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Accessing WWW/Internet: Example

check_links(URL,BadLinks) :-
        url_info(URL,URLInfo),
        fetch_url(URLInfo,[],Response),
        member(content_type(text,html,_),Response),
        member(content(Content),Response),
        html2terms(Content,Terms),
        check_source_links(Terms,URLInfo,[],BadLinks).

check_source_links([],_,BL,BL).
check_source_links([E|Es],BaseURL,BL0,BL) :-
        check_source_links1(E,BaseURL,BL0,BL1),
        check_source_links(Es,BaseURL,BL1,BL).
check_source_links1(env(a,AnchorAtts,_),BaseURL,BL0,BL) :-
        member((href=URL),AnchorAtts), !,
        check_link(URL,BaseURL,BL0,BL).
check_source_links1(env(_Name,_Atts,Env_html),BaseURL,BL0,BL) :- !,
        check_source_links(Env_html,BaseURL,BL0,BL).
check_source_links1(_,_,BL,BL).

check_link(URL,BaseURL,BL0,BL) :-
        url_info_relative(URL,BaseURL,URLInfo), !,
        fetch_url_status(URLInfo,Status,Phrase),
        ( Status \== success ->
          name(P,Phrase),
          name(U,URL),
          BL = [badlink(U,P)|BL0]
        ; BL = BL0
        ).
check_link(_,_,BL,BL).

fetch_url_status(URL,Status,Phrase) :-
        fetch_url(URL,[head,timeout(20)],Response), !,
        member(status(Status,_,Phrase),Response).
fetch_url_status(_,timeout,timeout).