# Module Lwt_stream

module Lwt_stream : sig..end

Data streams

type 'a t

A stream holding values of type 'a.

Naming convention: in this module, all functions applying a function to each element of a stream are suffixed by:

- _s when the function returns a thread and calls are serialised
- _p when the function returns a thread and calls are parallelised

This module may undergo redesign or deprecation in the future. See Flaws of Lwt_stream. In the meantime, you may want to consider using alternatives such as lwt-pipe. Lwt_stream will remain stable for the foreseeable future, but we would like to encourage users to try alternatives such as lwt-pipe, to collect feedback for those projects.

## Construction ¶

val from : (unit -> 'a option Lwt.t) -> 'a t

from f creates a stream from the given input function. f is called each time more input is needed, and the stream ends when f returns None.

If f, or the thread produced by f, raises an exception, that exception is forwarded to the consumer of the stream (for example, a caller of Lwt_stream.get). Note that this does not end the stream. A subsequent attempt to read from the stream will cause another call to f, which may succeed with a value.

val from_direct : (unit -> 'a option) -> 'a t

from_direct f does the same as Lwt_stream.from but with a function that does not return a thread. It is preferred that this function be used rather than wrapping f into a function which returns a thread.

The behavior when f raises an exception is the same as for Lwt_stream.from, except that f does not produce a thread.

exception Closed

Exception raised by the push function of a push-stream when pushing an element after the end of stream (= None) has been pushed.

val create : unit -> 'a t * ('a option -> unit)

create () returns a new stream and a push function.

To notify the stream's consumer of errors, either use a separate communication channel, or use a result stream. There is no way to push an exception into a push-stream.

val create_with_reference :

unit -> 'a t * ('a option -> unit) * ('b -> unit)

create_with_reference () returns a new stream and a push function. The last function allows a reference to be set to an external source. This prevents the external source from being garbage collected.

For example, to convert a reactive event to a stream:

```
let stream, push, set_ref = Lwt_stream.create_with_reference () in
set_ref (map_event push event)
```

exception Full

Exception raised by the push function of a bounded push-stream when the stream queue is full and a thread is already waiting to push an element.

class type ['a]bounded_push = object..end

Type of sources for bounded push-streams.

val create_bounded : int -> 'a t * 'a bounded_push

create_bounded size returns a new stream and a bounded push source. The stream can hold a maximum of size elements. When this limit is reached, pushing a new element will block until one is consumed.

Note that you cannot clone or parse (with Lwt_stream.parse) a bounded stream. These functions will raise Invalid_argument if you try to do so.

It raises Invalid_argument if size < 0.

val of_seq : 'a Stdlib.Seq.t -> 'a t

of_seq s creates a stream returning all elements of s. The elements are evaluated from s and pushed onto the stream as the stream is consumed.

val of_list : 'a list -> 'a t

of_list l creates a stream returning all elements of l. The elements are pushed into the stream immediately, resulting in a closed stream (in the sense of Lwt_stream.is_closed).

val of_array : 'a array -> 'a t

of_array a creates a stream returning all elements of a. The elements are pushed into the stream immediately, resulting in a closed stream (in the sense of Lwt_stream.is_closed).

val of_string : string -> char t

of_string str creates a stream returning all characters of str. The characters are pushed into the stream immediately, resulting in a closed stream (in the sense of Lwt_stream.is_closed).

val clone : 'a t -> 'a t

clone st clone the given stream. Operations on each stream will not affect the other.

For example:

```
# let st1 = Lwt_stream.of_list [1; 2; 3];;
val st1 : int Lwt_stream.t = <abstr>
# let st2 = Lwt_stream.clone st1;;
val st2 : int Lwt_stream.t = <abstr>
# lwt x = Lwt_stream.next st1;;
val x : int = 1
# lwt y = Lwt_stream.next st2;;
val y : int = 1
```

It raises Invalid_argument if st is a bounded push-stream.

## Destruction ¶

val to_list : 'a t -> 'a list Lwt.t

Returns the list of elements of the given stream

val to_string : char t -> string Lwt.t

Returns the word composed of all characters of the given stream

## Data retrieval ¶

exception Empty

Exception raised when trying to retrieve data from an empty stream.

val peek : 'a t -> 'a option Lwt.t

peek st returns the first element of the stream, if any, without removing it.

val npeek : int -> 'a t -> 'a list Lwt.t

npeek n st returns at most the first n elements of st, without removing them.

val get : 'a t -> 'a option Lwt.t

get st removes and returns the first element of the stream, if any.

val nget : int -> 'a t -> 'a list Lwt.t

nget n st removes and returns at most the first n elements of st.

val get_while : ('a -> bool) -> 'a t -> 'a list Lwt.t

val get_while_s : ('a -> bool Lwt.t) -> 'a t -> 'a list Lwt.t

get_while f st returns the longest prefix of st where all elements satisfy f.

val next : 'a t -> 'a Lwt.t

next st removes and returns the next element of the stream or fails with Lwt_stream.Empty, if the stream is empty.

val last_new : 'a t -> 'a Lwt.t

last_new st returns the last element that can be obtained without sleeping, or wait for one if none is available.

It fails with Lwt_stream.Empty if the stream has no more elements.

val junk : 'a t -> unit Lwt.t

junk st removes the first element of st.

val njunk : int -> 'a t -> unit Lwt.t

njunk n st removes at most the first n elements of the stream.

val junk_while : ('a -> bool) -> 'a t -> unit Lwt.t

val junk_while_s : ('a -> bool Lwt.t) -> 'a t -> unit Lwt.t

junk_while f st removes all elements at the beginning of the streams which satisfy f.

val junk_old : 'a t -> unit Lwt.t

junk_old st removes all elements that are ready to be read without yielding from st.

For example, the read_password function of Lwt_read_line uses it to flush keys previously typed by the user.

val get_available : 'a t -> 'a list

get_available st returns all available elements of l without blocking.

val get_available_up_to : int -> 'a t -> 'a list

get_available_up_to n st returns up to n elements of l without blocking.

val is_empty : 'a t -> bool Lwt.t

is_empty st returns whether the given stream is empty.

val is_closed : 'a t -> bool

is_closed st returns whether the given stream has been closed. A closed
stream is not necessarily empty. It may still contain unread elements. If
is_closed s = true, then all subsequent reads until the end of the
stream are guaranteed not to block.
**Since** 2.6.0

val closed : 'a t -> unit Lwt.t

closed st returns a thread that will sleep until the stream has been
closed.
**Since** 2.6.0

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

Deprecated.Use Lwt_stream.closed. on_termination st f executes f when the end of the stream st is reached. Note that the stream may still contain elements if Lwt_stream.peek or similar was used.

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

Deprecated.Use Lwt_stream.closed. Same as Lwt_stream.on_termination.

## Stream transversal ¶

Note: all the following functions are destructive.

For example:

```
# let st1 = Lwt_stream.of_list [1; 2; 3];;
val st1 : int Lwt_stream.t = <abstr>
# let st2 = Lwt_stream.map string_of_int st1;;
val st2 : string Lwt_stream.t = <abstr>
# lwt x = Lwt_stream.next st1;;
val x : int = 1
# lwt y = Lwt_stream.next st2;;
val y : string = "2"
```

val choose : 'a t list -> 'a t

choose l creates an stream from a list of streams. The resulting stream will return elements returned by any stream of l in an unspecified order.

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

val map_s : ('a -> 'b Lwt.t) -> 'a t -> 'b t

map f st maps the value returned by st with f

val filter : ('a -> bool) -> 'a t -> 'a t

val filter_s : ('a -> bool Lwt.t) -> 'a t -> 'a t

filter f st keeps only values, x, such that f x is true

val filter_map : ('a -> 'b option) -> 'a t -> 'b t

val filter_map_s : ('a -> 'b option Lwt.t) -> 'a t -> 'b t

filter_map f st filter and map st at the same time

val map_list : ('a -> 'b list) -> 'a t -> 'b t

val map_list_s : ('a -> 'b list Lwt.t) -> 'a t -> 'b t

map_list f st applies f on each element of st and flattens the lists returned

val fold : ('a -> 'b -> 'b) -> 'a t -> 'b -> 'b Lwt.t

val fold_s : ('a -> 'b -> 'b Lwt.t) -> 'a t -> 'b -> 'b Lwt.t

fold f s x fold_like function for streams.

val iter : ('a -> unit) -> 'a t -> unit Lwt.t

val iter_p : ('a -> unit Lwt.t) -> 'a t -> unit Lwt.t

val iter_s : ('a -> unit Lwt.t) -> 'a t -> unit Lwt.t

iter f s iterates over all elements of the stream.

val iter_n :

?max_concurrency:int ->

('a -> unit Lwt.t) -> 'a t -> unit Lwt.t

iter_n ?max_concurrency f s iterates over all elements of the stream s. Iteration is performed concurrently with up to max_threads concurrent instances of f.

Iteration is **not** guaranteed to be in order as this function will
attempt to always process max_concurrency elements from s at once.
**Raises** Invalid_argument if max_concurrency < 1.

max_concurrency : defaults to 1.

val find : ('a -> bool) -> 'a t -> 'a option Lwt.t

val find_s : ('a -> bool Lwt.t) -> 'a t -> 'a option Lwt.t

find f s find an element in a stream.

val find_map : ('a -> 'b option) -> 'a t -> 'b option Lwt.t

val find_map_s : ('a -> 'b option Lwt.t) -> 'a t -> 'b option Lwt.t

find_map f s find and map at the same time.

val combine : 'a t -> 'b t -> ('a * 'b) t

combine s1 s2 combines two streams. The stream will end when either stream ends.

val append : 'a t -> 'a t -> 'a t

append s1 s2 returns a stream which returns all elements of s1, then all elements of s2

val concat : 'a t t -> 'a t

concat st returns the concatenation of all streams of st.

val flatten : 'a list t -> 'a t

flatten st = map_list (fun l -> l) st

val wrap_exn : 'a t -> 'a Lwt.result t

wrap_exn s is a stream s' such that each time s yields a value v, s' yields Result.Ok v, and when the source of s raises an exception e, s' yields Result.Error e.

Note that push-streams (as returned by Lwt_stream.create) never raise exceptions.

If the stream source keeps raising the same exception e each time the
stream is read, s' is unbounded. Reading it will produce Result.Error e
indefinitely.
**Since** 2.7.0

## Parsing ¶

val parse : 'a t -> ('a t -> 'b Lwt.t) -> 'b Lwt.t

parse st f parses st with f. If f raise an exception, st is restored to its previous state.

It raises Invalid_argument if st is a bounded push-stream.

## Misc ¶

val hexdump : char t -> string t

hexdump byte_stream returns a stream which is the same as the output of hexdump -C.

Basically, here is a simple implementation of hexdump -C:

`let () = Lwt_main.run (Lwt_io.write_lines Lwt_io.stdout (Lwt_stream.hexdump (Lwt_io.read_lines Lwt_io.stdin)))`

## Deprecated ¶

type 'a result = | Value of 'a | Error of exn(* <<div class="odocwiki_info"|<<span class="odocwiki_warning"|Deprecated.>>~Replaced by <<a_api | val Lwt_stream.wrap_exn >>~, which uses <<a_api | type Lwt.result >>~. ~A value or an error~. >> *)

val map_exn : 'a t -> 'a result t

Deprecated.Use Lwt_stream.wrap_exn. map_exn s returns a stream that captures all exceptions raised by the source of the stream (the function passed to Lwt_stream.from).

Note that for push-streams (as returned by Lwt_stream.create) all elements of the mapped streams are values.

If the stream source keeps raising the same exception e each time the stream is read, the stream produced by map_exn is unbounded. Reading it will produce Lwt_stream.Error e indefinitely.