package lwt

  1. Overview
  2. Docs
Promises, concurrency, and parallelized I/O

Install

Dune Dependency

github.com Readme Changelog Edit opam file Versions (45)

Authors

  1. J
    Jérôme Vouillon
  2. J
    Jérémie Dimino

Maintainers

  1. Anton Bachin
  2. M
    Mauricio Fernandez <mfp@acm.org>
  3. S
    Simon Cruanes <simon.cruanes.2007@m4x.org>

Sources

3.3.0.tar.gz
sha256=a214b07b89822bb7e0291edbba56e3fb41dbb48b2353e41a7c85c459f832d3eb
md5=47bdf4b429da94419941ebe4354f505f

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.

Published: 08 Mar 2018

README

Lwt   

Lwt is OCaml's concurrent programming library. 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:

let () =
  let request =
    let%lwt 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%lwt () = write outgoing "GET / HTTP/1.1\r\n" in
      let%lwt () = write outgoing "Connection: close\r\n\r\n" in
      let%lwt response = read incoming in
      Lwt.return (Some response)))
  in

  let timeout =
    let%lwt () = 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 -package lwt.ppx -linkpkg -o request example.ml
   ./request *)

In the program, functions such as Lwt_io.write create promises. The let%lwt ... 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 and BuckleScript.

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 Gitter chat, ask in #ocaml, on discuss.ocaml.org, or on Stack Overflow. Please do ask! Even apparently simple questions often end up educating other users, not to mention enlightening the maintainers!

Subscribe to the announcements issue to get news about Lwt releases. It is less noisy than watching the whole repository. Announcements are also made in /r/ocaml, on the OCaml mailing list, and on discuss.ocaml.org.


Contributing

  • We maintain easy starter issues. These are thoroughly explained and hyperlinked. We hope that this makes working on Lwt accessible even to relative OCaml beginners :)

  • 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.

  • The overall development plan can be found in the roadmap.

  • Ask us anything, whether it's about working on Lwt, or any question at all about it :)

  • The documentation always needs proofreading and fixes.

  • Despite a lot of progress, Lwt still needs more tests.

  • 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!


License

Lwt is released under the LGPL, with an OpenSSL linking exception. See COPYING.


Related Libraries

  • alcotest A lightweight framework for unit testing

  • angstrom A library for building parsers with a focus on efficiency, concurrency, and reusability

  • cohttp A lightweight library for writing HTTP clients and servers

  • cstruct A library and syntax extension for interop with C-like structures

  • ezjsonm A library for easy interop with JSON

  • faraday A library for fast and memory-efficient serialization

  • logs A logging library with reporting decoupled from logging

  • lwt-parallel A library for distributed computing

  • opium A web toolkit that uses Sinatra-inspired middleware

Dependencies (7)

  1. result
  2. ppx_tools_versioned >= "5.0.1"
  3. ocaml-migrate-parsetree < "2.0.0"
  4. ocamlfind build & >= "1.7.3-1"
  5. jbuilder >= "1.0+beta14"
  6. cppo build & >= "1.1.0"
  7. ocaml >= "4.02.0" & < "4.08.0"

Dev Dependencies

None

  1. 0install >= "2.14"
  2. albatross
  3. alcotest-lwt
  4. alcotest-mirage
  5. ambient-context-lwt
  6. amqp-client >= "0.9.0" & < "1.0.2" | >= "1.1.0"
  7. amqp-client-lwt
  8. angstrom-lwt-unix
  9. anthill
  10. anycache-lwt
  11. arakoon < "1.8.6" | >= "1.8.8"
  12. archi-lwt
  13. arp >= "2.3.1"
  14. arp-mirage
  15. awa-lwt
  16. awa-mirage < "0.2.0"
  17. aws < "1.0.0"
  18. aws-s3-lwt
  19. awsm-lwt
  20. azure-cosmos-db
  21. baardskeerder
  22. balancer
  23. bap < "1.0.0"
  24. bap-server < "0.3.0"
  25. bimage-lwt
  26. biocaml = "0.4.0"
  27. bistro
  28. brotli < "1.2.0"
  29. brozip
  30. builder
  31. bun >= "0.3.3"
  32. c3
  33. calculon
  34. camltc
  35. canary
  36. capnp-rpc-lwt < "1.2.3"
  37. capnp-rpc-unix >= "0.9.0" & < "1.2.3"
  38. caqti-lwt < "2.0.1"
  39. carton-git < "0.7.2"
  40. carton-lwt
  41. cf-lwt
  42. channel
  43. charrua-client-lwt < "1.2.0"
  44. charrua-client-mirage < "1.2.0"
  45. charrua-core < "0.3"
  46. charrua-unix >= "0.3" & < "0.11.0" | >= "0.11.2"
  47. cmdtui-lambda-term
  48. coclobas
  49. cohttp-lwt < "6.0.0~alpha2"
  50. cohttp-lwt-jsoo
  51. cohttp-lwt-unix
  52. cohttp-lwt-unix-nossl
  53. cohttp-lwt-unix-ssl
  54. cohttp-mirage
  55. comby
  56. comby-semantic
  57. conan-lwt
  58. conduit-lwt
  59. conduit-lwt-unix
  60. core-lwt = "0.2.0"
  61. cowabloga >= "0.2.2"
  62. crunch >= "2.0.0"
  63. cstruct-lwt
  64. csv-lwt
  65. csvprovider
  66. ctypes >= "0.6.0" & < "0.21.1"
  67. ctypes-foreign >= "0.21.1"
  68. curly < "0.2.0"
  69. current_docker < "0.6.4"
  70. current_examples < "0.6.4"
  71. current_git < "0.6.4"
  72. current_github < "0.6.4"
  73. current_gitlab < "0.6.4"
  74. current_ocluster < "0.2"
  75. current_slack < "0.6.4"
  76. current_web < "0.6.4"
  77. DkSDKFFIOCaml_Std
  78. dap
  79. data-encoding < "0.1.1"
  80. datakit
  81. datakit-bridge-github
  82. datakit-bridge-local-git
  83. datakit-ci
  84. datakit-client >= "0.11.0"
  85. datakit-github
  86. datakit-server
  87. devkit
  88. dht < "0.2.0"
  89. distributed = "0.4.0"
  90. distributed-lwt
  91. dkim-mirage
  92. dns >= "0.19.1" & < "1.0.0"
  93. dns-forward >= "0.9.0"
  94. dns-forward-lwt-unix
  95. dns-lwt
  96. dnssd
  97. docker_hub
  98. dream
  99. dream-httpaf
  100. dream-pure
  101. dropbox
  102. dune_watch
  103. earlybird < "1.0.0"
  104. elasticsearch-cli
  105. eliom = "6.3.0"
  106. eris-lwt
  107. ethernet
  108. ezcurl-lwt
  109. ezirmin
  110. ezjsonm >= "0.4.2" & < "0.5.0"
  111. ezjsonm-lwt
  112. ezresto
  113. ezresto-directory
  114. faraday-lwt
  115. faraday-lwt-unix
  116. fat-filesystem >= "0.12.0"
  117. fiber-lwt
  118. flowtype >= "0.62.0"
  119. frenetic < "2.0.0"
  120. fswatch_lwt
  121. fuseau-lwt
  122. gamepad
  123. gdb
  124. gdbprofiler
  125. git != "1.4.3" & != "1.7.2"
  126. git-paf
  127. git-unix = "1.11.1" | >= "3.0.0" & < "3.10.0"
  128. github
  129. github-hooks
  130. github-unix >= "4.4.0"
  131. gitlab-unix
  132. gluten-lwt < "0.4.0"
  133. gluten-lwt-unix < "0.4.0"
  134. gluten-mirage < "0.4.0"
  135. graphql-lwt
  136. gufo
  137. h2-lwt < "0.10.0"
  138. h2-lwt-unix < "0.10.0"
  139. h2-mirage
  140. happy-eyeballs-lwt
  141. happy-eyeballs-mirage
  142. hardcaml < "1.1.0"
  143. hardcaml-examples >= "0.3.0"
  144. hardcaml-framework
  145. hardcaml-waveterm
  146. hiredis
  147. hl_yaml
  148. horned_worm < "0.3.1"
  149. http-lwt-client
  150. http-multipart-formdata >= "2.0.0" & < "3.0.0"
  151. http2https
  152. httpaf-lwt-unix
  153. httpun-lwt
  154. httpun-mirage
  155. httpun-ws-lwt
  156. hvsock
  157. i3ipc
  158. imaplet-lwt
  159. influxdb-lwt
  160. inotify >= "2.4"
  161. inquire < "0.3.0"
  162. iocaml < "0.4.6"
  163. iocaml-kernel >= "0.4.3"
  164. iocamljs-kernel
  165. ip2location
  166. ip2locationio
  167. ip2whois
  168. ipv6-multicast < "0.9"
  169. ipv6-multicast-lwt
  170. irc-client-lwt
  171. irc-client-lwt-ssl
  172. irc-client-tls
  173. irmin < "0.9.6" | = "0.9.10" | >= "0.11.0" & < "2.7.0"
  174. irmin-bench < "2.7.0"
  175. irmin-chunk < "2.7.0"
  176. irmin-containers < "2.7.0"
  177. irmin-fs >= "2.3.0" & < "2.7.0"
  178. irmin-git >= "2.3.0" & < "2.7.0"
  179. irmin-graphql >= "2.3.0" & < "2.7.0"
  180. irmin-http >= "2.3.0" & < "2.7.0"
  181. irmin-indexeddb
  182. irmin-layers < "2.7.0"
  183. irmin-mem >= "2.3.0"
  184. irmin-mirage-git >= "2.3.0" & < "2.7.0"
  185. irmin-mirage-graphql >= "2.3.0" & < "2.7.0"
  186. irmin-pack < "2.7.0"
  187. irmin-test >= "2.3.0" & < "2.7.0"
  188. irmin-unix >= "2.3.0" & < "2.7.0"
  189. irmin-watcher
  190. jitsu
  191. joolog
  192. jose < "0.9.0"
  193. js_of_ocaml < "3.0"
  194. js_of_ocaml-lwt
  195. jsoo_broadcastchannel
  196. jsoo_router
  197. jsoo_storage
  198. jupyter < "2.3.0"
  199. jupyter-archimedes < "2.3.2"
  200. jupyter-kernel
  201. KaSim >= "4.0.0"
  202. kafka >= "0.3" & < "0.5"
  203. kafka_lwt
  204. ke >= "0.5"
  205. ketrew >= "3.2.0"
  206. kinetic-client
  207. kubecaml
  208. lablqml < "0.6"
  209. lambda-runtime
  210. lambda-term >= "1.11" & < "1.13"
  211. launchd
  212. learn-ocaml < "0.13.0"
  213. learn-ocaml-client < "0.13.0"
  214. letsencrypt
  215. letsencrypt-app
  216. letsencrypt-dns
  217. libres3
  218. links < "0.9.5"
  219. lru_cache < "v0.16.0"
  220. lwt-binio
  221. lwt-canceler
  222. lwt-dllist
  223. lwt-exit
  224. lwt-parallel >= "0.1.2"
  225. lwt-pipe
  226. lwt-pipeline
  227. lwt-watcher
  228. lwt-zmq
  229. lwt_camlp4
  230. lwt_domain < "0.3.0"
  231. lwt_eio < "0.4"
  232. lwt_glib >= "1.0.1"
  233. lwt_log < "1.1.0"
  234. lwt_named_threads
  235. lwt_ppx
  236. lwt_ppx_let
  237. lwt_react >= "1.0.1"
  238. lwt_ssl >= "1.0.1"
  239. macaque_lwt
  240. maki
  241. mariadb < "0.5.1"
  242. markup = "0.7.6"
  243. markup-lwt
  244. mdx
  245. mechaml
  246. metrics-influx
  247. metrics-lwt
  248. metrics-mirage
  249. metrics-unix
  250. mindstorm-lwt
  251. mirage < "0.9.1" | >= "0.10.0" & < "2.7.0"
  252. mirage-block < "1.0.0"
  253. mirage-block-ccm
  254. mirage-block-lwt
  255. mirage-block-ramdisk
  256. mirage-block-solo5
  257. mirage-block-unix < "2.8.3"
  258. mirage-block-xen
  259. mirage-bootvar-solo5 >= "0.2.0"
  260. mirage-bootvar-unix
  261. mirage-bootvar-xen >= "0.4.0"
  262. mirage-channel-lwt
  263. mirage-clock-freestanding < "3.0.0"
  264. mirage-clock-lwt
  265. mirage-clock-unix >= "1.2.0" & < "3.0.0"
  266. mirage-console >= "2.1.1" & < "2.2.0"
  267. mirage-console-lwt
  268. mirage-console-solo5 >= "0.2.0"
  269. mirage-console-unix >= "2.2.0" & < "3.0.0"
  270. mirage-console-xen >= "5.0.0"
  271. mirage-console-xen-backend < "3.0.1"
  272. mirage-console-xen-cli
  273. mirage-dns < "3.0.0"
  274. mirage-entropy < "0.5.0"
  275. mirage-entropy-xen < "0.2.0"
  276. mirage-flow >= "1.0.3" & < "1.2.0"
  277. mirage-flow-lwt
  278. mirage-flow-rawlink
  279. mirage-flow-unix < "2.0.0"
  280. mirage-fs-lwt
  281. mirage-fs-unix != "1.2.1"
  282. mirage-http
  283. mirage-http-unix
  284. mirage-http-xen
  285. mirage-kv-lwt
  286. mirage-kv-unix < "3.0.0"
  287. mirage-logs
  288. mirage-nat < "3.0.0"
  289. mirage-net-fd
  290. mirage-net-lwt
  291. mirage-net-macosx
  292. mirage-net-solo5
  293. mirage-net-unix
  294. mirage-net-xen
  295. mirage-os-shim >= "3.0.0"
  296. mirage-profile
  297. mirage-protocols-lwt
  298. mirage-qubes < "0.9.4"
  299. mirage-qubes-ipv4 < "0.9.4"
  300. mirage-random-stdlib >= "0.1.0"
  301. mirage-solo5
  302. mirage-stack-lwt
  303. mirage-time-lwt
  304. mirage-time-unix < "2.0.0"
  305. mirage-types-lwt < "3.7.1"
  306. mirage-unix
  307. mirage-vnetif
  308. mirage-vnetif-stack
  309. mirage-www >= "1.1.0"
  310. mirage-xen
  311. mirror
  312. moonpool-lwt
  313. mpris = "0.1.1"
  314. mqtt = "0.0.2"
  315. mrmime >= "0.5.0"
  316. multipart-form-data
  317. multipart_form >= "0.2.0" & < "0.4.0"
  318. multipart_form-lwt < "0.6.0"
  319. mwt
  320. named-pipe
  321. nanomsg
  322. nbd = "2.1.1" | >= "3.0.0"
  323. nbd-tool
  324. nbd-unix
  325. netchannel
  326. nocrypto >= "0.5.4"
  327. noise
  328. nottui-lwt
  329. nproc
  330. nsq
  331. obrowser
  332. obuilder < "0.4"
  333. obus >= "1.1.8" & < "1.2.1"
  334. ocaml-variants >= "4.00.1+mirage-unix" & < "4.00.1+open-types"
  335. ocluster < "0.2"
  336. ocluster-api < "0.2"
  337. ocplib-concur
  338. ocplib-resto
  339. ocsigen-start = "1.1.0" | >= "4.1.0" & < "4.7.0"
  340. ocsigen-toolkit = "1.1.0"
  341. ocsigenserver >= "2.9"
  342. odoc >= "2.0.0" & < "2.1.0"
  343. ojquery
  344. ojs-base < "0.6.0"
  345. opam-compiler < "0.2.0"
  346. opam-publish = "0.3.5"
  347. opam-sync-github-prs
  348. openflow < "0.2.0"
  349. opium >= "0.11.0" & < "0.19.0"
  350. opium_kernel
  351. order-i3-xfce
  352. ordma
  353. osc-lwt
  354. oskel >= "0.3.0"
  355. otetris
  356. ounit-lwt < "2.2.0"
  357. ounit2-lwt
  358. ox < "1.1.0"
  359. paf
  360. paf-cohttp
  361. passage
  362. pcap-format >= "0.3.3" & < "0.5.0"
  363. pgx_lwt
  364. pgx_lwt_mirage
  365. pgx_lwt_unix < "2.0"
  366. plotkicadsch
  367. ppx_defer >= "0.4.0"
  368. ppx_deriving_rpc
  369. ppx_json_types
  370. ppx_netblob
  371. ppx_rapper_lwt
  372. ppx_sqlexpr
  373. prof_spacetime
  374. prometheus
  375. prometheus-app
  376. promise_jsoo_lwt
  377. protocol-9p >= "0.10.0"
  378. protocol-9p-unix
  379. pvem_lwt_unix >= "0.0.2"
  380. qcow < "0.11.0"
  381. qcow-format
  382. qcow-tool
  383. qfs >= "0.5" & < "0.7"
  384. quests
  385. rawlink < "2.1"
  386. rawlink-lwt
  387. redis-lwt
  388. resource-pooling
  389. resp >= "0.10.0"
  390. resp-mirage >= "0.10.0"
  391. resp-unix >= "0.10.0"
  392. resto
  393. resto-cohttp-client
  394. resto-cohttp-self-serving-client
  395. resto-cohttp-server
  396. resto-directory
  397. riak
  398. ringo-lwt
  399. river
  400. rpc >= "1.5.1" & < "7.1.0"
  401. rpclib-js
  402. rpclib-lwt
  403. sanddb
  404. scgi
  405. sendmail-lwt
  406. session-cohttp-lwt
  407. session-cookie-lwt
  408. session-postgresql-lwt
  409. sessions
  410. shared-block-ring
  411. shared-memory-ring < "2.0.0"
  412. shared-memory-ring-lwt
  413. skkserv-lite
  414. slacko
  415. slipshow
  416. socket-daemon
  417. speed
  418. spin < "0.6.0"
  419. spotify-web-api < "0.2.1"
  420. sqlexpr
  421. statsd-client
  422. stog >= "0.16.0" & < "0.19.0"
  423. syndic >= "1.4" & < "1.6.0"
  424. tar-format >= "0.4.1"
  425. tar-mirage < "2.2.0"
  426. tar-unix
  427. tcpip >= "3.1.1" & < "4.0.0"
  428. teash
  429. telegraml
  430. testrunner
  431. tezos-p2p >= "11.0" & < "13.0"
  432. tezos-stdlib < "9.0"
  433. tezos-stdlib-unix < "9.0"
  434. tftp
  435. themoviedb
  436. tls >= "0.10.1" & < "0.16.0"
  437. tls-lwt < "0.17.4"
  438. tls-mirage
  439. tlstunnel >= "0.2.0"
  440. transmission-rpc
  441. tube
  442. tuntap >= "1.0.0" & < "2.0.0"
  443. twirp_cohttp_lwt_unix
  444. typerex-lldb
  445. u2f
  446. usb
  447. uspf
  448. uspf-lwt
  449. utop >= "1.4.0"
  450. uwt >= "0.0.2"
  451. vchan >= "0.9.6" & < "2.0.0" | >= "2.0.3"
  452. vchan-unix
  453. vchan-xen
  454. vercel
  455. vhd-format >= "0.6.0" & < "0.9.1"
  456. vhd-format-lwt >= "0.12.0"
  457. vhd-tool < "0.12.0"
  458. vmnet
  459. vpnkit >= "0.1.1"
  460. vue-jsoo < "0.3"
  461. webauthn
  462. websocket < "2.3"
  463. websocket-lwt = "2.11"
  464. xe-unikernel-upload
  465. xen-api-client < "0.9.14"
  466. xen-block-driver
  467. xen-evtchn
  468. xen-evtchn-unix
  469. xen-gnt
  470. xen-gnt-unix >= "4.0.2"
  471. xenctrl
  472. xenstore
  473. xenstore_transport
  474. xentropyd
  475. yurt != "0.3" & < "0.5"
  476. zbar
  477. zmq-lwt

Conflicts (2)

  1. dune >= "1.7.0"
  2. ocaml-variants = "4.02.1+BER"
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