package lwt

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

Install 

dune-project
 Dependency

github.com Readme MIT License Edit opam file Versions (32)

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.

Added to opam-repository:

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" & < "5.5"
  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. atproto
  16. awa-mirage
  17. aws-lwt
  18. aws-s3-lwt >= "4.8.1"
  19. awskit-lwt
  20. awskit-lwt-unix
  21. awskit-s3-lwt
  22. awskit-s3-lwt-unix
  23. awsm-lwt
  24. awso-lwt
  25. azure-cosmos-db < "0.4.0"
  26. azure-cosmos-db-lwt
  27. balancer
  28. bastet_lwt
  29. bimage-lwt
  30. bistro
  31. bitflyer
  32. bizowie-api
  33. brisk-reconciler
  34. builder
  35. builder-web
  36. bun >= "0.3.3"
  37. cachet-lwt
  38. calculon
  39. caldav
  40. camltc
  41. canary
  42. capnp-rpc-lwt < "2.0"
  43. capnp-rpc-unix < "2.1"
  44. caqti-lwt
  45. caqti-mirage
  46. carton < "1.0.0"
  47. carton-git
  48. carton-lwt
  49. catala-format >= "0.2.0"
  50. cf-lwt
  51. chamelon
  52. chamelon-unix
  53. chamo
  54. charrua-client
  55. charrua-unix
  56. chess_com_api
  57. ciao_lwt
  58. clz
  59. cmdtui-lambda-term
  60. coap
  61. coap-server-lwt
  62. cohttp-curl-lwt
  63. cohttp-lwt
  64. cohttp-lwt-jsoo
  65. cohttp-lwt-unix
  66. cohttp-mirage
  67. cohttp-server-lwt-unix
  68. comby
  69. comby-semantic
  70. conan-lwt
  71. conduit-lwt
  72. conduit-lwt-unix
  73. cosmetyc_native
  74. cowabloga
  75. crunch
  76. cstruct-lwt
  77. csv-lwt
  78. ctypes >= "0.15.0" & < "0.21.1"
  79. ctypes-foreign >= "0.21.1"
  80. curl_lwt
  81. current
  82. current-albatross-deployer
  83. current_docker
  84. current_examples
  85. current_git
  86. current_github
  87. current_gitlab
  88. current_ocluster
  89. current_rpc
  90. current_slack
  91. current_web
  92. dap
  93. data-encoding < "0.1.1"
  94. devkit >= "1.2" & < "1.20261006"
  95. distributed-lwt
  96. dkim-bin < "0.8.0"
  97. dkim-lwt-unix
  98. dkim-mirage
  99. dlm
  100. dmarc
  101. dns-certify
  102. dns-cli
  103. dns-client < "7.0.3"
  104. dns-client-lwt
  105. dns-client-mirage
  106. dns-forward
  107. dns-forward-lwt-unix
  108. dns-lwt
  109. dns-mirage
  110. dns-resolver
  111. dns-server
  112. dns-stub
  113. dnsrobot
  114. dnssd
  115. docker_hub
  116. docteur >= "0.0.2"
  117. docteur-solo5
  118. docteur-unix >= "0.0.5"
  119. doi2bib
  120. dream
  121. dream-httpaf
  122. dream-mirage
  123. dream-pure
  124. dream-serve
  125. dropbox
  126. dune >= "3.17.2"
  127. dune-rpc-lwt
  128. earlybird
  129. elasticsearch-cli
  130. emoji = "2.0.0"
  131. equinoxe
  132. ethernet
  133. ez_api >= "1.2.0" & < "3.0.0"
  134. ezcurl-lwt
  135. ezjs_min < "0.2"
  136. ezjsonm-lwt
  137. ezresto
  138. ezresto-directory >= "0.5"
  139. faraday-lwt
  140. faraday-lwt-unix
  141. fat-filesystem
  142. fiber-lwt
  143. fsevents-lwt
  144. fswatch_lwt
  145. fuseau-lwt
  146. fxmacrodata
  147. gdbprofiler
  148. git
  149. git-cohttp
  150. git-cohttp-unix
  151. git-kv >= "0.2.0"
  152. git-mirage
  153. git-paf
  154. git-unix >= "3.2.0"
  155. github
  156. github-hooks
  157. github-unix >= "4.4.0"
  158. gitlab-unix
  159. gitlab_pipeline_notifier
  160. gluten-lwt
  161. gluten-lwt-unix < "0.4.0"
  162. gluten-mirage < "0.4.0"
  163. gmocoin
  164. granary
  165. granary_repl
  166. graphql-lwt
  167. gremlin
  168. grpc-lwt
  169. guardian
  170. gufo
  171. h1
  172. h1-lwt-unix
  173. h2-lwt
  174. h2-lwt-unix < "0.10.0"
  175. h2-mirage
  176. happy-eyeballs-lwt
  177. happy-eyeballs-mirage
  178. hidapi-lwt
  179. hiredis >= "0.6"
  180. hl_yaml
  181. hockmd
  182. http-lwt-client
  183. http-mirage-client
  184. http-multipart-formdata >= "2.0.0" & < "3.0.0"
  185. httpaf-lwt-unix
  186. httpun-lwt
  187. httpun-mirage
  188. httpun-ws-lwt
  189. hvsock
  190. i3ipc
  191. influxdb-lwt
  192. inotify >= "2.4"
  193. inquire < "0.3.0"
  194. interface-prime-lwt
  195. ip2location
  196. ip2locationio
  197. ip2whois
  198. ipv6-multicast-lwt
  199. irc-client-lwt
  200. irc-client-lwt-ssl
  201. irc-client-tls
  202. irmin
  203. irmin-bench
  204. irmin-chunk
  205. irmin-cli
  206. irmin-client
  207. irmin-containers
  208. irmin-fs
  209. irmin-git
  210. irmin-graphql
  211. irmin-http
  212. irmin-indexeddb
  213. irmin-layers
  214. irmin-mirage-git
  215. irmin-mirage-graphql
  216. irmin-pack
  217. irmin-server
  218. irmin-test
  219. irmin-unix
  220. irmin-watcher
  221. joolog
  222. jose < "0.9.0"
  223. js_of_ocaml-lwt
  224. jsoo_broadcastchannel
  225. jsoo_storage
  226. jupyter
  227. jupyter-kernel
  228. kafka < "0.5"
  229. kafka_lwt
  230. kappa-library
  231. ke >= "0.5"
  232. kinetic-client
  233. kubecaml
  234. lambda-runtime
  235. lambda-term
  236. lambda_streams_lwt
  237. launchd
  238. ldp
  239. learn-ocaml
  240. learn-ocaml-client
  241. ledgerwallet >= "0.4.0"
  242. letsencrypt < "2.0.0"
  243. letsencrypt-app
  244. letsencrypt-dns < "2.0.0"
  245. letters
  246. lichess_api
  247. links
  248. linol-lwt
  249. llama
  250. lru_cache
  251. lwt-canceler
  252. lwt-dllist
  253. lwt-exit
  254. lwt-parallel
  255. lwt-pipe
  256. lwt-pipeline
  257. lwt-watcher
  258. lwt_camlp4
  259. lwt_domain
  260. lwt_eio < "0.6"
  261. lwt_glib
  262. lwt_log
  263. lwt_ppx < "6.0.0~beta01"
  264. lwt_react
  265. lwt_retry
  266. lwt_ssl
  267. mariadb >= "1.2.0"
  268. markdown_monolith
  269. markup = "0.7.6"
  270. markup-lwt
  271. mdx
  272. mechaml
  273. mehari-lwt-unix
  274. mehari-mirage
  275. memtrace-mirage
  276. metrics-influx
  277. metrics-lwt
  278. metrics-unix
  279. migra
  280. mimic
  281. mindstorm-lwt
  282. mirage < "4.0.0"
  283. mirage-block >= "2.0.1"
  284. mirage-block-ccm
  285. mirage-block-combinators
  286. mirage-block-lwt
  287. mirage-block-partition
  288. mirage-block-ramdisk
  289. mirage-block-solo5
  290. mirage-block-unikraft
  291. mirage-block-unix >= "2.14.2"
  292. mirage-block-xen
  293. mirage-channel >= "4.0.1"
  294. mirage-channel-lwt
  295. mirage-clock-lwt
  296. mirage-clock-unix < "4.2.0"
  297. mirage-console-lwt
  298. mirage-crypto-rng < "0.11.3"
  299. mirage-crypto-rng-lwt
  300. mirage-crypto-rng-mirage
  301. mirage-device >= "2.0.0"
  302. mirage-flow >= "3.0.0"
  303. mirage-flow-combinators
  304. mirage-flow-lwt
  305. mirage-flow-unix
  306. mirage-fs >= "4.0.0"
  307. mirage-fs-lwt
  308. mirage-kv >= "3.0.1"
  309. mirage-kv-lwt
  310. mirage-kv-unix
  311. mirage-net >= "4.0.0"
  312. mirage-net-lwt
  313. mirage-net-macosx
  314. mirage-net-solo5
  315. mirage-net-unikraft
  316. mirage-net-unix
  317. mirage-net-xen
  318. mirage-profile
  319. mirage-protocols >= "7.0.0"
  320. mirage-protocols-lwt
  321. mirage-qubes
  322. mirage-qubes-ipv4
  323. mirage-runtime
  324. mirage-sleep
  325. mirage-solo5
  326. mirage-stack = "3.0.0"
  327. mirage-stack-lwt
  328. mirage-time >= "3.0.0"
  329. mirage-time-lwt
  330. mirage-time-unix
  331. mirage-types-lwt
  332. mirage-unikraft
  333. mirage-unix
  334. mirage-vnetif
  335. mirage-xen
  336. monorobot
  337. moonpool-lwt
  338. mqtt
  339. mrmime >= "0.5.0"
  340. multipart-form-data
  341. multipart_form >= "0.2.0" & < "0.4.0"
  342. multipart_form-cohttp-lwt < "0.6.0"
  343. multipart_form-lwt
  344. mwt
  345. naboris
  346. nbd >= "4.0.3"
  347. nbd-tool
  348. nbd-unix
  349. neo4j_bolt
  350. nocrypto
  351. nottui-lwt
  352. notty-community
  353. nproc
  354. nsq
  355. obuilder
  356. obus >= "1.2.1"
  357. ocaml-ai-sdk
  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. oframl
  369. ohttp-cohttp-lwt
  370. ojs_base
  371. omigrate
  372. oneffs
  373. opam-check-npm-deps >= "4.1.0"
  374. opam-publish >= "3.0.0"
  375. opencage
  376. opentelemetry-client-cohttp-lwt
  377. opentelemetry-client-ocurl-lwt
  378. opentelemetry-cohttp-lwt >= "0.4"
  379. opentelemetry-lwt
  380. opium
  381. opium-graphql
  382. opium_kernel
  383. opomodoro
  384. order-i3-xfce
  385. ordma
  386. oskel >= "0.3.0"
  387. ounit-lwt < "2.2.0"
  388. ounit2-lwt
  389. owork
  390. ozulip
  391. paf
  392. paf-cohttp
  393. passage < "0.1.8"
  394. pcap-format < "0.5.2"
  395. petrol
  396. pgn_parser
  397. pgx_lwt
  398. pgx_lwt_mirage
  399. pgx_lwt_unix < "2.0"
  400. piaf < "0.2.0"
  401. picos >= "0.3.0" & < "0.5.0"
  402. picos_lwt
  403. picos_meta
  404. plebeia >= "2.0.0"
  405. plist-xml-lwt
  406. plotkicadsch
  407. posix-getopt >= "4.0.2"
  408. ppx_defer >= "0.4.0"
  409. ppx_deriving_rpc
  410. ppx_rapper_lwt
  411. privatus-analytics-dream
  412. proc-smaps
  413. prof_spacetime
  414. prometheus < "2.0"
  415. prometheus-app
  416. prometheus-lwt
  417. promise_jsoo_lwt
  418. protocol-9p
  419. protocol-9p-unix
  420. proton
  421. pxshot
  422. qcow
  423. qcow-stream
  424. qcow-tool
  425. qcow-types
  426. qdrant < "0.2.0"
  427. qfs
  428. quests
  429. quickterface
  430. rawlink < "2.1"
  431. rawlink-lwt
  432. rdf_json_ld
  433. rdf_lwt
  434. redis-lwt
  435. reparse-lwt
  436. reparse-lwt-unix
  437. resource-pooling
  438. resp
  439. resp-mirage >= "0.10.0"
  440. resp-unix >= "0.10.0"
  441. resto
  442. resto-cohttp-client >= "0.4"
  443. resto-cohttp-self-serving-client
  444. resto-cohttp-server >= "0.4" & < "0.6" | >= "0.9"
  445. resto-directory >= "0.4"
  446. ringo-lwt
  447. river
  448. rock
  449. rpclib-js
  450. rpclib-lwt
  451. SZXX < "4.0.0"
  452. sanddb
  453. scgi
  454. sendmail-lwt
  455. sendmail-mirage
  456. serial
  457. server-reason-react < "0.4.0"
  458. session-cohttp-lwt
  459. session-cookie-lwt
  460. session-postgresql-lwt
  461. sessions
  462. shared-block-ring
  463. shared-memory-ring-lwt
  464. sherlodoc
  465. sihl < "0.2.0"
  466. slack
  467. slacko
  468. slipshow < "0.12.0"
  469. smtml >= "0.7.0"
  470. speed
  471. spin < "0.8.0"
  472. spoke
  473. statocaml
  474. stk
  475. stog
  476. swapfs
  477. syguslib-utils
  478. syndic >= "1.4" & < "1.6.0"
  479. tar-mirage
  480. tar-unix
  481. tcpip
  482. telegraml
  483. terminus
  484. testcontainers
  485. testo-lwt
  486. tezt
  487. tidy_email
  488. timmy-lwt
  489. tls >= "0.10.6" & < "0.16.0"
  490. tls-lwt
  491. tls-mirage
  492. tube
  493. tuntap
  494. twirp_cohttp_lwt_unix
  495. uring
  496. uspf
  497. uspf-lwt
  498. uspf-mirage
  499. utcp
  500. utop
  501. uwt
  502. vchan
  503. vchan-unix
  504. vchan-xen
  505. vercel
  506. vhd-format-lwt
  507. vmnet
  508. vue-jsoo < "0.3"
  509. wayland < "2.0"
  510. webauthn
  511. xen-evtchn
  512. xen-evtchn-unix
  513. xen-gnt
  514. xen-gnt-unix
  515. xenstore
  516. xenstore-tool
  517. xenstore_transport
  518. xlsx2csv
  519. yocaml_git
  520. yocaml_unix < "2.0.0"
  521. zarr-lwt
  522. zmq-lwt >= "5.2.1"

Conflicts

None

Build status

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This version is not in the latest run; the versions below are.

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