Warning: Reason support is experimental. We are looking for beta-tester and contributors.

Module Lwt

module Lwt: sig..end

Module Lwt: cooperative light-weight threads.

This module defines cooperative light-weight threads with their primitives. A light-weight thread represent a computation that may be not terminated, for example because it is waiting for some event to happen.

Lwt threads are cooperative in the sense that switching to another thread is awlays explicit (with Lwt.​wakeup or wekup_exn). When a thread is running, it executes as much as possible, and then returns (a value or an eror) or sleeps.

Note that inside a Lwt thread, exceptions must be raised with Lwt.​fail instead of raise. Also the try ... with ... construction will not catch Lwt errors. You must use Lwt.​catch instead.

Lwt also provides the syntax extension Pa_lwt to make code using Lwt more readable.

Definitions and basics

type +'a t

The type of threads returning a result of type 'a.

val return : 'a -> 'a t

return e is a thread whose return value is the value of the expression e.

val fail : exn -> 'a t

fail e is a thread that fails with the exception e.

val bind : 'a t -> ('a -> 'b t) -> 'b t

bind t f is a thread which first waits for the thread t to terminate and then, if the thread succeeds, behaves as the application of function f to the return value of t. If the thread t fails, bind t f also fails, with the same exception.

The expression bind t (fun x -> t') can intuitively be read as let x = t in t', and if you use the lwt.syntax syntax extension, you can write a bind operation like that: lwt x = t in t'.

Note that bind is also often used just for synchronization purpose: t' will not execute before t is terminated.

The result of a thread can be bound several time.

val (>>=) : 'a t -> ('a -> 'b t) -> 'b t

t >>= f is an alternative notation for bind t f.

val (=<<) : ('a -> 'b t) -> 'a t -> 'b t

f =<< t is t >>= f

val map : ('a -> 'b) -> 'a t -> 'b t

map f m map the result of a thread. This is the same as bind m (fun x -> return (f x))

val (>|=) : 'a t -> ('a -> 'b) -> 'b t

m >|= f is map f m

val (=|<) : ('a -> 'b) -> 'a t -> 'b t

f =|< m is map f m

Thread storage

type 'a key

Type of a key. Keys are used to store local values into threads

val new_key : unit -> 'a key

new_key () creates a new key.

val get : 'a key -> 'a option

get key returns the value associated with key in the current thread.

val with_value : 'a key -> 'a option -> (unit -> 'b) -> 'b

with_value key value f executes f with value associated to key. The previous value associated to key is restored after f terminates.

Exceptions handling

val catch : (unit -> 'a t) -> (exn -> 'a t) -> 'a t

catch t f is a thread that behaves as the thread t () if this thread succeeds. If the thread t () fails with some exception, catch t f behaves as the application of f to this exception.

val try_bind : (unit -> 'a t) -> ('a -> 'b t) -> (exn -> 'b t) -> 'b t

try_bind t f g behaves as bind (t ()) f if t does not fail. Otherwise, it behaves as the application of g to the exception associated to t ().

val finalize : (unit -> 'a t) -> (unit -> unit t) -> 'a t

finalize f g returns the same result as f () whether it fails or not. In both cases, g () is executed after f.

Multi-threads composition

val choose : 'a t list -> 'a t

choose l behaves as the first thread in l to terminate. If several threads are already terminated, one is choosen at random.

Note: Lwt.​choose leaves the local values of the current thread unchanged.

val nchoose : 'a t list -> 'a list t

nchoose l returns the value of all that have succcessfully terminated. If all threads are sleeping, it waits for at least one to terminates. If one the threads of l fails, nchoose fails with the same exception.

Note: Lwt.​nchoose leaves the local values of the current thread unchanged.

val join : unit t list -> unit t

join l waits for all threads in l to terminate. If one of the threads fails, then join l will fails with the same exception as the first one to terminate.

Note: Lwt.​join leaves the local values of the current thread unchanged.

val (<?>) : 'a t -> 'a t -> 'a t

t <?> t' is the same as choose [t; t']

val (<&>) : unit t -> unit t -> unit t

t <&> t' is the same as join [t; t']

val ignore_result : 'a t -> unit

ignore_result t start the thread t and ignores its result value if the thread terminates sucessfully. However, if the thread t fails, the exception is raised instead of being ignored.

You should use this function if you want to start a thread and don't care what its return value is, nor when it terminates (for instance, because it is looping). Note that if the thread t yields and later fails, the exception will not be raised at this point in the program.

Sleeping and resuming

type 'a u

The type of thread wakeners.

val wait : unit -> 'a t * 'a u

wait () is a pair of a thread which sleeps forever (unless it is resumed by one of the functions wakeup, wakeup_exn below) and the corresponding wakener. This thread does not block the execution of the remainder of the program (except of course, if another thread tries to wait for its termination).

val wakeup : 'a u -> 'a -> unit

wakeup t e makes the sleeping thread t terminate and return the value of the expression e.

val wakeup_exn : 'a u -> exn -> unit

wakeup_exn t e makes the sleeping thread t fail with the exception e.

val waiter_of_wakener : 'a u -> 'a t

Returns the thread associated to a wakener.

Threads state

type 'a state =

|Return of 'a(*The thread which has successfully terminated*)
|Fail of exn(*The thread raised an exception*)
|Sleep(*The thread is sleeping*)

State of a thread

val state : 'a t -> 'a state

state t returns the state of a thread

Cancelable threads

Cancelable threads are the same as regular threads except that they can be canceled.

exception Canceled

Canceled threads fails with this exception

val task : unit -> 'a t * 'a u

task () is the same as wait () except that threads created with task can be canceled.

val on_cancel : 'a t -> (unit -> unit) -> unit

on_cancel t f executes f when t is canceled. This is the same as catching Canceled.

val cancel : 'a t -> unit

cancel t cancels the threads t. This means that the deepest sleeping thread created with task and connected to t is wakeup with the exception Lwt.​Canceled.

For example, in the following code:

        let waiter, wakener = task () in
        cancel (waiter >> printl "plop")

waiter will be waked up with Lwt.​Canceled.

val pick : 'a t list -> 'a t

pick l is the same as Lwt.​choose, except that it cancels all sleeping threads when one terminates.

Note: Lwt.​pick leaves the local values of the current thread unchanged.

val npick : 'a t list -> 'a list t

npick l is the same as Lwt.​nchoose, except that it cancels all sleeping threads when one terminates.

Note: Lwt.​npick leaves the local values of the current thread unchanged.

val protected : 'a t -> 'a t

protected thread creates a new cancelable thread which behave as thread except that cancelling it does not cancel thread.


val pause : unit -> unit t

pause () is a sleeping thread which is wake up on the next call to Lwt.​wakeup_paused. A thread created with pause can be canceled.

val wakeup_paused : unit -> unit

wakeup_paused () wakes up all threads which suspended themselves with Lwt.​pause.

This function is called by the scheduler, before entering the main loop. You usually do not have to call it directly, except if you are writing a custom scheduler.

Note that if a paused thread resume and pause again, it will not be wakeup at this point.

val paused_count : unit -> int

paused_count () returns the number of thread currently paused.

val register_pause_notifier : (int -> unit) -> unit

register_pause_notifier f register a function f that will be called each time pause is called. The parameter passed to f is the new number of threads paused. It is usefull to be able to call Lwt.​wakeup_paused when there is no scheduler