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

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.

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

Conflicts

None