package lwt

  1. Overview
  2. Docs
Promises and event-driven I/O

Install

Dune Dependency

Authors

Maintainers

Sources

5.2.0.tar.gz
md5=d5783fcff4fbfa7f79c9303776e4d144

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: 09 Mar 2020

README

Lwt   

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:

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,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 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 (8)

  1. seq
  2. result
  3. ocplib-endian
  4. ocaml >= "4.02.0" & < "4.12"
  5. mmap >= "1.1.0"
  6. dune-configurator
  7. dune >= "1.7.0"
  8. cppo build & >= "1.1.0"

Dev Dependencies (2)

  1. ocamlfind dev & >= "1.7.3-1"
  2. bisect_ppx dev & >= "1.3.0"

  1. 0install >= "2.15.1"
  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 >= "2.2.1"
  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. bastet_lwt
  26. bimage-lwt
  27. biocaml = "0.4.0"
  28. bistro >= "0.4.0"
  29. brozip
  30. builder
  31. bun >= "0.3.3"
  32. c3
  33. calculon
  34. caldav
  35. camltc = "0.9.5" | >= "0.9.7.0"
  36. canary
  37. capnp-rpc-lwt < "1.2.3"
  38. capnp-rpc-unix >= "0.9.0" & < "1.2.3"
  39. caqti-lwt >= "0.11.0" & < "2.0.1"
  40. carton-git
  41. carton-lwt
  42. cf-lwt
  43. channel
  44. charrua-client >= "1.3.0"
  45. charrua-client-lwt
  46. charrua-client-mirage
  47. charrua-core < "0.3"
  48. charrua-unix >= "0.3" & != "0.10"
  49. cmdtui-lambda-term
  50. coclobas
  51. cohttp-lwt < "6.0.0~alpha2"
  52. cohttp-lwt-jsoo
  53. cohttp-lwt-unix >= "1.1.1"
  54. cohttp-lwt-unix-nossl
  55. cohttp-lwt-unix-ssl
  56. cohttp-mirage
  57. comby
  58. comby-semantic
  59. conan-lwt
  60. conduit-lwt
  61. conduit-lwt-unix
  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 < "0.6.4"
  70. current_docker < "0.6.4"
  71. current_examples < "0.6.4"
  72. current_git < "0.6.4"
  73. current_github < "0.6.4"
  74. current_gitlab < "0.6.4"
  75. current_ocluster < "0.2"
  76. current_slack < "0.6.4"
  77. current_web < "0.6.4"
  78. DkSDKFFIOCaml_Std
  79. dap
  80. data-encoding < "0.1.1"
  81. datakit
  82. datakit-bridge-github
  83. datakit-bridge-local-git
  84. datakit-ci
  85. datakit-client >= "0.11.0"
  86. datakit-github
  87. datakit-server
  88. devkit >= "1.2"
  89. dht < "0.2.0"
  90. distributed-lwt
  91. dkim-mirage
  92. dlm
  93. dns >= "0.19.1" & < "0.20.1"
  94. dns-certify
  95. dns-cli >= "4.6.3"
  96. dns-client < "7.0.0"
  97. dns-client-lwt
  98. dns-client-mirage
  99. dns-forward >= "0.9.0"
  100. dns-forward-lwt-unix
  101. dns-lwt
  102. dns-mirage
  103. dns-resolver
  104. dns-server
  105. dns-stub
  106. dnssd
  107. docker_hub
  108. dream
  109. dream-httpaf
  110. dream-pure
  111. dropbox
  112. dune_watch
  113. earlybird < "1.0.0"
  114. elasticsearch-cli >= "0.4"
  115. eris-lwt
  116. ethernet
  117. ez_api
  118. ezcurl-lwt
  119. ezirmin
  120. ezjs_min < "0.2"
  121. ezjsonm >= "0.4.2" & < "0.5.0"
  122. ezjsonm-lwt
  123. ezresto
  124. ezresto-directory >= "0.5"
  125. faraday-lwt
  126. faraday-lwt-unix >= "0.6.0"
  127. fat-filesystem >= "0.12.0"
  128. fiber-lwt
  129. flowtype >= "0.72.0"
  130. frenetic < "2.0.0"
  131. fsevents-lwt
  132. fswatch_lwt
  133. fuseau-lwt
  134. gamepad
  135. gdb
  136. gdbprofiler >= "0.3"
  137. git != "1.4.3" & != "1.7.2"
  138. git-paf
  139. git-unix = "1.11.1" | >= "3.0.0" & < "3.10.0"
  140. github
  141. github-hooks < "0.2.0" | >= "0.4.0"
  142. github-unix >= "4.4.0"
  143. gitlab-unix
  144. gluten-lwt
  145. gluten-lwt-unix < "0.4.0"
  146. gluten-mirage < "0.4.0"
  147. graphql-lwt
  148. gremlin
  149. gufo
  150. h2-lwt
  151. h2-lwt-unix < "0.10.0"
  152. h2-mirage
  153. happy-eyeballs-lwt
  154. happy-eyeballs-mirage
  155. hardcaml < "1.1.0"
  156. hardcaml-examples >= "0.3.0"
  157. hardcaml-framework
  158. hiredis != "0.4"
  159. hl_yaml
  160. horned_worm < "0.3.1"
  161. http-lwt-client
  162. http-multipart-formdata >= "2.0.0" & < "3.0.0"
  163. http2https
  164. httpaf-lwt-unix
  165. hvsock >= "1.0.2"
  166. i3ipc >= "0.1.4"
  167. imaplet-lwt
  168. influxdb-lwt
  169. inotify >= "2.4"
  170. inquire < "0.3.0"
  171. interface-prime-lwt
  172. iocaml < "0.4.6"
  173. iocaml-kernel >= "0.4.3" & < "0.4.6"
  174. iocamljs-kernel
  175. ip2location
  176. ip2locationio
  177. ipv6-multicast-lwt
  178. irc-client-lwt
  179. irc-client-lwt-ssl
  180. irc-client-tls
  181. irmin < "0.9.6" | = "0.9.10" | >= "0.11.0" & < "2.7.0"
  182. irmin-bench < "2.7.0"
  183. irmin-chunk < "2.7.0"
  184. irmin-containers < "2.7.0"
  185. irmin-fs >= "2.3.0" & < "2.7.0"
  186. irmin-git >= "2.3.0" & < "2.7.0"
  187. irmin-graphql >= "2.3.0" & < "2.7.0"
  188. irmin-http >= "2.3.0" & < "2.7.0"
  189. irmin-indexeddb
  190. irmin-layers < "2.7.0"
  191. irmin-mem >= "2.3.0"
  192. irmin-mirage-git >= "2.3.0" & < "2.7.0"
  193. irmin-mirage-graphql >= "2.3.0" & < "2.7.0"
  194. irmin-pack < "2.7.0"
  195. irmin-test >= "2.3.0" & < "2.7.0"
  196. irmin-unix >= "2.3.0" & < "2.7.0"
  197. irmin-watcher >= "0.3.0"
  198. jerboa
  199. jitsu
  200. joolog
  201. jose < "0.9.0"
  202. js_of_ocaml < "2.5"
  203. js_of_ocaml-lwt >= "3.5.0"
  204. jsoo_broadcastchannel
  205. jsoo_router
  206. jsoo_storage
  207. jupyter >= "2.3.0"
  208. jupyter-kernel >= "0.4"
  209. kafka >= "0.3" & < "0.5"
  210. kafka_lwt
  211. kappa-library
  212. ke >= "0.5"
  213. ketrew >= "3.2.0"
  214. kinetic-client < "0.0.3" | >= "0.0.9"
  215. kubecaml
  216. lablqml < "0.6"
  217. lambda-runtime
  218. lambda-term >= "1.13"
  219. lambda_streams_lwt
  220. launchd
  221. learn-ocaml >= "0.13.0"
  222. learn-ocaml-client >= "0.13.0"
  223. letsencrypt
  224. letsencrypt-app
  225. letsencrypt-dns
  226. letters
  227. libres3
  228. links != "0.9"
  229. linol-lwt
  230. lru_cache < "v0.16.0"
  231. lwt-binio < "0.2.0"
  232. lwt-canceler
  233. lwt-dllist
  234. lwt-exit
  235. lwt-parallel >= "1.0.0"
  236. lwt-pipe
  237. lwt-pipeline
  238. lwt-watcher
  239. lwt-zmq < "1.0.0"
  240. lwt_camlp4
  241. lwt_domain < "0.3.0"
  242. lwt_eio < "0.4"
  243. lwt_glib >= "1.0.1"
  244. lwt_log >= "1.1.0"
  245. lwt_ppx
  246. lwt_ppx_let
  247. lwt_react >= "1.0.1"
  248. lwt_ssl >= "1.0.1"
  249. mariadb < "0.5.1"
  250. markup = "0.7.6"
  251. markup-lwt
  252. mdx
  253. mechaml
  254. metrics-influx
  255. metrics-lwt
  256. metrics-mirage
  257. metrics-unix
  258. mindstorm-lwt
  259. mirage >= "0.4.1" & != "0.6.1" & < "0.8.0" | >= "0.10.0" & < "2.7.0"
  260. mirage-block < "1.0.0" | >= "2.0.0"
  261. mirage-block-ccm
  262. mirage-block-combinators
  263. mirage-block-lwt
  264. mirage-block-ramdisk
  265. mirage-block-solo5
  266. mirage-block-unix < "2.3.0" | = "2.8.2"
  267. mirage-block-xen
  268. mirage-bootvar-solo5 >= "0.2.0"
  269. mirage-bootvar-unix
  270. mirage-bootvar-xen >= "0.4.0"
  271. mirage-channel >= "4.0.0"
  272. mirage-channel-lwt
  273. mirage-clock-freestanding < "3.0.0"
  274. mirage-clock-lwt
  275. mirage-clock-unix >= "1.3.0" & < "3.0.0"
  276. mirage-console >= "3.0.0"
  277. mirage-console-lwt
  278. mirage-console-solo5 >= "0.2.0"
  279. mirage-console-unix >= "2.2.1"
  280. mirage-console-xen >= "5.0.0"
  281. mirage-console-xen-backend
  282. mirage-console-xen-cli
  283. mirage-crypto-entropy
  284. mirage-crypto-rng >= "0.7.0" & < "0.11.0"
  285. mirage-crypto-rng-lwt
  286. mirage-crypto-rng-mirage >= "0.8.8"
  287. mirage-device >= "2.0.0"
  288. mirage-dns < "3.0.0"
  289. mirage-entropy
  290. mirage-flow >= "1.0.3" & < "1.2.0" | >= "2.0.0"
  291. mirage-flow-combinators
  292. mirage-flow-lwt
  293. mirage-flow-rawlink
  294. mirage-flow-unix >= "1.3.0"
  295. mirage-fs >= "3.0.0"
  296. mirage-fs-lwt
  297. mirage-fs-unix < "1.1.1" | >= "1.3.0"
  298. mirage-http
  299. mirage-http-unix
  300. mirage-http-xen
  301. mirage-kv >= "3.0.0"
  302. mirage-kv-lwt
  303. mirage-kv-unix
  304. mirage-logs != "0.3.0"
  305. mirage-nat < "3.0.0"
  306. mirage-net >= "3.0.1"
  307. mirage-net-fd
  308. mirage-net-lwt
  309. mirage-net-macosx
  310. mirage-net-solo5
  311. mirage-net-unix >= "2.2.0"
  312. mirage-net-xen
  313. mirage-os-shim >= "3.0.0"
  314. mirage-profile
  315. mirage-protocols >= "4.0.0" & < "8.0.0"
  316. mirage-protocols-lwt
  317. mirage-qubes < "0.2" | >= "0.4"
  318. mirage-qubes-ipv4
  319. mirage-random-stdlib >= "0.1.0"
  320. mirage-runtime >= "3.7.0"
  321. mirage-solo5
  322. mirage-stack >= "2.0.0" & < "4.0.0"
  323. mirage-stack-lwt
  324. mirage-time >= "2.0.0"
  325. mirage-time-lwt
  326. mirage-time-unix
  327. mirage-types-lwt < "3.7.1"
  328. mirage-unix >= "3.0.0"
  329. mirage-vnetif
  330. mirage-vnetif-stack
  331. mirage-www >= "1.1.0"
  332. mirage-xen
  333. mirror
  334. monorobot
  335. mqtt >= "0.0.2"
  336. mrmime >= "0.5.0"
  337. multipart-form-data >= "0.2.0"
  338. multipart_form >= "0.2.0" & < "0.4.0"
  339. multipart_form-lwt
  340. mwt
  341. naboris
  342. nanomsg
  343. nbd = "2.1.1" | >= "4.0.3"
  344. nbd-tool
  345. nbd-unix
  346. netchannel
  347. nocrypto >= "0.5.4"
  348. nottui-lwt
  349. nproc
  350. nsq >= "0.4.0"
  351. obrowser
  352. obuilder < "0.4"
  353. obus >= "1.2.1"
  354. ocaml-variants >= "4.00.1+mirage-unix" & < "4.00.1+open-types"
  355. ocluster < "0.2"
  356. ocluster-api < "0.2"
  357. ocplib-concur
  358. ocplib-resto
  359. ocsigen-start >= "4.1.0" & < "4.7.0"
  360. ocsigenserver >= "2.10"
  361. ocsipersist
  362. ocsipersist-dbm
  363. ocsipersist-lib
  364. ocsipersist-pgsql
  365. ocsipersist-sqlite
  366. odoc >= "2.0.0" & < "2.1.0"
  367. ojquery
  368. ojs-base < "0.6.0"
  369. opam-compiler < "0.2.0"
  370. opam-sync-github-prs
  371. openflow < "0.2.0"
  372. opium >= "0.11.0" & != "0.16.0" & < "0.19.0"
  373. opium_kernel
  374. opomodoro
  375. order-i3-xfce
  376. ordma >= "0.0.3"
  377. osc-lwt
  378. oskel >= "0.3.0"
  379. ounit-lwt < "2.2.0"
  380. ounit2-lwt
  381. owork
  382. ox < "1.1.0"
  383. paf
  384. paf-cohttp
  385. pcap-format >= "0.3.3" & < "0.5.0"
  386. pgx_lwt
  387. pgx_lwt_mirage
  388. pgx_lwt_unix < "2.0"
  389. plist-xml-lwt
  390. plotkicadsch >= "0.4.0"
  391. ppx_defer >= "0.4.0"
  392. ppx_deriving_rpc
  393. ppx_json_types
  394. ppx_netblob
  395. ppx_rapper_lwt
  396. ppx_sqlexpr
  397. prof_spacetime
  398. prometheus
  399. prometheus-app
  400. promise_jsoo_lwt
  401. protocol-9p >= "0.10.0"
  402. protocol-9p-unix
  403. qcow >= "0.8.1"
  404. qcow-format < "0.3"
  405. qcow-tool
  406. qfs = "0.5" | >= "0.7"
  407. quests
  408. rawlink >= "1.0" & < "2.1"
  409. rawlink-lwt
  410. redis-lwt
  411. resource-pooling >= "0.3.2"
  412. resp
  413. resp-mirage >= "0.10.0"
  414. resp-unix >= "0.10.0"
  415. resto
  416. resto-cohttp-client >= "0.4"
  417. resto-cohttp-self-serving-client
  418. resto-cohttp-server >= "0.4"
  419. resto-directory >= "0.4"
  420. riak
  421. ringo-lwt
  422. river
  423. rpc >= "1.5.1" & < "7.1.0"
  424. rpclib-js
  425. rpclib-lwt
  426. SZXX < "2.0.0"
  427. sanddb
  428. scgi
  429. sendmail-lwt
  430. serial
  431. session-cohttp-lwt
  432. session-cookie-lwt
  433. session-postgresql-lwt >= "0.4.1"
  434. sessions
  435. shared-block-ring < "2.3.0" | >= "3.0.0"
  436. shared-memory-ring >= "1.2.0" & < "2.0.0"
  437. shared-memory-ring-lwt
  438. slack
  439. slacko
  440. socket-daemon < "0.3.0"
  441. spin < "0.6.0"
  442. spotify-web-api < "0.2.1"
  443. sqlexpr = "0.7.1" | >= "0.9.0"
  444. statsd-client
  445. stog >= "0.16.0" & < "0.19.0"
  446. syndic >= "1.4" & < "1.6.0"
  447. tar-format >= "0.4.1"
  448. tar-mirage < "2.2.0"
  449. tar-unix
  450. tcpip >= "3.1.1" & < "3.4.1" | >= "4.1.0"
  451. teash
  452. telegraml
  453. tezos-error-monad >= "8.0" & < "9.0"
  454. tezos-lwt-result-stdlib >= "9.0" & < "11.0"
  455. tezos-p2p >= "11.0" & < "13.0"
  456. tezos-stdlib < "9.3"
  457. tezos-stdlib-unix < "9.0"
  458. tftp
  459. themoviedb
  460. tls = "0.10.1" | >= "0.10.6" & < "0.16.0"
  461. tls-lwt < "0.17.4"
  462. tls-mirage
  463. transmission-rpc
  464. tube >= "4.3.0"
  465. tuntap >= "1.0.0" & < "1.7.0" | >= "2.0.0"
  466. typerex-lldb
  467. u2f
  468. uring
  469. uspf
  470. uspf-lwt
  471. utop >= "1.4.0"
  472. uwt >= "0.3.0"
  473. vchan >= "0.9.7" & < "2.0.0" | >= "2.0.3"
  474. vchan-unix
  475. vchan-xen
  476. vercel
  477. vhd-format >= "0.7.0" & < "0.8.0"
  478. vhd-format-lwt >= "0.12.0"
  479. vhd-tool < "0.12.0"
  480. vmnet >= "1.3.2"
  481. vpnkit >= "0.2.0"
  482. vue-jsoo < "0.3"
  483. webauthn
  484. websocket < "2.3"
  485. xe-unikernel-upload
  486. xen-api-client < "0.9.14"
  487. xen-block-driver
  488. xen-evtchn < "1.0.6" | >= "2.0.0"
  489. xen-evtchn-unix
  490. xen-gnt >= "2.2.3"
  491. xenctrl < "0.9.29" | >= "0.9.32"
  492. xenstore >= "1.3.0"
  493. xenstore_transport >= "1.0.0"
  494. xentropyd
  495. yurt < "0.3"
  496. zmq-lwt

Conflicts (1)

  1. ocaml-variants = "4.02.1+BER"