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

4.0.1.tar.gz
md5=d2d1dacf089d7948fd95d9665259a9b6

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: 14 Apr 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 (5)

  1. result
  2. ocamlfind build & >= "1.7.3-1"
  3. jbuilder >= "1.0+beta14"
  4. cppo build & >= "1.1.0"
  5. 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 >= "2.0.1"
  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-lwt
  18. aws-s3-lwt
  19. awsm-lwt
  20. azure-cosmos-db
  21. baardskeerder
  22. balancer
  23. bap < "1.0.0"
  24. bap-server != "0.2.0"
  25. bimage-lwt
  26. biocaml = "0.4.0"
  27. bistro >= "0.4.0"
  28. brozip
  29. builder
  30. bun >= "0.3.3"
  31. c3
  32. calculon
  33. caldav
  34. camltc = "0.9.5" | >= "0.9.7.0"
  35. canary
  36. capnp-rpc-lwt < "1.2.3"
  37. capnp-rpc-unix >= "0.9.0" & < "1.2.3"
  38. caqti-lwt >= "0.11.0" & < "2.0.1"
  39. carton-git
  40. carton-lwt
  41. cf-lwt
  42. channel
  43. charrua-client >= "1.3.0"
  44. charrua-client-lwt
  45. charrua-client-mirage
  46. charrua-core < "0.3"
  47. charrua-unix >= "0.3" & != "0.10"
  48. cmdtui-lambda-term
  49. coclobas
  50. cohttp-lwt < "6.0.0~alpha2"
  51. cohttp-lwt-jsoo
  52. cohttp-lwt-unix >= "1.1.1"
  53. cohttp-lwt-unix-nossl
  54. cohttp-lwt-unix-ssl
  55. cohttp-mirage
  56. comby
  57. comby-semantic
  58. conan-lwt
  59. conduit-lwt
  60. conduit-lwt-unix
  61. core-lwt >= "0.3.0"
  62. cowabloga >= "0.2.2"
  63. crunch >= "2.0.0"
  64. cstruct-lwt
  65. csv-lwt
  66. csvprovider
  67. ctypes >= "0.15.0" & < "0.21.1"
  68. ctypes-foreign >= "0.21.1"
  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 >= "0.6"
  88. dht < "0.2.0"
  89. distributed-lwt
  90. dkim-mirage
  91. dns >= "0.19.1" & < "0.20.1"
  92. dns-cli >= "4.6.3"
  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 >= "0.4"
  105. eris-lwt
  106. ethernet
  107. ez_api
  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 >= "0.6.0"
  116. fat-filesystem >= "0.12.0"
  117. fiber-lwt
  118. flowtype >= "0.72.0"
  119. frenetic < "2.0.0"
  120. fswatch_lwt
  121. fuseau-lwt
  122. gamepad
  123. gdb
  124. gdbprofiler >= "0.3"
  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 < "0.2.0" | >= "0.4.0"
  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. hiredis
  146. hl_yaml
  147. horned_worm < "0.3.1"
  148. http-lwt-client
  149. http-multipart-formdata >= "2.0.0" & < "3.0.0"
  150. http2https
  151. httpaf-lwt-unix
  152. hvsock >= "1.0.2"
  153. i3ipc >= "0.1.4"
  154. imaplet-lwt
  155. influxdb-lwt
  156. inotify >= "2.4"
  157. inquire < "0.3.0"
  158. iocaml < "0.4.6"
  159. iocaml-kernel >= "0.4.3" & < "0.4.6"
  160. iocamljs-kernel
  161. ip2location
  162. ip2locationio
  163. ipv6-multicast-lwt
  164. irc-client-lwt
  165. irc-client-lwt-ssl
  166. irc-client-tls
  167. irmin < "0.9.6" | = "0.9.10" | >= "0.11.0" & < "2.7.0"
  168. irmin-bench < "2.7.0"
  169. irmin-chunk < "2.7.0"
  170. irmin-containers < "2.7.0"
  171. irmin-fs >= "2.3.0" & < "2.7.0"
  172. irmin-git >= "2.3.0" & < "2.7.0"
  173. irmin-graphql >= "2.3.0" & < "2.7.0"
  174. irmin-http >= "2.3.0" & < "2.7.0"
  175. irmin-indexeddb
  176. irmin-layers < "2.7.0"
  177. irmin-mem >= "2.3.0"
  178. irmin-mirage-git >= "2.3.0" & < "2.7.0"
  179. irmin-mirage-graphql >= "2.3.0" & < "2.7.0"
  180. irmin-pack < "2.7.0"
  181. irmin-test >= "2.3.0" & < "2.7.0"
  182. irmin-unix >= "2.3.0" & < "2.7.0"
  183. irmin-watcher >= "0.3.0"
  184. jitsu
  185. joolog
  186. jose < "0.9.0"
  187. js_of_ocaml < "2.5"
  188. js_of_ocaml-lwt >= "3.2.0"
  189. jsoo_broadcastchannel
  190. jsoo_router
  191. jsoo_storage
  192. jupyter >= "2.3.0"
  193. jupyter-kernel >= "0.4"
  194. kafka >= "0.3" & < "0.5"
  195. kafka_lwt
  196. ke >= "0.5"
  197. ketrew >= "3.2.0"
  198. kinetic-client < "0.0.3" | >= "0.0.9"
  199. kubecaml
  200. lablqml < "0.6"
  201. lambda-runtime
  202. lambda-term >= "1.13" & < "3.0.0" | = "3.2.0"
  203. launchd
  204. learn-ocaml >= "0.13.0"
  205. learn-ocaml-client >= "0.13.0"
  206. letsencrypt
  207. letsencrypt-app
  208. letsencrypt-dns
  209. libres3
  210. links < "0.9.5"
  211. lru_cache < "v0.16.0"
  212. lwt-binio
  213. lwt-canceler
  214. lwt-dllist
  215. lwt-exit
  216. lwt-parallel >= "1.0.0"
  217. lwt-pipe
  218. lwt-pipeline
  219. lwt-watcher
  220. lwt-zmq < "1.0.0" | >= "2.0.1"
  221. lwt_camlp4
  222. lwt_domain < "0.3.0"
  223. lwt_eio < "0.4"
  224. lwt_glib >= "1.0.1"
  225. lwt_log >= "1.1.0"
  226. lwt_named_threads
  227. lwt_ppx
  228. lwt_ppx_let
  229. lwt_react >= "1.0.1"
  230. lwt_ssl >= "1.0.1"
  231. mariadb < "0.5.1"
  232. markup = "0.7.6"
  233. markup-lwt
  234. mdx
  235. mechaml
  236. metrics-influx
  237. metrics-lwt
  238. metrics-mirage
  239. metrics-unix
  240. mindstorm-lwt
  241. mirage >= "0.4.1" & != "0.6.1" & < "0.8.0" | >= "0.10.0" & < "2.7.0"
  242. mirage-block < "1.0.0" | >= "2.0.0"
  243. mirage-block-ccm
  244. mirage-block-combinators
  245. mirage-block-lwt
  246. mirage-block-ramdisk
  247. mirage-block-solo5
  248. mirage-block-unix < "2.3.0" | = "2.8.2"
  249. mirage-block-xen
  250. mirage-bootvar-solo5 >= "0.2.0"
  251. mirage-bootvar-unix
  252. mirage-bootvar-xen >= "0.4.0"
  253. mirage-channel >= "4.0.0"
  254. mirage-channel-lwt
  255. mirage-clock-freestanding < "3.0.0"
  256. mirage-clock-lwt
  257. mirage-clock-unix >= "1.2.0" & < "3.0.0"
  258. mirage-console >= "2.1.1" & < "2.2.0" | >= "3.0.0"
  259. mirage-console-lwt
  260. mirage-console-solo5 >= "0.2.0"
  261. mirage-console-unix >= "2.2.1"
  262. mirage-console-xen >= "5.0.0"
  263. mirage-console-xen-backend
  264. mirage-console-xen-cli
  265. mirage-crypto-entropy
  266. mirage-crypto-rng >= "0.7.0" & < "0.11.0"
  267. mirage-crypto-rng-lwt
  268. mirage-crypto-rng-mirage >= "0.8.8"
  269. mirage-device >= "2.0.0"
  270. mirage-dns < "3.0.0"
  271. mirage-entropy
  272. mirage-flow >= "1.0.3" & < "1.2.0" | >= "2.0.0"
  273. mirage-flow-combinators
  274. mirage-flow-lwt
  275. mirage-flow-rawlink
  276. mirage-flow-unix
  277. mirage-fs >= "3.0.0"
  278. mirage-fs-lwt
  279. mirage-fs-unix < "1.1.1" | >= "1.3.0"
  280. mirage-http
  281. mirage-http-unix
  282. mirage-http-xen
  283. mirage-kv >= "3.0.0"
  284. mirage-kv-lwt
  285. mirage-kv-unix < "3.0.0"
  286. mirage-logs
  287. mirage-nat < "3.0.0"
  288. mirage-net >= "3.0.1"
  289. mirage-net-fd
  290. mirage-net-lwt
  291. mirage-net-macosx
  292. mirage-net-solo5
  293. mirage-net-unix >= "2.2.0"
  294. mirage-net-xen
  295. mirage-os-shim >= "3.0.0"
  296. mirage-profile
  297. mirage-protocols >= "4.0.0" & < "8.0.0"
  298. mirage-protocols-lwt
  299. mirage-qubes < "0.2" | >= "0.4" & < "0.9.4"
  300. mirage-qubes-ipv4 < "0.9.4"
  301. mirage-random-stdlib >= "0.1.0"
  302. mirage-runtime >= "3.7.0"
  303. mirage-solo5
  304. mirage-stack >= "2.0.0" & < "4.0.0"
  305. mirage-stack-lwt
  306. mirage-time >= "2.0.0"
  307. mirage-time-lwt
  308. mirage-time-unix
  309. mirage-types-lwt < "3.7.1"
  310. mirage-unix >= "3.0.0"
  311. mirage-vnetif
  312. mirage-vnetif-stack
  313. mirage-www >= "1.1.0"
  314. mirage-xen
  315. mirror
  316. moonpool-lwt
  317. mqtt = "0.0.2"
  318. mrmime >= "0.5.0"
  319. multipart-form-data >= "0.2.0"
  320. multipart_form >= "0.2.0" & < "0.4.0"
  321. multipart_form-lwt < "0.6.0"
  322. mwt
  323. named-pipe >= "0.3"
  324. nanomsg
  325. nbd = "2.1.1" | >= "3.0.0"
  326. nbd-tool
  327. nbd-unix
  328. netchannel
  329. nocrypto >= "0.5.4"
  330. noise
  331. nottui-lwt
  332. nproc
  333. nsq >= "0.4.0"
  334. obrowser
  335. obuilder < "0.4"
  336. obus >= "1.1.8" & < "1.2.1"
  337. ocaml-variants >= "4.00.1+mirage-unix" & < "4.00.1+open-types"
  338. ocluster < "0.2"
  339. ocluster-api < "0.2"
  340. ocplib-concur
  341. ocplib-resto
  342. ocsigen-start >= "4.1.0" & < "4.7.0"
  343. ocsigenserver >= "2.10"
  344. odoc >= "2.0.0" & < "2.1.0"
  345. ojquery
  346. ojs-base < "0.6.0"
  347. opam-compiler < "0.2.0"
  348. opam-publish = "0.3.5"
  349. opam-sync-github-prs
  350. openflow < "0.2.0"
  351. opium >= "0.11.0" & != "0.16.0" & < "0.19.0"
  352. opium_kernel
  353. opomodoro
  354. order-i3-xfce
  355. ordma >= "0.0.3"
  356. osc-lwt
  357. oskel >= "0.3.0"
  358. ounit-lwt < "2.2.0"
  359. ounit2-lwt
  360. ox < "1.1.0"
  361. paf
  362. paf-cohttp
  363. pcap-format >= "0.3.3" & < "0.5.0"
  364. pgx_lwt
  365. pgx_lwt_mirage
  366. pgx_lwt_unix < "2.0"
  367. plotkicadsch >= "0.4.0"
  368. ppx_defer >= "0.4.0"
  369. ppx_deriving_rpc
  370. ppx_json_types
  371. ppx_netblob
  372. ppx_rapper_lwt
  373. ppx_sqlexpr
  374. prof_spacetime
  375. prometheus
  376. prometheus-app
  377. promise_jsoo_lwt
  378. protocol-9p >= "0.10.0"
  379. protocol-9p-unix
  380. qcow >= "0.8.1"
  381. qcow-format < "0.3"
  382. qcow-tool
  383. qfs = "0.5" | >= "0.7"
  384. quests
  385. rawlink >= "1.0" & < "2.1"
  386. rawlink-lwt
  387. redis-lwt
  388. resource-pooling >= "0.3.2"
  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. serial
  407. session-cohttp-lwt
  408. session-cookie-lwt
  409. session-postgresql-lwt >= "0.4.1"
  410. sessions
  411. shared-block-ring < "2.3.0" | >= "3.0.0"
  412. shared-memory-ring >= "1.2.0" & < "2.0.0"
  413. shared-memory-ring-lwt
  414. slacko
  415. socket-daemon < "0.3.0"
  416. spin < "0.6.0"
  417. spotify-web-api < "0.2.1"
  418. sqlexpr = "0.7.1" | >= "0.9.0"
  419. statsd-client
  420. stog >= "0.16.0" & < "0.19.0"
  421. syndic >= "1.4" & < "1.6.0"
  422. tar-format >= "0.4.1"
  423. tar-mirage < "2.2.0"
  424. tar-unix
  425. tcpip >= "3.1.1"
  426. teash
  427. telegraml
  428. tezos-p2p < "8.0" | >= "11.0" & < "13.0"
  429. tezos-stdlib < "9.3"
  430. tezos-stdlib-unix < "9.0"
  431. tftp
  432. themoviedb
  433. tls >= "0.10.1" & < "0.16.0"
  434. tls-lwt < "0.17.4"
  435. tls-mirage
  436. tlstunnel >= "0.2.0"
  437. transmission-rpc
  438. tube >= "4.3.0"
  439. tuntap >= "1.0.0" & < "2.0.0"
  440. typerex-lldb
  441. u2f
  442. uspf
  443. uspf-lwt
  444. utop >= "1.4.0"
  445. uwt >= "0.3.0"
  446. vchan >= "0.9.7" & < "2.0.0" | >= "2.0.3"
  447. vchan-unix
  448. vchan-xen
  449. vercel
  450. vhd-format >= "0.7.0" & < "0.8.0"
  451. vhd-format-lwt >= "0.12.0"
  452. vhd-tool < "0.12.0"
  453. vmnet >= "1.3.2"
  454. vpnkit >= "0.2.0"
  455. vue-jsoo < "0.3"
  456. webauthn
  457. websocket < "2.3"
  458. xe-unikernel-upload
  459. xen-api-client < "0.9.14"
  460. xen-block-driver
  461. xen-evtchn != "1.0.6"
  462. xen-evtchn-unix
  463. xen-gnt >= "2.2.3"
  464. xen-gnt-unix >= "4.0.2"
  465. xenctrl < "0.9.29" | >= "0.9.32"
  466. xenstore >= "1.3.0"
  467. xenstore_transport >= "1.0.0"
  468. xentropyd
  469. yurt < "0.3"
  470. zbar
  471. zmq-lwt

Conflicts (1)

  1. ocaml-variants = "4.02.1+BER"