package lwt

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Main loop and event queue

This module controls the ``main-loop'' of Lwt.

val run : 'a Lwt.t -> 'a p calls the Lwt scheduler, performing I/O until p resolves. p returns the value in p if p is fulfilled. If p is rejected with an exception instead, p raises that exception.

Every native and bytecode program that uses Lwt should call this function at its top level. It implements the Lwt main loop.


let main () = Lwt_io.write_line Lwt_io.stdout "hello world"

let () = (main ()) is not available when targeting JavaScript, because the environment (such as Node.js or the browser's script engine) implements the I/O loop.

On Unix, calling installs a SIGCHLD handler, which is needed for the implementations of Lwt_unix.waitpid and Lwt_unix.wait4. As a result, programs that call and also use non-Lwt system calls need to handle those system calls failing with EINTR.

Nested calls to are not allowed. That is, do not call in a callback triggered by a promise that is resolved by an outer invocation of If your program makes such a call, will raise Failure. This should be considered a logic error (i.e., code making such a call is inherently broken).

It is not safe to call in a function registered with Pervasives.at_exit, use Lwt_main.at_exit instead.

val yield : unit -> unit Lwt.t

yield () is a pending promise that is fulfilled after Lwt finishes calling all currently ready callbacks, i.e. it is fulfilled on the next “tick.”

yield is now deprecated in favor of the more general Lwt.pause in order to avoid discrepancies in resolution (see below) and stay compatible with other execution environments such as js_of_ocaml.

Currently, paused promises are resolved more frequently than yielded promises. The difference is unintended but existing applications could depend on it. Unifying the two pools of promises into one in the future would eliminate possible discrepancies and simplify the code.

  • deprecated Use Lwt.pause instead
val abandon_yielded_and_paused : unit -> unit

Causes promises created with Lwt.pause and Lwt_main.yield to remain forever pending.

yield is now deprecated in favor of the more general Lwt.pause. Once yield is phased out, this function will be deprecated as well.

This is meant for use with Lwt.fork, as a way to "abandon" more promise chains that are pending in your process.

module type Hooks = sig ... end

Hook sequences. Each module of this type is a set of hooks, to be run by Lwt at certain points during execution. See modules Enter_iter_hooks, Leave_iter_hooks, and Exit_hooks.

module Enter_iter_hooks : Hooks with type 'return_value kind = 'return_value

Hooks, of type unit -> unit, that are called before each iteration of the Lwt main loop.

module Leave_iter_hooks : Hooks with type 'return_value kind = 'return_value

Hooks, of type unit -> unit, that are called after each iteration of the Lwt main loop.

module Exit_hooks : Hooks with type 'return_value kind = 'return_value Lwt.t

Promise-returning hooks, of type unit -> unit Lwt.t, that are called at process exit. Exceptions raised by these hooks are ignored.

val enter_iter_hooks : (unit -> unit) Lwt_sequence.t
val leave_iter_hooks : (unit -> unit) Lwt_sequence.t
val exit_hooks : (unit -> unit Lwt.t) Lwt_sequence.t
val at_exit : (unit -> unit Lwt.t) -> unit

Lwt_main.at_exit hook is the same as ignore (Lwt_main.Exit_hooks.add_first hook).


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