# 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.

    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`.