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Module Lwt_main

module Lwt_main : sig..end

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 () = Lwt_main.run @@ main ()

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

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 Lwt_main.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.

val enter_iter_hooks : (unit -> unit) Lwt_sequence.t

Functions that are called before the main iteration.

val leave_iter_hooks : (unit -> unit) Lwt_sequence.t

Functions that are called after the main iteration.

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

Sets of functions executed just before the program exit.


  • each hook is called exactly one time
  • exceptions raised by hooks are ignored
val at_exit : (unit -> unit Lwt.t) -> unit

at_exit hook adds hook at the left of exit_hooks