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Asked: 2026-06-06T06:12:57+00:00

When I attempt to use SWI-Prolog’s http_post/4 , as follows: :- use_module(library(http/http_client). update(URL, Arg)

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When I attempt to use SWI-Prolog’s http_post/4, as follows:

:- use_module(library(http/http_client).

update(URL, Arg) :-
    http_post(URL, form([update = Arg), _, [status_code(204)]).

When I query this rule, and watch the TCP traffic, I see the HTTP POST request and reply with the expected 204 status code both occur immediately. However, Prolog hangs for up to 30 seconds before returning back ‘true’. What is happening that prevents the rule from immediately returning?

I’ve tried this variant as well, but it also hangs:

:- use_module(library(http/http_client).

update(URL, Arg) :-
    http_post(URL, form([update = Arg), Reply, [status_code(204)]),
    close(Reply).

I have a similar issue with http_delete/3, but not with http_get/3.

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1 Answer

  1. Editorial Team

    library docs state that http_post

    It is equivalent to http_get/3, except for providing an input document, which is posted using http_post_data/3.

    http_get has timeout(+Timeout) in its options. That could help to lower the latency, but as it is set to +infinite by default, I fear will not solve the issue. Seems like the service you are calling keeps the connection alive up to some timeout.

    Personally I had to use http_open, instead of http_post, when calling Google API services on https…

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