Module Lwt_stream

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 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 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 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 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 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 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 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 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 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

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

deprecated

Use closed.

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

Same as on_termination.

deprecated

Use closed.

Stream transversal

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.

parameter max_concurrency

defaults to 1.

raises Invalid_argument

if max_concurrency < 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 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

A value or an error.

deprecated

Replaced by wrap_exn, which uses Lwt.result.

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

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

Note that for push-streams (as returned by 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.

deprecated

Use wrap_exn.