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

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.

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 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 Stdlib.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 parse) a bounded stream. These functions will raise Invalid_argument if you try to do so.

It raises Invalid_argument if size < 0.

return a creates a stream containing the value a and being immediately closed stream (in the sense of is_closed).

• since 5.5.0

return_lwt l creates a stream returning the value that l resolves to. The value is pushed into the stream immediately after the promise becomes resolved and the stream is then immediately closed (in the sense of is_closed).

If, instead, l becomes rejected, then the stream is closed without any elements in it. Attempting to fetch elements from it will raise Empty.

• since 5.5.0

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.

• since 4.2.0

of_lwt_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.

• since 5.5.0

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 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 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 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 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 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.

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

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.

Same as on_termination.

• deprecated

  Use closed.

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.

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.

• since 3.3.0

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 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 begin
    Lwt_io.write_lines
      Lwt_io.stdout
      (Lwt_stream.hexdump (Lwt_io.read_lines Lwt_io.stdin))
  end

A value or an error.

• deprecated

  Replaced by wrap_exn, which uses Lwt.result.

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.