Question

Recently I tried the book “Building Expert Systems in Prolog” which is freely available at http://www.amzi.com/ExpertSystemsInProlog.

There is a code called native shell at the appendix along with a bird identification database. The problem is that after consulting with the shell and running:

main.

load.

solve.

It asks “nostrils:external_tubular?”

If you answer no, then an stack overflow occurs. The problem is probably at the section blow:

prove(true,_) :- !.
prove((Goal,Rest),Hist) :-
    prov(Goal,[Goal|Hist]),
    prove(Rest,Hist).
prove(Goal,Hist) :-
    prov(Goal,[Goal|Hist]).

prov(true,_) :- !.
prov(menuask(X,Y,Z),Hist) :- menuask(X,Y,Z,Hist), !.
prov(ask(X,Y),Hist) :- ask(X,Y,Hist), !.
prov(Goal,Hist) :-
    clause(Goal,Body),
    prove(Body,Hist).

Can somebody please please help me? Any help is highly appreciated.

Full code of the shell: http://www.amzi.com/ExpertSystemsInProlog/code/native/native.pro

Full code for database: http://www.amzi.com/ExpertSystemsInProlog/code/native/birds.nkb

I’m using swi prolog.

Answer 1

I found the problem to be here:

prove((Goal,Rest),Hist) :-
    prov(Goal,[Goal|Hist]),
    prove(Rest,Hist).
prove(Goal,Hist) :-
    prov(Goal,[Goal|Hist]).

Since swi-prolog allows the head ((x,y,z), [h]) to match the second prove predicate, somehow in the inference process the prove(Goal,Hist) is triggered which will cause the infinite loop to occur. It took me 8 hours to analyze the trace.

Therefore to fix the issue, we need a cut to be placed in the beginning of prove((Goal,Rest),Hist). The code should look like this:

prove((Goal,Rest),Hist) :-
    !,
    prov(Goal,[Goal|Hist]),
    prove(Rest,Hist).
prove(Goal,Hist) :-
    prov(Goal,[Goal|Hist]).