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

5.8.0.tar.gz
md5=2682558f405ab7c8638eeb16d0f9f46f
sha512=7f6548a1b1dbfdbc98d9352151ca7be97fa2ab63dbcc429208ce8d08308eee13f7fce31e0cca53f8880233959a60212d622270dd51bf164c3ee272f179769bd9

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 Oct 2024

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 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" & < "5.5"
  5. dune >= "1.12"

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 < "0.4.0"
  20. azure-cosmos-db-lwt
  21. balancer
  22. bastet_lwt
  23. bimage-lwt
  24. bistro
  25. brisk-reconciler
  26. brozip
  27. builder
  28. builder-web
  29. bun >= "0.3.3"
  30. cachet-lwt
  31. calculon
  32. caldav
  33. camltc
  34. canary
  35. capnp-rpc-lwt < "2.0"
  36. capnp-rpc-unix < "2.1"
  37. caqti-lwt
  38. caqti-mirage
  39. carton < "1.0.0"
  40. carton-git
  41. carton-lwt
  42. catala-format >= "0.2.0"
  43. cf-lwt
  44. chamelon
  45. chamelon-unix
  46. chamo
  47. charrua-client
  48. charrua-unix
  49. chess_com_api
  50. ciao_lwt
  51. clz
  52. cmdtui-lambda-term
  53. coap
  54. coap-server-lwt
  55. cohttp-curl-lwt
  56. cohttp-lwt
  57. cohttp-lwt-jsoo
  58. cohttp-lwt-unix
  59. cohttp-mirage
  60. cohttp-server-lwt-unix
  61. comby
  62. comby-semantic
  63. conan-lwt
  64. conduit-lwt
  65. conduit-lwt-unix
  66. cowabloga
  67. crunch
  68. cstruct-lwt
  69. csv-lwt
  70. ctypes >= "0.15.0" & < "0.21.1"
  71. ctypes-foreign >= "0.21.1"
  72. curl_lwt
  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
  82. current_slack
  83. current_web
  84. dap
  85. data-encoding < "0.1.1"
  86. devkit >= "1.2"
  87. distributed-lwt
  88. dkim-bin < "0.8.0"
  89. dkim-lwt-unix
  90. dkim-mirage
  91. dlm
  92. dmarc
  93. dns-certify
  94. dns-cli
  95. dns-client < "7.0.3"
  96. dns-client-lwt
  97. dns-client-mirage
  98. dns-forward
  99. dns-forward-lwt-unix
  100. dns-lwt
  101. dns-mirage
  102. dns-resolver
  103. dns-server
  104. dns-stub
  105. dnsrobot
  106. dnssd
  107. docker_hub
  108. docteur >= "0.0.2"
  109. docteur-solo5
  110. docteur-unix >= "0.0.5"
  111. doi2bib
  112. dream
  113. dream-httpaf
  114. dream-pure
  115. dream-serve
  116. dropbox
  117. dune >= "3.17.2"
  118. dune-rpc-lwt
  119. earlybird
  120. elasticsearch-cli
  121. emoji = "2.0.0"
  122. equinoxe
  123. ethernet
  124. ez_api >= "1.2.0"
  125. ezcurl-lwt
  126. ezjs_min < "0.2"
  127. ezjsonm-lwt
  128. ezresto
  129. ezresto-directory >= "0.5"
  130. faraday-lwt
  131. faraday-lwt-unix
  132. fat-filesystem
  133. fiber-lwt
  134. fsevents-lwt
  135. fswatch_lwt
  136. fuseau-lwt
  137. gdbprofiler
  138. git
  139. git-cohttp
  140. git-cohttp-unix
  141. git-mirage
  142. git-paf
  143. git-unix >= "3.2.0"
  144. github
  145. github-hooks
  146. github-unix >= "4.4.0"
  147. gitlab-unix
  148. gitlab_pipeline_notifier
  149. gluten-lwt
  150. gluten-lwt-unix < "0.4.0"
  151. gluten-mirage < "0.4.0"
  152. graphql-lwt
  153. gremlin
  154. grpc-lwt
  155. guardian
  156. gufo
  157. h1
  158. h1-lwt-unix
  159. h2-lwt
  160. h2-lwt-unix < "0.10.0"
  161. h2-mirage
  162. happy-eyeballs-lwt
  163. happy-eyeballs-mirage
  164. hidapi-lwt
  165. hiredis >= "0.6"
  166. hl_yaml
  167. hockmd
  168. http-lwt-client
  169. http-mirage-client
  170. http-multipart-formdata >= "2.0.0" & < "3.0.0"
  171. httpaf-lwt-unix
  172. httpun-lwt
  173. httpun-mirage
  174. httpun-ws-lwt
  175. hvsock
  176. i3ipc
  177. influxdb-lwt
  178. inotify >= "2.4"
  179. inquire < "0.3.0"
  180. interface-prime-lwt
  181. ip2location
  182. ip2locationio
  183. ip2whois
  184. ipv6-multicast-lwt
  185. irc-client-lwt
  186. irc-client-lwt-ssl
  187. irc-client-tls
  188. irmin
  189. irmin-bench
  190. irmin-chunk
  191. irmin-cli
  192. irmin-client
  193. irmin-containers
  194. irmin-fs
  195. irmin-git
  196. irmin-graphql
  197. irmin-http
  198. irmin-indexeddb
  199. irmin-layers
  200. irmin-mirage-git
  201. irmin-mirage-graphql
  202. irmin-pack
  203. irmin-server
  204. irmin-test
  205. irmin-unix
  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. lambda-runtime
  221. lambda-term
  222. lambda_streams_lwt
  223. launchd
  224. ldp
  225. learn-ocaml
  226. learn-ocaml-client
  227. ledgerwallet >= "0.4.0"
  228. letsencrypt < "2.0.0"
  229. letsencrypt-app
  230. letsencrypt-dns < "2.0.0"
  231. letters
  232. lichess_api
  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 < "0.6"
  247. lwt_glib
  248. lwt_log
  249. lwt_ppx < "6.0.0~beta01"
  250. lwt_react
  251. lwt_retry
  252. lwt_ssl
  253. mariadb >= "1.2.0"
  254. markdown_monolith
  255. markup = "0.7.6"
  256. markup-lwt
  257. mdx
  258. mechaml
  259. mehari-lwt-unix
  260. mehari-mirage
  261. memtrace-mirage
  262. metrics-influx
  263. metrics-lwt
  264. metrics-unix
  265. mimic
  266. mindstorm-lwt
  267. mirage < "4.0.0"
  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-unikraft
  276. mirage-block-unix >= "2.14.2"
  277. mirage-block-xen
  278. mirage-channel >= "4.0.1"
  279. mirage-channel-lwt
  280. mirage-clock-lwt
  281. mirage-clock-unix < "4.2.0"
  282. mirage-console-lwt
  283. mirage-crypto-rng < "0.11.3"
  284. mirage-crypto-rng-lwt
  285. mirage-crypto-rng-mirage
  286. mirage-device >= "2.0.0"
  287. mirage-flow >= "3.0.0"
  288. mirage-flow-combinators
  289. mirage-flow-lwt
  290. mirage-flow-unix
  291. mirage-fs >= "4.0.0"
  292. mirage-fs-lwt
  293. mirage-kv >= "3.0.1"
  294. mirage-kv-lwt
  295. mirage-kv-unix
  296. mirage-net >= "4.0.0"
  297. mirage-net-lwt
  298. mirage-net-macosx
  299. mirage-net-solo5
  300. mirage-net-unikraft
  301. mirage-net-unix
  302. mirage-net-xen
  303. mirage-profile
  304. mirage-protocols >= "7.0.0"
  305. mirage-protocols-lwt
  306. mirage-qubes
  307. mirage-qubes-ipv4
  308. mirage-runtime
  309. mirage-sleep
  310. mirage-solo5
  311. mirage-stack = "3.0.0"
  312. mirage-stack-lwt
  313. mirage-time >= "3.0.0"
  314. mirage-time-lwt
  315. mirage-time-unix
  316. mirage-types-lwt
  317. mirage-unikraft
  318. mirage-unix
  319. mirage-vnetif
  320. mirage-xen
  321. monorobot
  322. moonpool-lwt
  323. mqtt
  324. mrmime >= "0.5.0"
  325. multipart-form-data
  326. multipart_form >= "0.2.0" & < "0.4.0"
  327. multipart_form-cohttp-lwt < "0.6.0"
  328. multipart_form-lwt
  329. mwt
  330. naboris
  331. nbd >= "4.0.3"
  332. nbd-tool
  333. nbd-unix
  334. neo4j_bolt
  335. nocrypto
  336. nottui-lwt
  337. notty-community
  338. nproc
  339. nsq
  340. obuilder
  341. obus >= "1.2.1"
  342. ocluster
  343. ocluster-api
  344. ocluster-worker
  345. ocplib-resto
  346. ocsigenserver
  347. ocsipersist
  348. ocsipersist-dbm
  349. ocsipersist-lib
  350. ocsipersist-pgsql
  351. ocsipersist-sqlite
  352. oframl
  353. ojs_base
  354. omigrate
  355. oneffs
  356. opam-check-npm-deps >= "4.1.0"
  357. opam-publish >= "3.0.0"
  358. opencage
  359. opentelemetry-client-cohttp-lwt
  360. opentelemetry-client-ocurl-lwt
  361. opentelemetry-cohttp-lwt >= "0.4"
  362. opentelemetry-lwt
  363. opium
  364. opium-graphql
  365. opium_kernel
  366. opomodoro
  367. order-i3-xfce
  368. ordma
  369. oskel >= "0.3.0"
  370. ounit-lwt < "2.2.0"
  371. ounit2-lwt
  372. owork
  373. ozulip
  374. paf
  375. paf-cohttp
  376. passage < "0.1.8"
  377. pcap-format < "0.5.2"
  378. petrol
  379. pgn_parser
  380. pgx_lwt
  381. pgx_lwt_mirage
  382. pgx_lwt_unix < "2.0"
  383. piaf < "0.2.0"
  384. picos >= "0.3.0" & < "0.5.0"
  385. picos_lwt
  386. picos_meta
  387. plebeia >= "2.0.0"
  388. plist-xml-lwt
  389. plotkicadsch
  390. posix-getopt >= "4.0.2"
  391. ppx_defer >= "0.4.0"
  392. ppx_deriving_rpc
  393. ppx_rapper_lwt
  394. proc-smaps
  395. prof_spacetime
  396. prometheus
  397. prometheus-app
  398. promise_jsoo_lwt
  399. protocol-9p
  400. protocol-9p-unix
  401. proton
  402. pxshot
  403. qcow
  404. qcow-stream
  405. qcow-tool
  406. qcow-types
  407. qdrant < "0.2.0"
  408. qfs >= "0.5"
  409. quests
  410. quickterface
  411. rawlink < "2.1"
  412. rawlink-lwt
  413. rdf_json_ld
  414. rdf_lwt
  415. redis-lwt
  416. reparse-lwt
  417. reparse-lwt-unix
  418. resource-pooling
  419. resp
  420. resp-mirage >= "0.10.0"
  421. resp-unix >= "0.10.0"
  422. resto
  423. resto-cohttp-client >= "0.4"
  424. resto-cohttp-self-serving-client
  425. resto-cohttp-server >= "0.4" & < "0.6" | >= "0.9"
  426. resto-directory >= "0.4"
  427. riak
  428. ringo-lwt
  429. river
  430. rock
  431. rpclib-js
  432. rpclib-lwt
  433. SZXX < "4.0.0"
  434. sanddb
  435. scgi
  436. sendmail-lwt
  437. sendmail-mirage
  438. serial
  439. server-reason-react < "0.4.0"
  440. session-cohttp-lwt
  441. session-cookie-lwt
  442. session-postgresql-lwt
  443. sessions
  444. shared-block-ring
  445. shared-memory-ring-lwt
  446. sherlodoc
  447. sihl < "0.2.0"
  448. slack
  449. slacko
  450. slipshow
  451. smtml >= "0.7.0"
  452. speed
  453. spin < "0.8.0"
  454. spoke
  455. stk
  456. stog
  457. swapfs
  458. syguslib-utils
  459. syndic >= "1.4" & < "1.6.0"
  460. tar-mirage
  461. tar-unix
  462. tcpip
  463. telegraml
  464. terminus
  465. testcontainers
  466. testo-lwt
  467. tezt
  468. tidy_email
  469. timmy-lwt
  470. tls >= "0.10.6" & < "0.16.0"
  471. tls-lwt
  472. tls-mirage
  473. tube
  474. tuntap
  475. twirp_cohttp_lwt_unix
  476. uring
  477. uspf
  478. uspf-lwt
  479. uspf-mirage
  480. utcp
  481. utop
  482. uwt
  483. vchan
  484. vchan-unix
  485. vchan-xen
  486. vercel
  487. vhd-format-lwt
  488. vmnet
  489. vue-jsoo < "0.3"
  490. wayland < "2.0"
  491. webauthn
  492. xen-evtchn
  493. xen-evtchn-unix
  494. xen-gnt
  495. xen-gnt-unix
  496. xenstore
  497. xenstore-tool
  498. xenstore_transport
  499. xlsx2csv
  500. yocaml_git
  501. yocaml_unix < "2.0.0"
  502. zarr-lwt
  503. zmq-lwt >= "5.2.1"

Conflicts

None