Module Lwt_stream

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.

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.

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

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.

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 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.

Type of sources for bounded push-streams.

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.

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).

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).

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).

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.

Returns the list of elements of the given stream

Returns the word composed of all characters of the given stream

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

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

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

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

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

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

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

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.

junk st removes the first element of st.

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

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

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.

get_available st returns all available elements of l without blocking.

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

is_empty st returns whether the given stream is empty.

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

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

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.

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

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.

map f st maps the value returned by st with f

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

filter_map f st filter and map st at the same time

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

fold f s x fold_like function for streams.

iter f s iterates over all elements of the stream.

find f s find an element in a stream.

find_map f s find and map at the same time.

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

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

concat st returns the concatenation of all streams of st.

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

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

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.

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.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.