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 Lwt.​wakeup_exn). When a thread is running, it executes as much as possible, and then returns (a value or an error) 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 times.

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 maps 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

Pre-allocated threads

val return_unit : unit t

return_unit = return ()

val return_none : 'a option t

return_none = return None

val return_nil : 'a list t

return_nil = return []

val return_true : bool t

return_true = return true

val return_false : bool t

return_false = return false

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. key is restored to its previous value 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 transforms the result into an Lwt thread. If f raises an exception, it is caught and converted to an Lwt exception.

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 incorrect:

wrap1 f (Hashtbl.find table key)

and should be written as:

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 chosen 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 async : (unit -> 'a t) -> unit

async f starts a thread without waiting for the result. If it fails (now or later), the exception is given to Lwt.​async_exception_hook.

You should use this function if you want to start a thread that might fail and don't care what its return value is, nor when it terminates (for instance, because it is looping).

val ignore_result : 'a t -> unit

ignore_result t is like Pervasives.ignore t except that:

  • if t already failed, it raises the exception now,
  • if t is sleeping and fails later, the exception will be given to Lwt.​async_exception_hook.
val async_exception_hook : (exn -> unit) Pervasives.ref

Function called when a asynchronous exception is thrown.

The default behavior is to print an error message with a backtrace if available and to exit the program.

The behavior is undefined if this function raise an exception.

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 woken up immediately.

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

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

val waiter_of_wakener : 'a u -> 'a t

Returns the thread associated to a wakener.

type +'a result

Either a value of type 'a, either an exception.

val make_value : 'a -> 'a result

value x creates a result containing the value x.

val make_error : exn -> 'a result

error e creates a result containing the exception e.

val of_result : 'a result -> 'a t

Returns a thread from a result.

val wakeup_result : 'a u -> 'a result -> unit

wakeup_result t r makes the sleeping thread t terminate with the result r.

val wakeup_later_result : 'a u -> 'a result -> unit

Same as Lwt.​wakeup_result but it is not guaranteed that the thread will be woken up immediately.

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

val is_sleeping : 'a t -> bool

is_sleeping t returns true iff t is sleeping.

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. f will be executed before all other threads waiting on t.

If f raises an exception it is given to Lwt.​async_exception_hook.

val add_task_r : 'a u Lwt_sequence.t -> 'a t

add_task_r seq creates a sleeping thread, adds its wakener to the right of seq and returns its waiter. When the thread is canceled, it is removed from seq.

val add_task_l : 'a u Lwt_sequence.t -> 'a t

add_task_l seq creates a sleeping thread, adds its wakener to the left of seq and returns its waiter. When the thread is canceled, it is removed from seq.

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.

val no_cancel : 'a t -> 'a t

no_cancel thread creates a thread which behave as thread except that it cannot be canceled.

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 resumes and pauses again, it will not be woken up at this point.

val paused_count : unit -> int

paused_count () returns the number of currently paused threads.

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. If f raises an exception it is given to Lwt.​async_exception_hook.

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

on_failure t f executes f when t terminates and fails. If f raises an exception it is given to Lwt.​async_exception_hook.

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

on_termination t f executes f when t terminates. If f raises an exception it is given to Lwt.​async_exception_hook.

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

on_any t f g executes f or g when t terminates. If f or g raises an exception it is given to Lwt.​async_exception_hook.