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

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. 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 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