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 (34)

Authors

  1. J
    Jérôme Vouillon
  2. J
    Jérémie Dimino

Maintainers

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

Sources

6.0.0-beta01.tar.gz
md5=6bb978c590433a218a53722b985a01d4
sha512=529c8c920716cf204efd60a2bad3f817eed6eed120a8e8a8dae763ab7f4c45799a74004d0d0a2b8ec08f37c81b77f1a4813426a25f256b254dfa846c86debe44

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"
  4. ocaml >= "4.14" & < "5.5"
  5. dune >= "3.15"

Dev Dependencies (2)

  1. odoc with-doc & >= "2.3"
  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. cowabloga
  74. crunch
  75. cstruct-lwt
  76. csv-lwt
  77. ctypes >= "0.15.0" & < "0.21.1"
  78. ctypes-foreign >= "0.21.1"
  79. curl_lwt
  80. current
  81. current-albatross-deployer
  82. current_docker
  83. current_examples
  84. current_git
  85. current_github
  86. current_gitlab
  87. current_ocluster
  88. current_rpc
  89. current_slack
  90. current_web
  91. dap
  92. data-encoding < "0.1.1"
  93. devkit >= "1.2"
  94. distributed-lwt
  95. dkim-bin < "0.8.0"
  96. dkim-lwt-unix
  97. dkim-mirage
  98. dlm
  99. dmarc
  100. dns-certify
  101. dns-cli
  102. dns-client < "7.0.3"
  103. dns-client-lwt
  104. dns-client-mirage
  105. dns-forward
  106. dns-forward-lwt-unix
  107. dns-lwt
  108. dns-mirage
  109. dns-resolver
  110. dns-server
  111. dns-stub
  112. dnsrobot
  113. dnssd
  114. docker_hub
  115. docteur >= "0.0.2"
  116. docteur-solo5
  117. docteur-unix >= "0.0.5"
  118. doi2bib
  119. dream
  120. dream-httpaf
  121. dream-pure
  122. dream-serve < "1.0.1"
  123. dropbox
  124. dune >= "3.17.2"
  125. dune-rpc-lwt
  126. earlybird
  127. elasticsearch-cli
  128. emoji = "2.0.0"
  129. equinoxe
  130. ethernet
  131. ez_api >= "1.2.0" & < "3.0.0"
  132. ezcurl-lwt
  133. ezjs_min < "0.2"
  134. ezjsonm-lwt
  135. ezresto
  136. ezresto-directory >= "0.5"
  137. faraday-lwt
  138. faraday-lwt-unix
  139. fat-filesystem
  140. fiber-lwt
  141. fsevents-lwt
  142. fswatch_lwt
  143. fuseau-lwt
  144. fxmacrodata
  145. gdbprofiler
  146. git
  147. git-cohttp
  148. git-cohttp-unix
  149. git-kv >= "0.2.0"
  150. git-mirage
  151. git-paf
  152. git-unix >= "3.2.0"
  153. github
  154. github-hooks
  155. github-unix >= "4.4.0"
  156. gitlab-unix
  157. gitlab_pipeline_notifier
  158. gluten-lwt
  159. gluten-lwt-unix < "0.4.0"
  160. gluten-mirage < "0.4.0"
  161. gmocoin
  162. granary
  163. granary_repl
  164. graphql-lwt
  165. gremlin
  166. grpc-lwt
  167. guardian
  168. gufo
  169. h1
  170. h1-lwt-unix
  171. h2-lwt
  172. h2-lwt-unix < "0.10.0"
  173. h2-mirage
  174. happy-eyeballs-lwt
  175. happy-eyeballs-mirage
  176. hidapi-lwt
  177. hiredis >= "0.6"
  178. hl_yaml
  179. hockmd
  180. http-lwt-client
  181. http-mirage-client
  182. http-multipart-formdata >= "2.0.0" & < "3.0.0"
  183. httpaf-lwt-unix
  184. httpun-lwt
  185. httpun-mirage
  186. httpun-ws-lwt
  187. hvsock
  188. i3ipc
  189. influxdb-lwt
  190. inotify >= "2.4"
  191. inquire < "0.3.0"
  192. interface-prime-lwt
  193. ip2location
  194. ip2locationio
  195. ip2whois
  196. ipv6-multicast-lwt
  197. irc-client-lwt
  198. irc-client-lwt-ssl
  199. irc-client-tls
  200. irmin
  201. irmin-bench
  202. irmin-chunk
  203. irmin-cli
  204. irmin-client
  205. irmin-containers
  206. irmin-fs
  207. irmin-git
  208. irmin-graphql
  209. irmin-http
  210. irmin-indexeddb
  211. irmin-layers
  212. irmin-mirage-git
  213. irmin-mirage-graphql
  214. irmin-pack
  215. irmin-server
  216. irmin-test
  217. irmin-unix
  218. irmin-watcher
  219. joolog
  220. jose < "0.9.0"
  221. js_of_ocaml-lwt
  222. jsoo_broadcastchannel
  223. jsoo_storage
  224. jupyter
  225. jupyter-kernel
  226. kafka < "0.5"
  227. kafka_lwt
  228. kappa-library
  229. ke >= "0.5"
  230. kinetic-client
  231. kubecaml
  232. lambda-runtime
  233. lambda-term
  234. lambda_streams_lwt
  235. launchd
  236. ldp
  237. learn-ocaml
  238. learn-ocaml-client
  239. ledgerwallet >= "0.4.0"
  240. letsencrypt < "2.0.0"
  241. letsencrypt-app
  242. letsencrypt-dns < "2.0.0"
  243. letters
  244. lichess_api
  245. links
  246. llama
  247. lru_cache
  248. lwt-canceler
  249. lwt-dllist
  250. lwt-exit
  251. lwt-parallel
  252. lwt-pipe
  253. lwt-pipeline
  254. lwt-watcher
  255. lwt_camlp4
  256. lwt_direct >= "6.0.0~beta01"
  257. lwt_domain
  258. lwt_eio < "0.6"
  259. lwt_glib
  260. lwt_log
  261. lwt_ppx
  262. lwt_react
  263. lwt_retry
  264. lwt_ssl
  265. mariadb >= "1.2.0"
  266. markdown_monolith
  267. markup = "0.7.6"
  268. markup-lwt
  269. mdx
  270. mechaml
  271. mehari-lwt-unix
  272. mehari-mirage
  273. memtrace-mirage
  274. metrics-influx
  275. metrics-lwt
  276. metrics-unix
  277. migra
  278. mimic
  279. mindstorm-lwt
  280. mirage < "4.0.0"
  281. mirage-block >= "2.0.1"
  282. mirage-block-ccm
  283. mirage-block-combinators
  284. mirage-block-lwt
  285. mirage-block-partition
  286. mirage-block-ramdisk
  287. mirage-block-solo5
  288. mirage-block-unikraft
  289. mirage-block-unix >= "2.14.2"
  290. mirage-block-xen
  291. mirage-channel >= "4.0.1"
  292. mirage-channel-lwt
  293. mirage-clock-lwt
  294. mirage-clock-unix < "4.2.0"
  295. mirage-console-lwt
  296. mirage-crypto-rng < "0.11.3"
  297. mirage-crypto-rng-lwt
  298. mirage-crypto-rng-mirage
  299. mirage-device >= "2.0.0"
  300. mirage-flow >= "3.0.0"
  301. mirage-flow-combinators
  302. mirage-flow-lwt
  303. mirage-flow-unix
  304. mirage-fs >= "4.0.0"
  305. mirage-fs-lwt
  306. mirage-kv >= "3.0.1"
  307. mirage-kv-lwt
  308. mirage-kv-unix
  309. mirage-net >= "4.0.0"
  310. mirage-net-lwt
  311. mirage-net-macosx
  312. mirage-net-solo5
  313. mirage-net-unikraft
  314. mirage-net-unix
  315. mirage-net-xen
  316. mirage-profile
  317. mirage-protocols >= "7.0.0"
  318. mirage-protocols-lwt
  319. mirage-qubes
  320. mirage-qubes-ipv4
  321. mirage-runtime
  322. mirage-sleep
  323. mirage-solo5
  324. mirage-stack = "3.0.0"
  325. mirage-stack-lwt
  326. mirage-time >= "3.0.0"
  327. mirage-time-lwt
  328. mirage-time-unix
  329. mirage-types-lwt
  330. mirage-unikraft
  331. mirage-unix
  332. mirage-vnetif
  333. mirage-xen
  334. monorobot
  335. moonpool-lwt < "0.11"
  336. mqtt
  337. mrmime >= "0.5.0"
  338. multipart-form-data
  339. multipart_form >= "0.2.0" & < "0.4.0"
  340. multipart_form-cohttp-lwt < "0.6.0"
  341. multipart_form-lwt
  342. mwt
  343. naboris
  344. nbd >= "4.0.3"
  345. nbd-tool
  346. nbd-unix
  347. neo4j_bolt
  348. nocrypto
  349. nottui-lwt
  350. notty-community
  351. nproc
  352. nsq
  353. obuilder
  354. obus >= "1.2.1"
  355. ocaml-ai-sdk
  356. ocluster
  357. ocluster-api
  358. ocluster-worker
  359. ocplib-resto
  360. ocsigenserver
  361. ocsipersist
  362. ocsipersist-dbm
  363. ocsipersist-lib
  364. ocsipersist-pgsql
  365. ocsipersist-sqlite
  366. oframl
  367. ojs_base
  368. omigrate
  369. oneffs
  370. opam-check-npm-deps >= "4.1.0"
  371. opam-publish >= "3.0.0"
  372. opencage
  373. opentelemetry-client-cohttp-lwt
  374. opentelemetry-client-ocurl-lwt
  375. opentelemetry-cohttp-lwt >= "0.4"
  376. opentelemetry-lwt
  377. opium
  378. opium-graphql
  379. opium_kernel
  380. opomodoro
  381. order-i3-xfce
  382. ordma
  383. oskel >= "0.3.0"
  384. ounit-lwt < "2.2.0"
  385. ounit2-lwt
  386. owork
  387. ozulip
  388. paf
  389. paf-cohttp
  390. passage < "0.1.8"
  391. pcap-format < "0.5.2"
  392. petrol
  393. pgn_parser
  394. pgx_lwt
  395. pgx_lwt_mirage
  396. pgx_lwt_unix < "2.0"
  397. piaf < "0.2.0"
  398. picos >= "0.3.0" & < "0.5.0"
  399. picos_lwt
  400. picos_meta
  401. plebeia >= "2.0.0"
  402. plotkicadsch
  403. posix-getopt >= "4.0.2"
  404. ppx_defer >= "0.4.0"
  405. ppx_deriving_rpc
  406. ppx_rapper_lwt
  407. proc-smaps
  408. prof_spacetime
  409. prometheus < "2.0"
  410. prometheus-app
  411. prometheus-lwt
  412. promise_jsoo_lwt
  413. protocol-9p
  414. protocol-9p-unix
  415. proton
  416. pxshot
  417. qcow
  418. qcow-stream
  419. qcow-tool
  420. qcow-types
  421. qdrant < "0.2.0"
  422. qfs
  423. quests
  424. quickterface
  425. rawlink < "2.1"
  426. rawlink-lwt
  427. rdf_json_ld
  428. rdf_lwt
  429. redis-lwt
  430. reparse-lwt
  431. reparse-lwt-unix
  432. resource-pooling
  433. resp
  434. resp-mirage >= "0.10.0"
  435. resp-unix >= "0.10.0"
  436. resto
  437. resto-cohttp-client >= "0.4"
  438. resto-cohttp-self-serving-client
  439. resto-cohttp-server >= "0.4" & < "0.6" | >= "0.9"
  440. resto-directory >= "0.4"
  441. ringo-lwt
  442. river
  443. rock
  444. rpclib-js
  445. rpclib-lwt
  446. SZXX < "4.0.0"
  447. sanddb
  448. scgi
  449. sendmail-lwt
  450. sendmail-mirage
  451. serial
  452. server-reason-react
  453. session-cohttp-lwt
  454. session-cookie-lwt
  455. session-postgresql-lwt
  456. sessions
  457. shared-block-ring
  458. shared-memory-ring-lwt
  459. sherlodoc
  460. sihl < "0.2.0"
  461. slack
  462. slacko
  463. slipshow
  464. smtml >= "0.7.0"
  465. speed
  466. spin < "0.8.0"
  467. spoke
  468. statocaml
  469. stk
  470. stog
  471. swapfs
  472. syguslib-utils
  473. syndic >= "1.4" & < "1.6.0"
  474. tar-mirage
  475. tar-unix
  476. tcpip
  477. telegraml
  478. terminus
  479. testcontainers
  480. testo-lwt
  481. tezt
  482. tidy_email
  483. timmy-lwt
  484. tls >= "0.10.6" & < "0.16.0"
  485. tls-lwt
  486. tls-mirage
  487. tube
  488. tuntap
  489. twirp_cohttp_lwt_unix
  490. uring
  491. uspf
  492. uspf-lwt
  493. uspf-mirage
  494. utcp
  495. utop
  496. uwt
  497. vchan
  498. vchan-unix
  499. vchan-xen
  500. vercel
  501. vhd-format-lwt
  502. vmnet
  503. vue-jsoo < "0.3"
  504. wayland < "2.0"
  505. webauthn
  506. xen-evtchn
  507. xen-evtchn-unix
  508. xen-gnt
  509. xen-gnt-unix
  510. xenstore
  511. xenstore-tool
  512. xenstore_transport
  513. xlsx2csv
  514. yocaml_git
  515. yocaml_unix < "2.0.0"
  516. zarr-lwt
  517. zmq-lwt >= "5.2.1"

Conflicts

None