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

5.8.1.tar.gz
md5=d0f824f75ce5297975aec75366fed36c
sha512=c43fff2e9e808dc49ddf09caf7040e512ba36aebe2c600d524095ced11c196c3ac619953a11cb3e3a9f7c6c99e798d43bf179d5f790ab258fb9f746fae4c1125

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 >= "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 >= "4.8.1"
  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. gdbprofiler
  144. git
  145. git-cohttp
  146. git-cohttp-unix
  147. git-mirage
  148. git-paf
  149. git-unix >= "3.2.0"
  150. github
  151. github-hooks
  152. github-unix >= "4.4.0"
  153. gitlab-unix
  154. gitlab_pipeline_notifier
  155. gluten-lwt
  156. gluten-lwt-unix < "0.4.0"
  157. gluten-mirage < "0.4.0"
  158. gmocoin
  159. granary
  160. granary_repl
  161. graphql-lwt
  162. gremlin
  163. grpc-lwt
  164. guardian
  165. gufo
  166. h1
  167. h1-lwt-unix
  168. h2-lwt
  169. h2-lwt-unix < "0.10.0"
  170. h2-mirage
  171. happy-eyeballs-lwt
  172. happy-eyeballs-mirage
  173. hidapi-lwt
  174. hiredis >= "0.6"
  175. hl_yaml
  176. hockmd
  177. http-lwt-client
  178. http-mirage-client
  179. http-multipart-formdata >= "2.0.0" & < "3.0.0"
  180. httpaf-lwt-unix
  181. httpun-lwt
  182. httpun-mirage
  183. httpun-ws-lwt
  184. hvsock
  185. i3ipc
  186. influxdb-lwt
  187. inotify >= "2.4"
  188. inquire < "0.3.0"
  189. interface-prime-lwt
  190. ip2location
  191. ip2locationio
  192. ip2whois
  193. ipv6-multicast-lwt
  194. irc-client-lwt
  195. irc-client-lwt-ssl
  196. irc-client-tls
  197. irmin
  198. irmin-bench
  199. irmin-chunk
  200. irmin-cli
  201. irmin-client
  202. irmin-containers
  203. irmin-fs
  204. irmin-git
  205. irmin-graphql
  206. irmin-http
  207. irmin-indexeddb
  208. irmin-layers
  209. irmin-mirage-git
  210. irmin-mirage-graphql
  211. irmin-pack
  212. irmin-server
  213. irmin-test
  214. irmin-unix
  215. irmin-watcher
  216. joolog
  217. jose < "0.9.0"
  218. js_of_ocaml-lwt
  219. jsoo_broadcastchannel
  220. jsoo_storage
  221. jupyter
  222. jupyter-kernel
  223. kafka < "0.5"
  224. kafka_lwt
  225. kappa-library
  226. ke >= "0.5"
  227. kinetic-client
  228. kubecaml
  229. lambda-runtime
  230. lambda-term
  231. lambda_streams_lwt
  232. launchd
  233. ldp
  234. learn-ocaml
  235. learn-ocaml-client
  236. ledgerwallet >= "0.4.0"
  237. letsencrypt < "2.0.0"
  238. letsencrypt-app
  239. letsencrypt-dns < "2.0.0"
  240. letters
  241. lichess_api
  242. links
  243. linol-lwt
  244. llama
  245. lru_cache
  246. lwt-canceler
  247. lwt-dllist
  248. lwt-exit
  249. lwt-parallel
  250. lwt-pipe
  251. lwt-pipeline
  252. lwt-watcher
  253. lwt_camlp4
  254. lwt_domain
  255. lwt_eio < "0.6"
  256. lwt_glib
  257. lwt_log
  258. lwt_ppx < "6.0.0~beta01"
  259. lwt_react
  260. lwt_retry
  261. lwt_ssl
  262. mariadb >= "1.2.0"
  263. markdown_monolith
  264. markup = "0.7.6"
  265. markup-lwt
  266. mdx
  267. mechaml
  268. mehari-lwt-unix
  269. mehari-mirage
  270. memtrace-mirage
  271. metrics-influx
  272. metrics-lwt
  273. metrics-unix
  274. migra
  275. mimic
  276. mindstorm-lwt
  277. mirage < "4.0.0"
  278. mirage-block >= "2.0.1"
  279. mirage-block-ccm
  280. mirage-block-combinators
  281. mirage-block-lwt
  282. mirage-block-partition
  283. mirage-block-ramdisk
  284. mirage-block-solo5
  285. mirage-block-unikraft
  286. mirage-block-unix >= "2.14.2"
  287. mirage-block-xen
  288. mirage-channel >= "4.0.1"
  289. mirage-channel-lwt
  290. mirage-clock-lwt
  291. mirage-clock-unix < "4.2.0"
  292. mirage-console-lwt
  293. mirage-crypto-rng < "0.11.3"
  294. mirage-crypto-rng-lwt
  295. mirage-crypto-rng-mirage
  296. mirage-device >= "2.0.0"
  297. mirage-flow >= "3.0.0"
  298. mirage-flow-combinators
  299. mirage-flow-lwt
  300. mirage-flow-unix
  301. mirage-fs >= "4.0.0"
  302. mirage-fs-lwt
  303. mirage-kv >= "3.0.1"
  304. mirage-kv-lwt
  305. mirage-kv-unix
  306. mirage-net >= "4.0.0"
  307. mirage-net-lwt
  308. mirage-net-macosx
  309. mirage-net-solo5
  310. mirage-net-unikraft
  311. mirage-net-unix
  312. mirage-net-xen
  313. mirage-profile
  314. mirage-protocols >= "7.0.0"
  315. mirage-protocols-lwt
  316. mirage-qubes
  317. mirage-qubes-ipv4
  318. mirage-runtime
  319. mirage-sleep
  320. mirage-solo5
  321. mirage-stack = "3.0.0"
  322. mirage-stack-lwt
  323. mirage-time >= "3.0.0"
  324. mirage-time-lwt
  325. mirage-time-unix
  326. mirage-types-lwt
  327. mirage-unikraft
  328. mirage-unix
  329. mirage-vnetif
  330. mirage-xen
  331. monorobot
  332. moonpool-lwt
  333. mqtt
  334. mrmime >= "0.5.0"
  335. multipart-form-data
  336. multipart_form >= "0.2.0" & < "0.4.0"
  337. multipart_form-cohttp-lwt < "0.6.0"
  338. multipart_form-lwt
  339. mwt
  340. naboris
  341. nbd >= "4.0.3"
  342. nbd-tool
  343. nbd-unix
  344. neo4j_bolt
  345. nocrypto
  346. nottui-lwt
  347. notty-community
  348. nproc
  349. nsq
  350. obuilder
  351. obus >= "1.2.1"
  352. ocluster
  353. ocluster-api
  354. ocluster-worker
  355. ocplib-resto
  356. ocsigenserver
  357. ocsipersist
  358. ocsipersist-dbm
  359. ocsipersist-lib
  360. ocsipersist-pgsql
  361. ocsipersist-sqlite
  362. oframl
  363. ojs_base
  364. omigrate
  365. oneffs
  366. opam-check-npm-deps >= "4.1.0"
  367. opam-publish >= "3.0.0"
  368. opencage
  369. opentelemetry-client-cohttp-lwt
  370. opentelemetry-client-ocurl-lwt
  371. opentelemetry-cohttp-lwt >= "0.4"
  372. opentelemetry-lwt
  373. opium
  374. opium-graphql
  375. opium_kernel
  376. opomodoro
  377. order-i3-xfce
  378. ordma
  379. oskel >= "0.3.0"
  380. ounit-lwt < "2.2.0"
  381. ounit2-lwt
  382. owork
  383. ozulip
  384. paf
  385. paf-cohttp
  386. passage < "0.1.8"
  387. pcap-format < "0.5.2"
  388. petrol
  389. pgn_parser
  390. pgx_lwt
  391. pgx_lwt_mirage
  392. pgx_lwt_unix < "2.0"
  393. piaf < "0.2.0"
  394. picos >= "0.3.0" & < "0.5.0"
  395. picos_lwt
  396. picos_meta
  397. plebeia >= "2.0.0"
  398. plist-xml-lwt
  399. plotkicadsch
  400. posix-getopt >= "4.0.2"
  401. ppx_defer >= "0.4.0"
  402. ppx_deriving_rpc
  403. ppx_rapper_lwt
  404. proc-smaps
  405. prof_spacetime
  406. prometheus
  407. prometheus-app
  408. prometheus-lwt
  409. promise_jsoo_lwt
  410. protocol-9p
  411. protocol-9p-unix
  412. proton
  413. pxshot
  414. qcow
  415. qcow-stream
  416. qcow-tool
  417. qcow-types
  418. qdrant < "0.2.0"
  419. qfs
  420. quests
  421. quickterface
  422. rawlink < "2.1"
  423. rawlink-lwt
  424. rdf_json_ld
  425. rdf_lwt
  426. redis-lwt
  427. reparse-lwt
  428. reparse-lwt-unix
  429. resource-pooling
  430. resp
  431. resp-mirage >= "0.10.0"
  432. resp-unix >= "0.10.0"
  433. resto
  434. resto-cohttp-client >= "0.4"
  435. resto-cohttp-self-serving-client
  436. resto-cohttp-server >= "0.4" & < "0.6" | >= "0.9"
  437. resto-directory >= "0.4"
  438. ringo-lwt
  439. river
  440. rock
  441. rpclib-js
  442. rpclib-lwt
  443. SZXX < "4.0.0"
  444. sanddb
  445. scgi
  446. sendmail-lwt
  447. sendmail-mirage
  448. serial
  449. server-reason-react < "0.4.0"
  450. session-cohttp-lwt
  451. session-cookie-lwt
  452. session-postgresql-lwt
  453. sessions
  454. shared-block-ring
  455. shared-memory-ring-lwt
  456. sherlodoc
  457. sihl < "0.2.0"
  458. slack
  459. slacko
  460. slipshow < "0.12.0"
  461. smtml >= "0.7.0"
  462. speed
  463. spin < "0.8.0"
  464. spoke
  465. stk
  466. stog
  467. swapfs
  468. syguslib-utils
  469. syndic >= "1.4" & < "1.6.0"
  470. tar-mirage
  471. tar-unix
  472. tcpip
  473. telegraml
  474. terminus
  475. testcontainers
  476. testo-lwt
  477. tezt
  478. tidy_email
  479. timmy-lwt
  480. tls >= "0.10.6" & < "0.16.0"
  481. tls-lwt
  482. tls-mirage
  483. tube
  484. tuntap
  485. twirp_cohttp_lwt_unix
  486. uring
  487. uspf
  488. uspf-lwt
  489. uspf-mirage
  490. utcp
  491. utop
  492. uwt
  493. vchan
  494. vchan-unix
  495. vchan-xen
  496. vercel
  497. vhd-format-lwt
  498. vmnet
  499. vue-jsoo < "0.3"
  500. wayland < "2.0"
  501. webauthn
  502. xen-evtchn
  503. xen-evtchn-unix
  504. xen-gnt
  505. xen-gnt-unix
  506. xenstore
  507. xenstore-tool
  508. xenstore_transport
  509. xlsx2csv
  510. yocaml_git
  511. yocaml_unix < "2.0.0"
  512. zarr-lwt
  513. zmq-lwt >= "5.2.1"

Conflicts

None