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Module Lwt


module Lwt: sig .. end
Module Lwt: cooperative light-weight threads.

Execution order A thread executes as much as possible. Switching to another thread is always explicit.

Exception handling


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'.

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 = 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 = map f m
val (=|<) : ('a -> 'b) -> 'a t -> 'b t
f =|< m = map f m
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 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.
val join : unit t list -> unit t
join l wait for all threads in l to terminate. If fails if one of the threads fail.
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.
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 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.

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

Cancelling threads

exception Canceled
Canceled threads fails with this exception
val task : unit -> 'a t * 'a u
task () creates a sleeping thread that can be canceled using Lwt.cancel
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 () to which t is connected is wakeup with the exception Lwt.Canceled.

For example, in the following code:

        let w = task () in        cancel (w >> printl "plop")

w will be waked up with Lwt.Canceled.

val select : 'a t list -> 'a t
select l is the same as choose but it cancels all sleeping threads when one terminates.