
  95. dns-client-mirage
  96. dns-forward
  97. dns-forward-lwt-unix
  98. dns-lwt
  99. dns-mirage
  100. dns-resolver
  101. dns-server
  102. dns-stub
  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. ezresto-directory >= "0.5"
  127. faraday-lwt
  128. faraday-lwt-unix
  129. fat-filesystem
  130. fiber-lwt
  131. fsevents-lwt
  132. fswatch_lwt
  133. fuseau-lwt
  134. gdbprofiler
  135. git
  136. git-cohttp
  137. git-cohttp-unix
  138. git-kv >= "0.2.0"
  139. git-mirage
  140. git-paf
  141. git-unix >= "3.2.0"
  142. github
  143. github-hooks
  144. github-unix >= "4.4.0"
  145. gitlab-unix
  146. gitlab_pipeline_notifier
  147. gluten-lwt
  148. gluten-lwt-unix < "0.4.0"
  149. gluten-mirage < "0.4.0"
  150. graphql-lwt
  151. gremlin
  152. grpc-lwt
  153. guardian
  154. gufo
  155. h1
  156. h1-lwt-unix
  157. h2-lwt
  158. h2-lwt-unix < "0.10.0"
  159. h2-mirage
  160. happy-eyeballs-lwt
  161. happy-eyeballs-mirage
  162. hidapi-lwt
  163. hiredis >= "0.6"
  164. hl_yaml
  165. hockmd
  166. http-lwt-client
  167. http-mirage-client
  168. http-multipart-formdata >= "2.0.0" & < "3.0.0"
  169. httpaf-lwt-unix
  170. httpun-lwt
  171. httpun-mirage
  172. httpun-ws-lwt
  173. hvsock
  174. i3ipc
  175. influxdb-lwt
  176. inotify >= "2.4"
  177. inquire < "0.3.0"
  178. interface-prime-lwt
  179. ip2location
  180. ip2locationio
  181. ip2whois
  182. ipv6-multicast-lwt
  183. irc-client-lwt
  184. irc-client-lwt-ssl
  185. irc-client-tls
  186. irmin
  187. irmin-bench
  188. irmin-chunk
  189. irmin-cli
  190. irmin-client
  191. irmin-containers
  192. irmin-fs
  193. irmin-git
  194. irmin-graphql
  195. irmin-http
  196. irmin-indexeddb
  197. irmin-layers
  198. irmin-mirage-git
  199. irmin-mirage-graphql
  200. irmin-pack
  201. irmin-server
  202. irmin-test
  203. irmin-unix
  204. irmin-watcher
  205. joolog
  206. jose < "0.9.0"
  207. js_of_ocaml-lwt >= "3.5.0"
  208. jsoo_broadcastchannel
  209. jsoo_storage
  210. jupyter
  211. jupyter-kernel
  212. kafka < "0.5"
  213. kafka_lwt
  214. kappa-library
  215. ke >= "0.5"
  216. kinetic-client
  217. kubecaml
  218. lambda-runtime
  219. lambda-term
  220. lambda_streams_lwt
  221. launchd
  222. ldp
  223. learn-ocaml
  224. learn-ocaml-client
  225. ledgerwallet >= "0.4.0"
  226. letsencrypt
  227. letsencrypt-app
  228. letsencrypt-dns
  229. letters
  230. lichess_api
  231. links >= "0.9.1"
  232. linol-lwt
  233. llama
  234. lru_cache
  235. lwt-canceler
  236. lwt-dllist
  237. lwt-exit
  238. lwt-parallel
  239. lwt-pipe
  240. lwt-pipeline
  241. lwt-watcher
  242. lwt_camlp4
  243. lwt_domain
  244. lwt_eio
  245. lwt_glib
  246. lwt_log
  247. lwt_ppx < "6.0.0~beta01"
  248. lwt_react
  249. lwt_retry
  250. lwt_ssl
  251. mariadb >= "1.2.0"
  252. markdown_monolith
  253. markup = "0.7.6"
  254. markup-lwt
  255. mdx
  256. mechaml
  257. mehari-lwt-unix
  258. mehari-mirage
  259. memtrace-mirage
  260. metrics-influx
  261. metrics-lwt
  262. metrics-unix
  263. mimic
  264. mindstorm-lwt
  265. mirage < "4.0.0"
  266. mirage-block >= "2.0.1"
  267. mirage-block-ccm
  268. mirage-block-combinators
  269. mirage-block-lwt
  270. mirage-block-partition
  271. mirage-block-ramdisk
  272. mirage-block-solo5
  273. mirage-block-unikraft
  274. mirage-block-unix >= "2.14.2"
  275. mirage-block-xen
  276. mirage-channel >= "4.0.1"
  277. mirage-channel-lwt
  278. mirage-clock-lwt
  279. mirage-clock-unix < "4.2.0"
  280. mirage-console-lwt
  281. mirage-crypto-rng < "0.11.3"
  282. mirage-crypto-rng-lwt
  283. mirage-crypto-rng-mirage
  284. mirage-device >= "2.0.0"
  285. mirage-flow >= "3.0.0"
  286. mirage-flow-combinators
  287. mirage-flow-lwt
  288. mirage-flow-unix
  289. mirage-fs >= "4.0.0"
  290. mirage-fs-lwt
  291. mirage-kv >= "3.0.1"
  292. mirage-kv-lwt
  293. mirage-kv-unix
  294. mirage-net >= "4.0.0"
  295. mirage-net-lwt
  296. mirage-net-macosx
  297. mirage-net-solo5
  298. mirage-net-unikraft
  299. mirage-net-unix
  300. mirage-net-xen
  301. mirage-profile
  302. mirage-protocols >= "7.0.0"
  303. mirage-protocols-lwt
  304. mirage-qubes
  305. mirage-qubes-ipv4
  306. mirage-runtime
  307. mirage-sleep
  308. mirage-solo5
  309. mirage-stack = "3.0.0"
  310. mirage-stack-lwt
  311. mirage-time >= "3.0.0"
  312. mirage-time-lwt
  313. mirage-time-unix
  314. mirage-types-lwt
  315. mirage-unikraft
  316. mirage-unix
  317. mirage-vnetif
  318. mirage-xen
  319. monorobot
  320. moonpool-lwt
  321. mqtt
  322. mrmime >= "0.5.0"
  323. multipart-form-data
  324. multipart_form >= "0.2.0" & < "0.4.0"
  325. multipart_form-cohttp-lwt < "0.6.0"
  326. multipart_form-lwt
  327. mwt
  328. naboris
  329. nbd >= "4.0.3"
  330. nbd-tool
  331. nbd-unix
  332. neo4j_bolt
  333. nocrypto
  334. nottui-lwt
  335. notty-community
  336. nproc
  337. nsq
  338. obuilder
  339. obus >= "1.2.1"
  340. ocluster
  341. ocluster-api
  342. ocluster-worker
  343. ocplib-resto
  344. ocsigenserver
  345. ocsipersist
  346. ocsipersist-dbm
  347. ocsipersist-lib
  348. ocsipersist-pgsql
  349. ocsipersist-sqlite
  350. oframl
  351. ojs_base
  352. omigrate
  353. oneffs
  354. opam-check-npm-deps >= "4.1.0"
  355. opencage
  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 < "0.1.8"
  373. pcap-format < "0.5.2"
  374. petrol
  375. pgn_parser
  376. pgx_lwt
  377. pgx_lwt_mirage
  378. pgx_lwt_unix < "2.0"
  379. piaf < "0.2.0"
  380. picos >= "0.3.0" & < "0.5.0"
  381. picos_lwt
  382. picos_meta
  383. plebeia >= "2.0.0"
  384. plist-xml-lwt
  385. plotkicadsch
  386. ppx_defer >= "0.4.0"
  387. ppx_deriving_rpc
  388. ppx_rapper_lwt
  389. proc-smaps
  390. prof_spacetime
  391. prometheus
  392. prometheus-app
  393. promise_jsoo_lwt
  394. protocol-9p
  395. protocol-9p-unix
  396. proton
  397. pxshot
  398. qcow
  399. qcow-stream
  400. qcow-tool
  401. qcow-types
  402. qdrant < "0.2.0"
  403. qfs >= "0.5"
  404. quests
  405. rawlink < "2.1"
  406. rawlink-lwt
  407. rdf_json_ld
  408. rdf_lwt
  409. redis-lwt
  410. reparse-lwt
  411. reparse-lwt-unix
  412. resource-pooling
  413. resp
  414. resp-mirage >= "0.10.0"
  415. resp-unix >= "0.10.0"
  416. resto
  417. resto-cohttp-client >= "0.4"
  418. resto-cohttp-self-serving-client
  419. resto-cohttp-server >= "0.4" & < "0.6" | >= "0.9"
  420. resto-directory >= "0.4"
  421. riak
  422. ringo-lwt
  423. river
  424. rock
  425. rpclib-js
  426. rpclib-lwt
  427. SZXX < "4.0.0"
  428. sanddb
  429. scgi
  430. sendmail-lwt
  431. sendmail-mirage
  432. serial
  433. server-reason-react < "0.4.0"
  434. session-cohttp-lwt
  435. session-cookie-lwt
  436. session-postgresql-lwt
  437. sessions
  438. shared-block-ring
  439. shared-memory-ring-lwt
  440. sherlodoc
  441. sihl < "0.2.0"
  442. slack
  443. slacko
  444. slipshow
  445. smtml >= "0.7.0"
  446. speed
  447. spin < "0.8.0"
  448. spoke
  449. statocaml
  450. stk
  451. stog
  452. swapfs
  453. syguslib-utils
  454. syndic >= "1.4" & < "1.6.0"
  455. tar-mirage
  456. tar-unix
  457. tcpip
  458. telegraml
  459. terminus
  460. testo-lwt
  461. tezt
  462. tidy_email
  463. timmy-lwt
  464. tls >= "0.10.6" & < "0.16.0"
  465. tls-lwt
  466. tls-mirage
  467. tube
  468. tuntap
  469. twirp_cohttp_lwt_unix
  470. uring
  471. uspf
  472. uspf-lwt
  473. uspf-mirage
  474. utcp
  475. utop
  476. uwt
  477. vchan
  478. vchan-unix
  479. vchan-xen
  480. vercel
  481. vhd-format-lwt
  482. vmnet
  483. vpnkit
  484. vue-jsoo < "0.3"
  485. wayland < "2.0"
  486. webauthn
  487. xen-evtchn
  488. xen-evtchn-unix
  489. xen-gnt
  490. xen-gnt-unix
  491. xenstore
  492. xenstore-tool
  493. xenstore_transport
  494. xlsx2csv
  495. yocaml_git
  496. yocaml_unix < "2.0.0"
  497. zarr-lwt
  498. zmq-lwt >= "5.2.1"

Conflicts

None