How to implement a solve predicate for this “moving blocks” Prolog exercise?

Question

I am studying Prolog using Ivan Bratko book: Programming for Artificial Intelligence and I am finding some difficulties to implement the final part of an exercise proposed

Answer 1

There will be loops in space search graph, then you can switch to some form of bound search. The easier I know it's bounded depth search:

?- length(Situation,_), solve([[c,a,b],[],[]], Situation).
Situation = [[[c, a, b], [], []], [[a, b], [c], []], [[b], [a], [c]], [[], [b, c], [a]], [[], [a, b|...], []]] .

length/2 enumerates unbound lists of growing length. So we get a result.

Note that this will still loop if there are no solutions from initial state to goal/1. If this is bad, I think solve/2 will need a service solve2/2 predicate, that will get the path, to enable the usual \+ memberchk(NewState, Visited) after the nondeterministic s/2 call. Then it will be (untested)

solve(N, SearchPath) :- solve2([N], SearchPath).

solve2([N|Visited], [N|Visited]) :- goal(N).
solve2([N|Visited], Path) :-
   s(N,N1),
   \+ memberchk(N1, Visited),
   solve2([N1,N|Visited], Path).