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

Conflicts

None