# 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. You can also use Lwt.wrap for functions that may raise
normal exception.

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.

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

wrap f calls f and transform the result into a monad. If f
raise an exception, it is catched by Lwt.

This is actually the same as:

try return (f ()) with exn -> fail exn

val wrap1 : ('a -> 'b) -> 'a -> 'b t

wrap1 f x applies f on x and returns the result as a
thread. If the application of f to x raise an exception it
is catched and a thread is returned.

Note that you must use Lwt.wrap instead of Lwt.wrap1 if the
evaluation of x may raise an exception.

for example the following code is not ok:

wrap1 f (Hashtbl.find table key)

you should write instead:

wrap (fun () -> f (Hashtbl.find table key))

val wrap2 : ('a -> 'b -> 'c) -> 'a -> 'b -> 'c t

val wrap3 : ('a -> 'b -> 'c -> 'd) -> 'a -> 'b -> 'c -> 'd t

val wrap4 : ('a -> 'b -> 'c -> 'd -> 'e) -> 'a -> 'b -> 'c -> 'd -> 'e t

val wrap5 :

('a -> 'b -> 'c -> 'd -> 'e -> 'f) ->

'a -> 'b -> 'c -> 'd -> 'e -> 'f t

val wrap6 :

('a -> 'b -> 'c -> 'd -> 'e -> 'f -> 'g) ->

'a -> 'b -> 'c -> 'd -> 'e -> 'f -> 'g t

val wrap7 :

('a -> 'b -> 'c -> 'd -> 'e -> 'f -> 'g -> 'h) ->

'a -> 'b -> 'c -> 'd -> 'e -> 'f -> 'g -> 'h t

###### 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 nchoose_split : 'a t list -> ('a list * 'a t list) t

nchoose_split l does the same as Lwt.nchoose but also retrurns
the list of threads that have not yet terminated.

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 wakeup_later : 'a u -> 'a -> unit

Same as Lwt.wakeup but it is not guaranteed that the thread will
be wakeup immediately.

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

Same as Lwt.wakeup_exn but it is not guaranteed that the thread
will be wakeup immediately.

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
woken up with the exception Lwt.Canceled.

For example, in the following code:

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

waiter will be woken 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.

###### Pause ¶

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

###### Misc ¶

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

on_success t f executes f when t terminates without
failing. This is the same as:

ignore_result (bind t (fun x -> f x; return ()))

but a bit more efficient.

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

on_failure t f executes f when t terminates and
fails. This is the same as:

ignore_result (catch t (fun e -> f e; return ()))

but a bit more efficient.

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

on_termination t f executes f when t terminates. This is
the same as:

ignore_result (finalize (fun () -> t) (fun () -> f (); return ()))

but a bit more efficient.