Question

I am trying to solve the The Farmer, Goat, Wolf, Cabbage riddle in Prolog using the Breadth First technique and I am running into some issues. When I try to gather all the valid combinations for the second level of the tree it fails. Here is the relevant code,

extend([Node|Path], NewPaths) :-
    bagof([NewNode, Node|Path],
        (s(Node, NewNode), not(member(NewNode, [Node|Path]))),
        NewPaths),
    !.
extend(Path, []).

s(state(X,X,W,C), state(Y,Y,W,C))
    :- opp(X,Y), not(unsafe(state(Y,Y,W,C))).
s(state(X,G,X,C), state(Y,G,Y,C))
    :- opp(X,Y), not(unsafe(state(Y,G,Y,C))). 
s(state(X,G,W,X), state(Y,G,W,Y))
    :- opp(X,Y), not(unsafe(state(Y,G,W,Y))).
s(state(X,G,W,C), state(Y,G,W,C))
    :- opp(X,Y), not(unsafe(state(Y,G,W,C))).
s(state(F,G,W,C), state(F,G,W,C))
    :- fail.

opp(e,w).
opp(w,e).

unsafe(state(X,Y,Y,C)) :- opp(X,Y).
unsafe(state(X,Y,W,Y)) :- opp(X,Y).

not(P) :-
    P, !, fail
    ;
    true.

The extend predicate is where I am seeing the issues. When I run it on the first level, it works fine,

?- extend([state(e,e,e,e)],[X]).
X = [state(w,w,e,e),state(e,e,e,e)]

When I run the second level, it fails,

?- extend([state(w,w,e,e)],[X]).
no

It should return something like the following,

X = [state(e,w,e,e),state(w,w,e,e),state(e,e,e,e)]

Thanks in advance for all your help, as it is much appreciated.

Regards,

Darian

Was it helpful?

Solution

If I query your code I get

?- extend([state(w,w,e,e)],X).
X = [[state(e, e, e, e), state(w, w, e, e)], [state(e, w, e, e), state(w, w, e, e)]]

X is a list of lists. Then I simplified the extend/2 predicate,

extend([Node|Path], [Node|NewPaths]) :-
    bagof(NewNode,
        (s(Node, NewNode), not(member(NewNode, Path))),
        NewPaths).

and I get

?- extend([state(w,w,e,e)],X).
X = [state(w, w, e, e), state(e, e, e, e), state(e, w, e, e)].

Note that I'm not using [X] querying extend/2.

BTW this clause is useless

s(state(F,G,W,C), state(F,G,W,C))
    :- fail.
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