swi prolog mysql + web

Question

Sorry for my English. I want to create a simple website that will take data from the mysql database and display it on the page. I have two problems:

1) work with a database made in separate modules:

% database.pl
:- module(database,
 [ create_db_connect/0,
 use_database/0,
 query_to_database/1,
 disconnect_database/0
 ]).

:- use_module(library(odbc)).

create_db_connect :-
 odbc_connect('test', _,
       [ user('root'),
         password('123')
         alias(myblog),
         open(once)
 ]).

use_database :-
 odbc_query(myblog, 'use test', _).

query_to_database(X):-
 odbc_query(myblog, 'SELECT data FROM testtable where id = 4', row(X)).

disconnect_database :- odbc_disconnect(myblog).

import this module in the main file:

% el.pl
:- use_module(library(http/thread_httpd)).
:- use_module(library(http/http_dispatch)).
:- use_module(library(http/html_write)).
:- use_module(library(http/http_parameters)).
:- use_module(library(http/html_head)).

:- use_module(database).

:- http_handler(root(.), home, []).

server(Port): -
 http_server(http_dispatch, [port(Port)]).


home(_Request): -
 reply_html_page (
 title('Sql'),
 [\ main_page
 ]).

main_page -->
 create_db_connect,
 use_database,
 query_to_database(X),
 disconnect_database,
 html(div('id="tab_c2"', p('~w')-[X])).

In this case, get the error:

Warning: The predicates below are not defined. If these are defined
Warning: at runtime using assert / 1, use: - dynamic Name / Arity.
Warning:
Warning: create_db_connect/2, which is referenced by
Warning: /root/prologDev/el.pl:56:17: 1-st clause of main_page/2

But why? I defined it in the module database.pl!

2) Although I do not like the decision, but I adjusted the module database.pl on this:

:- module (database,
 [ create_db_connect/2,
   use_database/2,
   query_to_database/3,
   disconnect_database/2
 ]).

:- use_module(library(odbc)).

create_db_connect(_, _) :-
 odbc_connect('test', _,
 [ user('root'),
   password('123'),
   alias(myblog),
   open(once)
 ]).

use_database(_, _) :-
 odbc_query(myblog, 'use test', _).
query_to_database(X, _, _) :-
 odbc_query(myblog, 'SELECT data FROM testtable where id = 4', row(X)).
disconnect_database(_, _) :- odbc_disconnect(myblog).

And now, the page is empty. When I stop swipl whit halt, an error occurs: Error in my_thread_global_end (): 1 threads did not exit.

what i`m doing wrong?

Answer 1

Note that main_page//0 is a non-terminal, not a predicate. To call your database predicates from the main_page//0, you need to write something like:

main_page -->
   {create_db_connect,
   use_database,
   query_to_database(X),
   disconnect_database},
   html(div('id="tab_c2"', p('~w')-[X])).

The {}/1 construct allows you to call predicates from (the body of) grammar rules. Without it, create_db_connect and the others would be interpreted as calls to other non-terminals (create_db_connect//0, ...).

Typically, non-terminals are expanded into predicates by appending two arguments. Hence the warnings you got:

Warning: create_db_connect/2, which is referenced by
Warning: /root/prologDev/el.pl:56:17: 1-st clause of main_page/2