How to import modules from prolog

[michal-escola]

Hi I am wondering how I can use module http/thread_httpd from Prolog?
use_module(library(http/thread_httpd) is not working. Any ideas?

[pmoura]

For the general answer, you can refer to the following Handbook section:

https://logtalk.org/manuals/userman/predicates.html#calling-prolog-predicates

For the particular case you asked, the first think to notice is that, inside an object (or category, use_module/1-2 directives are Logtalk directives, not Prolog directives. You want to read:

https://logtalk.org/manuals/refman/directives/use_module_1.html
https://logtalk.org/manuals/refman/directives/use_module_2.html

The advised solution is to load any required Prolog modules from you application loader.lgt file. In this case, you could have e.g.

% HTTP and JSON libraries
:- use_module(library(http/thread_httpd)).
:- use_module(library(http/http_dispatch)).
:- use_module(library(http/http_json)).
% lambda library to simplify working with end-points
% (the http_handler/1 goal second argument is a closure that takes a request as argument)
:- use_module(library(yall)).
:- use_module(library(apply_macros)).

% load Logtalk resources
:- initialization((
    set_logtalk_flag(optimize, on),
    ...,
    logtalk_load(my_http_handler),
    ...
)).

% define end-points
:- initialization((
    % root end-point
    http_handler(/, [Request]>>(my_http_handler::request(root, Request)), [spawn([])]),
    % other end-points
    http_handler('/foo', [Request]>>(my_http_handler::request(foo, Request)), [spawn([])]),
    http_handler('/bar', [Request]>>(my_http_handler::request(bar, Request)), [spawn([])]),
    ...
)).

% start the server
:- initialization(http_server(http_dispatch, [port(8888)])).

Then perhaps in a my_http_handler.lgt file:

:- object(my_http_handler).

    :- public(request/2).
    :- mode(request(+atom, ++nonvar), one).
    :- info(request/2, [
        comment is 'Handle endpoint requests.',
        argnames is ['Endpoint', 'Request']
    ]).

    :- use_module(http_json, [
        http_read_json_dict/2, reply_json_dict/1, reply_json_dict/2
    ]).

    request(EndPoint, Request) :-
        http_read_json_dict(Request, Dict),
        ...
    ...

:- end_object.

This is, of course, one of several possible scenarios. But hopefully will help you get started.