/usr/lib/swipl/library/http/websocket.pl
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 http_open_websocket(+URL, -WebSocket, +Options) is det
Establish a client websocket connection. This predicate calls http_open/3 with additional headers to negotiate a websocket connection. In addition to the options processed by http_open/3, the following options are recognised:
subprotocols(+List)
List of subprotocols that are acceptable. The selected protocol is available as ws_property(WebSocket, subprotocol(Protocol).

Note that clients often provide an Origin header and some servers require this field. See RFC 6455 for details. By default this predicate does not set Origin. It may be set using the request_header option of http_open/3, e.g. by passing this in the Options list:

request_header('Origin' = 'https://www.swi-prolog.org')

The following example exchanges a message with the html5rocks.websocket.org echo service:

?- URL = 'ws://html5rocks.websocket.org/echo',
   http_open_websocket(URL, WS, []),
   ws_send(WS, text('Hello World!')),
   ws_receive(WS, Reply),
   ws_close(WS, 1000, "Goodbye").
URL = 'ws://html5rocks.websocket.org/echo',
WS = <stream>(0xe4a440,0xe4a610),
Reply = websocket{data:"Hello World!", opcode:text}.
Arguments:
WebSocket- is a stream pair (see stream_pair/3)