http_upgrade_to_websocket/3

Availability::- use_module(library(http/websocket)).

http_upgrade_to_websocket(:Goal, +Options, +Request)

Create a websocket connection running call(Goal, WebSocket), where WebSocket is a socket-pair. Options:

guarded(+Boolean): If true (default), guard the execution of Goal and close the websocket on both normal and abnormal termination of Goal. If false, Goal itself is responsible for the created websocket. This can be used to create a single thread that manages multiple websockets using I/O multiplexing.
subprotocols(+List): List of acceptable subprotocols.
timeout(+TimeOut): Timeout to apply to the input stream. Default is infinite.

Note that the Request argument is the last for cooperation with http_handler/3. A simple echo server that can be accessed at =/ws/= can be implemented as:

:- use_module(library(http/websocket)).
:- use_module(library(http/thread_httpd)).
:- use_module(library(http/http_dispatch)).

:- http_handler(root(ws),
                http_upgrade_to_websocket(echo, []),
                [spawn([])]).

echo(WebSocket) :-
    ws_receive(WebSocket, Message),
    (   Message.opcode == close
    ->  true
    ;   ws_send(WebSocket, Message),
        echo(WebSocket)
    ).

throws: switching_protocols(Goal, Options). The recovery from this exception causes the HTTP infrastructure to call call(Goal, WebSocket).
See also: http_switch_protocol/2.