package lwt

  1. Overview
  2. Docs

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
  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. hvsock
  154. i3ipc
  155. imaplet-lwt
  156. influxdb-lwt
  157. inotify >= "2.4"
  158. inquire < "0.3.0"
  159. iocaml < "0.4.6"
  160. iocaml-kernel >= "0.4.3"
  161. iocamljs-kernel
  162. ip2location
  163. ip2locationio
  164. ipv6-multicast < "0.9"
  165. ipv6-multicast-lwt
  166. irc-client-lwt
  167. irc-client-lwt-ssl
  168. irc-client-tls
  169. irmin < "0.9.6" | = "0.9.10" | >= "0.11.0" & < "2.7.0"
  170. irmin-bench < "2.7.0"
  171. irmin-chunk < "2.7.0"
  172. irmin-containers < "2.7.0"
  173. irmin-fs >= "2.3.0" & < "2.7.0"
  174. irmin-git >= "2.3.0" & < "2.7.0"
  175. irmin-graphql >= "2.3.0" & < "2.7.0"
  176. irmin-http >= "2.3.0" & < "2.7.0"
  177. irmin-indexeddb
  178. irmin-layers < "2.7.0"
  179. irmin-mem >= "2.3.0"
  180. irmin-mirage-git >= "2.3.0" & < "2.7.0"
  181. irmin-mirage-graphql >= "2.3.0" & < "2.7.0"
  182. irmin-pack < "2.7.0"
  183. irmin-test >= "2.3.0" & < "2.7.0"
  184. irmin-unix >= "2.3.0" & < "2.7.0"
  185. irmin-watcher
  186. jitsu
  187. joolog
  188. jose < "0.9.0"
  189. js_of_ocaml < "3.0"
  190. js_of_ocaml-lwt
  191. jsoo_broadcastchannel
  192. jsoo_router
  193. jsoo_storage
  194. jupyter < "2.3.0"
  195. jupyter-archimedes < "2.3.2"
  196. jupyter-kernel
  197. KaSim >= "4.0.0"
  198. kafka >= "0.3" & < "0.5"
  199. kafka_lwt
  200. ke >= "0.5"
  201. ketrew >= "3.2.0"
  202. kinetic-client
  203. kubecaml
  204. lablqml < "0.6"
  205. lambda-runtime
  206. lambda-term >= "1.11" & < "1.13"
  207. launchd
  208. learn-ocaml < "0.13.0"
  209. learn-ocaml-client < "0.13.0"
  210. letsencrypt
  211. letsencrypt-app
  212. letsencrypt-dns
  213. libres3
  214. links < "0.9.5"
  215. lru_cache < "v0.16.0"
  216. lwt-binio
  217. lwt-canceler
  218. lwt-dllist
  219. lwt-exit
  220. lwt-parallel >= "0.1.2"
  221. lwt-pipe
  222. lwt-pipeline
  223. lwt-watcher
  224. lwt-zmq
  225. lwt_camlp4
  226. lwt_domain < "0.3.0"
  227. lwt_eio < "0.4"
  228. lwt_glib >= "1.0.1"
  229. lwt_log < "1.1.0"
  230. lwt_named_threads
  231. lwt_ppx
  232. lwt_ppx_let
  233. lwt_react >= "1.0.1"
  234. lwt_ssl >= "1.0.1"
  235. macaque_lwt
  236. maki
  237. mariadb < "0.5.1"
  238. markup = "0.7.6"
  239. markup-lwt
  240. mdx
  241. mechaml
  242. metrics-influx
  243. metrics-lwt
  244. metrics-mirage
  245. metrics-unix
  246. mindstorm-lwt
  247. mirage < "0.9.1" | >= "0.10.0" & < "2.7.0"
  248. mirage-block < "1.0.0"
  249. mirage-block-ccm
  250. mirage-block-lwt
  251. mirage-block-ramdisk
  252. mirage-block-solo5
  253. mirage-block-unix < "2.8.3"
  254. mirage-block-xen
  255. mirage-bootvar-solo5 >= "0.2.0"
  256. mirage-bootvar-unix
  257. mirage-bootvar-xen >= "0.4.0"
  258. mirage-channel-lwt
  259. mirage-clock-freestanding < "3.0.0"
  260. mirage-clock-lwt
  261. mirage-clock-unix >= "1.2.0" & < "3.0.0"
  262. mirage-console >= "2.1.1" & < "2.2.0"
  263. mirage-console-lwt
  264. mirage-console-solo5 >= "0.2.0"
  265. mirage-console-unix >= "2.2.0" & < "3.0.0"
  266. mirage-console-xen >= "5.0.0"
  267. mirage-console-xen-backend < "3.0.1"
  268. mirage-console-xen-cli
  269. mirage-dns < "3.0.0"
  270. mirage-entropy < "0.5.0"
  271. mirage-entropy-xen < "0.2.0"
  272. mirage-flow >= "1.0.3" & < "1.2.0"
  273. mirage-flow-lwt
  274. mirage-flow-rawlink
  275. mirage-flow-unix < "2.0.0"
  276. mirage-fs-lwt
  277. mirage-fs-unix != "1.2.1"
  278. mirage-http
  279. mirage-http-unix
  280. mirage-http-xen
  281. mirage-kv-lwt
  282. mirage-kv-unix < "3.0.0"
  283. mirage-logs
  284. mirage-nat < "3.0.0"
  285. mirage-net-fd
  286. mirage-net-lwt
  287. mirage-net-macosx
  288. mirage-net-solo5
  289. mirage-net-unix
  290. mirage-net-xen
  291. mirage-os-shim >= "3.0.0"
  292. mirage-profile
  293. mirage-protocols-lwt
  294. mirage-qubes < "0.9.4"
  295. mirage-qubes-ipv4 < "0.9.4"
  296. mirage-random-stdlib >= "0.1.0"
  297. mirage-solo5
  298. mirage-stack-lwt
  299. mirage-time-lwt
  300. mirage-time-unix < "2.0.0"
  301. mirage-types-lwt < "3.7.1"
  302. mirage-unix
  303. mirage-vnetif
  304. mirage-vnetif-stack
  305. mirage-www >= "1.1.0"
  306. mirage-xen
  307. mirror
  308. moonpool-lwt
  309. mpris = "0.1.1"
  310. mqtt = "0.0.2"
  311. mrmime >= "0.5.0"
  312. multipart-form-data
  313. multipart_form >= "0.2.0" & < "0.4.0"
  314. multipart_form-lwt < "0.6.0"
  315. mwt
  316. named-pipe
  317. nanomsg
  318. nbd = "2.1.1" | >= "3.0.0"
  319. nbd-tool
  320. nbd-unix
  321. netchannel
  322. nocrypto >= "0.5.4"
  323. noise
  324. nottui-lwt
  325. nproc
  326. nsq
  327. obrowser
  328. obuilder < "0.4"
  329. obus >= "1.1.8" & < "1.2.1"
  330. ocaml-variants >= "4.00.1+mirage-unix" & < "4.00.1+open-types"
  331. ocluster < "0.2"
  332. ocluster-api < "0.2"
  333. ocplib-concur
  334. ocplib-resto
  335. ocsigen-start = "1.1.0" | >= "4.1.0" & < "4.7.0"
  336. ocsigen-toolkit = "1.1.0"
  337. ocsigenserver >= "2.9"
  338. odoc >= "2.0.0" & < "2.1.0"
  339. ojquery
  340. ojs-base < "0.6.0"
  341. opam-compiler < "0.2.0"
  342. opam-publish = "0.3.5"
  343. opam-sync-github-prs
  344. openflow < "0.2.0"
  345. opium >= "0.11.0" & < "0.19.0"
  346. opium_kernel
  347. order-i3-xfce
  348. ordma
  349. osc-lwt
  350. oskel >= "0.3.0"
  351. otetris
  352. ounit-lwt < "2.2.0"
  353. ounit2-lwt
  354. ox < "1.1.0"
  355. paf
  356. paf-cohttp
  357. pcap-format >= "0.3.3" & < "0.5.0"
  358. pgx_lwt
  359. pgx_lwt_mirage
  360. pgx_lwt_unix < "2.0"
  361. plotkicadsch
  362. ppx_defer >= "0.4.0"
  363. ppx_deriving_rpc
  364. ppx_json_types
  365. ppx_netblob
  366. ppx_rapper_lwt
  367. ppx_sqlexpr
  368. prof_spacetime
  369. prometheus
  370. prometheus-app
  371. promise_jsoo_lwt
  372. protocol-9p >= "0.10.0"
  373. protocol-9p-unix
  374. pvem_lwt_unix >= "0.0.2"
  375. qcow < "0.11.0"
  376. qcow-format
  377. qcow-tool
  378. qfs >= "0.5" & < "0.7"
  379. quests
  380. rawlink < "2.1"
  381. rawlink-lwt
  382. redis-lwt
  383. resource-pooling
  384. resp >= "0.10.0"
  385. resp-mirage >= "0.10.0"
  386. resp-unix >= "0.10.0"
  387. resto
  388. resto-cohttp-client
  389. resto-cohttp-self-serving-client
  390. resto-cohttp-server
  391. resto-directory
  392. riak
  393. ringo-lwt
  394. river
  395. rpc >= "1.5.1" & < "7.1.0"
  396. rpclib-js
  397. rpclib-lwt
  398. sanddb
  399. scgi
  400. sendmail-lwt
  401. session-cohttp-lwt
  402. session-cookie-lwt
  403. session-postgresql-lwt
  404. sessions
  405. shared-block-ring
  406. shared-memory-ring < "2.0.0"
  407. shared-memory-ring-lwt
  408. skkserv-lite
  409. slacko
  410. socket-daemon
  411. spin < "0.6.0"
  412. spotify-web-api < "0.2.1"
  413. sqlexpr
  414. statsd-client
  415. stog >= "0.16.0" & < "0.19.0"
  416. syndic >= "1.4" & < "1.6.0"
  417. tar-format >= "0.4.1"
  418. tar-mirage < "2.2.0"
  419. tar-unix
  420. tcpip >= "3.1.1" & < "4.0.0"
  421. teash
  422. telegraml
  423. testrunner
  424. tezos-p2p >= "11.0" & < "13.0"
  425. tezos-stdlib < "9.0"
  426. tezos-stdlib-unix < "9.0"
  427. tftp
  428. themoviedb
  429. tls >= "0.10.1" & < "0.16.0"
  430. tls-lwt < "0.17.4"
  431. tls-mirage
  432. tlstunnel >= "0.2.0"
  433. transmission-rpc
  434. tube
  435. tuntap >= "1.0.0" & < "2.0.0"
  436. typerex-lldb
  437. u2f
  438. usb
  439. uspf
  440. uspf-lwt
  441. utop >= "1.4.0"
  442. uwt >= "0.0.2"
  443. vchan >= "0.9.6" & < "2.0.0" | >= "2.0.3"
  444. vchan-unix
  445. vchan-xen
  446. vercel
  447. vhd-format >= "0.6.0" & < "0.9.1"
  448. vhd-format-lwt >= "0.12.0"
  449. vhd-tool < "0.12.0"
  450. vmnet
  451. vpnkit >= "0.1.1"
  452. vue-jsoo < "0.3"
  453. webauthn
  454. websocket < "2.3"
  455. websocket-lwt = "2.11"
  456. xe-unikernel-upload
  457. xen-api-client < "0.9.14"
  458. xen-block-driver
  459. xen-evtchn
  460. xen-evtchn-unix
  461. xen-gnt
  462. xen-gnt-unix >= "4.0.2"
  463. xenctrl
  464. xenstore
  465. xenstore_transport
  466. xentropyd
  467. yurt != "0.3" & < "0.5"
  468. zbar
  469. zmq-lwt

Conflicts (2)

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

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