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

run p calls the Lwt scheduler repeatedly until p resolves, and returns the value of p if it is fulfilled. If p is rejected with an exception, that exception is raised.

Every native or bytecode program that uses Lwt should always use this function for evaluating a promise at the top level (such as its main function or main loop), otherwise promises that depend on I/O operations will not be resolved.


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

let () = @@ main ()

When targeting JavaScript, is not available, but neither it's necessary since the JS environment automatically takes care of the I/O considerations.

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.

Note that you should avoid using run inside threads

  • The calling threads will not resume before run returns.
  • Successive invocations of run are serialized: an invocation of run will not terminate before all subsequent invocations are terminated.

Note also that it is not safe to call run in a function registered with Pervasives.at_exit, use the at_exit function of this module instead.

val yield : unit -> unit Lwt.t

yield () is a threads which suspends itself and then resumes as soon as possible and terminates.

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).